ANTILOCK BRAKE SYSTEM

There are a lot of ABS-equipped vehicles on the road today that will eventually need ABS replacement parts as they age.

What kind of parts fail? Wheel speed sensors are probably the most often replaced component. Most wheel speed sensors (WSS) are located at the wheels, which makes them vulnerable to road splash, corrosion and road hazards. The WSS wires and connectors are also often a source of trouble.

On many vehicles, each wheel has its own speed sensor. These are called "four channel" ABS systems. On others, a common sensor is used for the rear wheels (which may be mounted in the differential or transmission) but each front wheel still has its own wheel speed sensor. These are called "three-channel" ABS systems.

Another variation is the "single-channel" rear-wheel-only ABS system that is used on many rear-wheel-drive pickups and vans. This includes Ford "Rear Antilock Brakes" (RABS) and GM and Dodge "Rear-Wheel Anti-Lock" (RWAL). The front wheels on these trucks have no speed sensors and only a single speed sensor mounted in the differential or transmission for both rear wheels. Rear-wheel antilock systems are typically used on applications where vehicle loading can affect rear wheel traction, which is why it's used on pickup trucks and vans.

Most ABS systems use magnetic wheel speed sensors that generate a frequency signal as the notches on the sensor ring pass by the tip of the sensor. The sensor ring is usually mounted on the back of the brake rotor or outer CV joint.

On some vehicles (many GM models, for example), the wheel speed sensor is built into the sealed wheel hub assembly - making it very expensive to replace if it fails because the entire hub has to be changed. On most vehicles, though, the wheel speed sensor can be replaced separately if it fails.

The WSS signal is sent to the ABS control module, where the signal pulses are counted so the module can monitor wheel speed. If the tip of the WSS becomes coated with debris, or there's a poor wiring connection between the sensor and module, or the "air gap" between the end of the sensor and the sensor ring is too large, it may prevent the WSS from generating a good signal. An internal short or open in the sensor itself can also render it useless. The ABS control module will detect the problem and set a fault code that identifies the "bad" WSS circuit (a number that corresponds to left front, for example.) This does not mean the sensor itself is bad, only that there is a signal problem in that WSS circuit.

The troublesome WSS circuit can be diagnosed by measuring the resistance of the WSS with an ohmmeter, by checking wiring continuity and inspecting the wiring and connectors, or by spinning the wheel and checking the sensor's output signal.

If a WSS sensor is lost, the ABS system can't monitor wheel speed and can't tell if ABS is needed or not. So it will set a code and disable itself. As long as the warning light remains on, the ABS system will stay "off line." Only when the fault has been fixed and the code cleared will the warning light go out, allowing the ABS system to become active again.

ABS systems are designed to be as "fail-safe" as possible. All ABS systems run their own self-diagnostics and will set codes and deactivate themselves when a serious fault is detected. An illuminated ABS warning light may seem ominous, but most vehicles should still be safe to drive and have normal braking. The only exceptions are some older vehicles with "integral" ABS systems that use the ABS pump and accumulator to provide power-assisted braking. On these vehicles, the brakes should still work, but without the benefit of any power assist if the ABS system goes down.

If the brake warning light is also on, however, it may indicate a more serious problem such as loss of brake fluid or a low fluid level. So if both warning lights are on (ABS and brakes), the vehicle should not be driven until the problem can be investigated.

Another ABS component that often fails is the hydraulic modulator assembly that contains the ABS solenoid valves, or in the case of GM ABS-IV systems, the motor pack and valve assembly. This is the heart of the ABS system that controls the cycling of hydraulic pressure to the wheels. On some systems, the ABS solenoid valves can be replaced separately if one fails.

But on many systems, the entire modulator must be replaced if anything inside fails. Failures may be caused by electrical faults (opens or shorts in solenoids), by internal corrosion or by debris in the brake fluid that jams a solenoid valve or prevents it from fully closing.

Modulators can be tricky to replace because they must be bled after they have been installed to remove all air from the circuits. On some vehicles, they may require using a scan tool to cycle the ABS solenoids. On others, manual bleeder screws may be provided for this purpose. Either way, your customer will also need brake fluid to refill the system. The old fluid should also be flushed out to get rid of any sediment or contamination that may cause future problems.

On systems that use a high-pressure pump and accumulator to provide power-assisted braking or to reapply pressure during an ABS stop, a defective pump or relay can cause a loss of pressure. So can a leaky accumulator. The ABS system monitors these components and will set a code if it detects a problem.

The accumulator can be dangerous to replace because it may contain up to 1,500 or more pounds of pressure. Your customers should be warned to never open up an ABS system that may still be under pressure. The accumulator must first be discharged by pumping the brake pedal 40 times with the ignition key off. Once all pressure has been relieved, it can be safety removed and replaced.

ABS control modules can also fail, though they don't fail that often. Most ABS problems are electrical (bad WSS, pump relay, pump motor, wiring or solenoid) or mechanical (sticking, jammed or leaking ABS valve.) If a module does go bad, it may cause erratic or abnormal operation of the system (such as ABS braking during a normal stop.) ABS should only come into play when traction conditions are marginal or during sudden "panic" stops. The rest of the time, it should have no effect on normal driving or braking.

When ABS comes into play, the module energizes the solenoid valves in the hydraulic modulator to hold, release and reapply hydraulic pressure to the brakes. Sealing off the affected brake circuit prevents any additional pressure from being applied to the brake. The module then energizes a vent solenoid to release pressure from the line. This allows the brake to release momentarily so the tire can regain its grip. The vent valve is then closed and the pressure valve reopened to reapply the brake. This rapid cycling produces a pulsating effect that can usually be felt in the brake pedal during an ABS stop. The driver may also hear a buzzing or chattering noise from the ABS hydraulic unit - which is normal and tells the driver that the ABS system is active.

 BRAKING SYSTEM

The brake system includes the master cylinder, power booster, disc brake calipers and rotors, wheel cylinders and brake drums, brake hardware, brake hoses and lines, various valves, disc brake pads and drum shoes, and brake fluid. On vehicles equipped with anti-lock brakes (ABS), additional parts include up to four wheel speed sensors, the ABS hydraulic modulator and control module, and a pump motor and accumulator (not used on some systems).

When the driver steps on the brake pedal, it moves a rod in the master cylinder forward to push a pair of pistons against fluid in the primary and secondary chambers. Brake systems are split into two separate hydraulic circuits. Each circuit operates two of the four brakes (both fronts, both rears or a diagonal pair). This is a safety requirement, so if one circuit fails, at least two brakes will continue to operate so the vehicle can still be stopped.

Brake fluid carries the hydraulic pressure created in the master cylinder through the brake lines to the front calipers and rear calipers or drums to apply the brakes. Most older vehicles have front disc brakes and drums in the rear, but many newer cars, SUVs and trucks have disc brakes in the front and rear. When pressure reaches the brakes, the pads are squeezed against the rotors, and if the vehicle has drums in the rear, the shoes are pushed out against the drums to generate friction and stop the vehicle.

On vehicles equipped with power brakes, a brake booster located behind the master cylinder on the firewall multiplies the force of the brake pedal input using engine vacuum and a large diaphragm. This reduces the pedal effort needed to stop the vehicle. On some older vehicles with integral ABS systems, the ABS pump and accumulator provide power assist.

Additional parts involved in the braking process include a "pressure differential valve" (a safety switch that turns on the brake warning lamp if there's a loss of pressure in either brake circuit), a "proportioning valve" to reduce pressure to the rear brakes for more balanced braking (not used on all vehicles), and on some, a "load-sensing proportioning valve" to increase or decrease hydraulic pressure to the rear brakes based on vehicle loading.

The major wear components in the brake system are the disc brake pads and drum shoes. Every time the brakes are applied these parts are subjected to friction and wear. Lining life depends on how the vehicle is driven. Stop-and-go city driving, mountain driving and aggressive driving all involve more frequent braking, harder braking and higher brake temperatures - all of which add up to more wear and shorter lining life. Highway driving and gradual, light braking produce less lining wear and longer lining life.

On most vehicles, the front linings wear out twice as fast as the rear linings, so when the linings need to be replaced for the first time it's usually only the front pads that need to be changed. Replacement linings should be the same or better than the original linings. Semi-metallic linings are often used in high-heat applications because they can withstand high operating temperatures without fading or wearing excessively. Other high-temperature friction materials include linings with ceramic content. Pads and shoes with non-asbestos organic (NAO) linings are typically used for lower heat applications such as rear brakes and front brakes on rear-wheel drive cars and trucks.
When linings are replaced, the rotors and drums may need to be resurfaced or replaced depending on their condition. If a rotor is worn to minimum thickness or a drum is worn to maximum diameter, replacement is necessary. Most rotors are made of cast iron, but "composite" rotors have a thin, stamped steel center hat attached to a cast iron rotor ring. Composite rotors are more difficult to resurface and more prone to pedal pulsation problems than one-piece cast rotors. They are also more expensive than cast. Composite rotors can be replaced with cast rotors as long as both rotors are replaced at the same time (don't intermix different kinds of rotors side-to-side).

Most front rotors are vented, while most rear rotors are not because more cooling is usually needed for the harder-working front brakes. Most rotors are interchangeable left to right, but some are directional, so pay close attention to the catalog listings when looking up part numbers. New rotors are ready to install and do not require additional resurfacing (turning rotors unnecessarily shortens the service life).

Other parts that wear out over time include hydraulic components such as the calipers, wheel cylinders and master cylinder. Most calipers have one or two pistons, but some have up to four pistons mounted in a rigid housing. Most calipers are cast iron (though some are aluminum) with steel or molded phenolic (plastic) pistons. Most calipers are a "floating" design with slides or bushings that allow the caliper to move sideways and center itself over the rotor when the brakes are applied.

Others are a "fixed" design with rigid mounts and do not move. If the slides or bushings on a floating caliper become badly corroded or worn, it may prevent the caliper from sliding, causing uneven pad wear. The inside pad will wear faster than the outside pad. If a piston in either type of caliper sticks, the caliper may not release. This causes the brake to drag, puts rapid pad wear on one side, and an uneven braking and a pull to one side when the brakes are applied.

Leaky piston seals will allow brake fluid to contaminate the brake linings. Leaky calipers must be rebuilt or replaced. Loaded calipers come ready to install with new pads. Bare calipers do not include pads. On vehicles with four-wheel disc brakes, the rear calipers may also include some type of parking brake mechanism. This makes the calipers more complicated and expensive to replace.

The wheel cylinders inside drum brakes have two opposing pistons that move outward when pressure is applied. The wheel cylinder is mounted on the brake backing plate and has dust seals over the pistons to keep out dust and water. Each piston has a cup-shaped seal for the fluid inside. Common problems with wheel cylinders include fluid leaks and sticking. Wheel cylinders can be rebuilt or replaced. Leaking fluid can contaminate the brake shoes, requiring their replacement as well.

Wear in the master cylinder may allow fluid to leak past the piston or shaft seals. One symptom that might indicate a bad master cylinder is a brake pedal that slowly sinks to the floor when braking at a stop light. Leaks or failure to hold pressure require rebuilding or replacing the master cylinder. Rebuilding aluminum master cylinders is not recommended. On some older vehicles with ABS, the master cylinder is part of the ABS modulator and is very expensive to replace.

Rubber brake hoses can also deteriorate with age and leak. Any hose that is cracked, bulging, leaking or damaged should be replaced without delay because of the danger of brake failure should the hose leak. Steel brake lines can corrode internally or externally. Replacement brake lines must be steel with double-flared or ISO end fittings.

Brake fluid also wears out over time and should be replaced when the brakes are serviced. The main issue here is moisture contamination that causes a breakdown of corrosion inhibitors in the fluid and lowers the fluid's boiling temperature (which increases the risk of fluid boil and pedal fade under hard use). DOT 3 and DOT 4 brake fluid are the two main types, and both are glycol-based hydraulic fluids. DOT 5 fluid is a silicone-based fluid and is used only for special applications (like older vehicles that sit for long periods of time or are operated in extremely wet environments). DOT 4 has a higher temperature rating than DOT 3 and is used in many European vehicles. Use the type of fluid specified by the vehicle manufacturer.

Related items that may also need to be replaced when servicing the brakes include the wheel bearings and seals. On older vehicles with serviceable wheel bearings, the grease seals should always be replaced when the bearings are cleaned and repacked with grease. Special high-temperature wheel bearing grease is required (never ordinary chassis grease).

Disc and drum brake hardware should also be replaced when the brakes are serviced. Drum hardware includes the return springs, hold-down springs, self-adjusters and other cables, clips or springs used in the brake assembly. Return springs that pull the shoes back away from the drum when the brakes are released may become weak with age, allowing the brakes to drag.

Self-adjusters can become corroded and stick, which can cause increased pedal travel as the shoes wear. On disc brakes, the hardware includes slides and bushings that can become worn and corroded, and anti-rattle clips and springs that reduce noise. A high-temperature, moly-based brake grease should be used to lubricate slides, bushings and shoe pads on drum-brake backing plates

COOLING SYSTEMS

The cooling system includes the radiator, radiator cap, coolant reservoir, fan, water pump, thermostat, hoses, belts and antifreeze. Related parts include the coolant sensor and fan relay.
The radiators in most late-model vehicles are aluminum with plastic end tanks. Most older vehicles have copper/brass radiators. Both types are vulnerable to internal corrosion caused by coolant neglect. Replacement radiators should have the same cooling capacity (or better) than the original, and the same hose connections. Cooling capacity is determined by the thickness of the radiator, the number of fins and tubes and/or the design of the fins and tubes. Increased cooling capacity is recommended for towing and performance applications.

The water pump is the heart of the cooling system. It is belt-driven and consists of an impeller mounted on a shaft inside a cast or stamped steel housing. The pump is usually mounted on the front of the engine. On some overhead cam (OHC) engines, the pump is mounted under the timing belt and requires considerable labor to replace. For this reason, you should recommend replacing the pump if the timing belt is being replaced for scheduled maintenance (recommended every 60,000 to 100,000 miles, depending on the application).

The service life of the water pump and timing belt is about the same, so changing both at the same time can save the vehicle owner money on future repairs. Water pumps don't last forever, and leaks around the pump shaft seal and bearing can quickly lead to overheating. Any water pump that is leaking, making noise or has excessive shaft play should be replaced. Replacement options include remanufactured and new pumps.

The water pump may also be driven by a V-belt or a flat serpentine belt. The same belt may also drive other engine accessories. Belts deteriorate with age and should be replaced if frayed, cracked, glazed or oil-soaked. Replacement belt length and width must be the same as the original. On vehicles with serpentine belts, the automatic tensioner may also need to be replaced if it is sticking, making noise or cannot maintain proper belt tension. Belt idler pulleys should also be replaced if noisy, worn or sticking.

For temperature control, the cooling system requires a thermostat. It is usually located in a housing where the upper radiator hose connects to the engine. The thermostat does two things: 1. It allows the engine to warm up quickly (which reduces cold emissions and fuel consumption) and 2. Maintains a consistent operating temperature (also important for low emissions, good fuel economy and performance). The thermostat has a temperature-sensitive valve that remains closed and blocks the flow of coolant until the engine reaches 195 to 210 degrees. It then opens and allows coolant to flow to the radiator. The thermostat continues to cycle open and shut so the engine can run within a certain temperature range. This is very important on late-model vehicles with computer engine controls because engine temperature affects the fuel feedback control loop, emissions, fuel economy and performance.

If the thermostat sticks shut, the engine will overheat. If it fails to close, the engine will be slow to warm-up and the heater may not put out much heat when the weather turns cold. Fuel economy, emissions and engine wear will also suffer. Under no circumstances should an engine run without a thermostat. Replacement thermostats should have the same rating as the original. A slightly hotter thermostat may be used during cold weather for increased heater output, but a colder thermostat should not be used on engines with computer controls. Other items that may be needed when changing a thermostat include a new thermostat housing and gasket or sealer.

To improve cooling, a fan is needed to pull air through the radiator when the vehicle is stopped or traveling at low speed. Older rear-wheel-drive vehicles may have a belt-driven fan with or without a clutch. The clutch allows some slippage and is used to reduce fan noise at high RPM and to improve fuel economy by reducing drag. Excessive slippage in the clutch, however, may reduce airflow and cause the engine to overheat at low speed. Most newer vehicles have one or two electric cooling fans mounted behind the radiator, and a few have hydraulic fans driven by power steering fluid. Electric fans are powered through a relay and controlled by a coolant temperature switch or the engine computer. A failure of the fan motor, fan relay, coolant temperature switch or a wiring problem that prevents the fan from coming on can cause the engine to overheat.

The major hoses in the cooling system include the upper and lower radiator hoses (the lower one usually attaches to the water pump and the upper usually attaches to the thermostat housing), plus a pair of heater hoses (one inlet and one outlet) and various connecting hoses and bypass hoses depending on the application. Replacement hoses must be the same length and diameter as the original. New clamps should also be installed when hoses are replaced. Hoses should be replaced if leaking, cracked or bulging. Electro-chemical degradation due to coolant neglect can cause hoses to fail from the inside out.

The coolant is a mixture of ethylene glycol antifreeze and water (typically a 50/50 mix). This combination provides freezing protection down to -34 degrees and boilover protection up to 265 degrees with a 15 psi radiator cap. The condition of the coolant is just as important as the strength because corrosion can attack the system from within if the coolant is neglected. The recommended replacement interval for traditional "green" antifreeze is two years or 30,000 miles. For the newer extended life coolants, the change interval can be as long as five years or 150,000 miles. Most long-life coolants use organic acid technology (OAT) additives that are different from those used in standard antifreeze. Long-life coolants may contain special dyes to distinguish them from ordinary antifreeze, such as the orange dye in General Motor's Dex-Cool coolant. Other manufacturers use different colors. Even so, the type of coolant can't always be determined by its color, so color is not a reliable indicator of the type of coolant in a vehicle.The cooling system should be topped off or refilled with the same type of antifreeze as the original.

 COOLING SYSTEM  ANTIFREEZE COLORS

Have you ever seen the inside of an aluminum water pump that was not adequately protected by the corrosion inhibitors in the coolant? Or a radiator or heater core that failed from the inside out because of internal corrosion? These kinds of parts failures are all too common. Yet they can be easily prevented by using the "right" coolant and changing the coolant before trouble starts to heat up.

All types of antifreeze contain corrosion inhibiting chemicals to protect bare metal surfaces from electrolytic attack. Though auto makers disagree on which chemical additives work best in their vehicles, essentially any kind of antifreeze will work in any vehicle. But how well will it protect the cooling system? And for how long? And will it void the OEM warranty? These are important questions that need to be answered before choosing an antifreeze for a particular vehicle application.

There are essentially three basic types of antifreeze corrosion additives for passenger cars and light trucks:

• "IAT" (Inorganic Acid Technology) is the traditional "green" formula antifreeze. This is the stuff General Motors used until 1996, Chrysler used until 2001 and Ford used until 2002 in its trucks, and 2003 in its passenger cars. The green additive package contains phosphate and silicates, and provides good protection for cast iron and aluminum engine parts, as well as copper/brass radiators in older vehicles and aluminum radiators in newer vehicles. The corrosion-fighting chemicals are fast-acting but wear out after two to three years or 36,000 miles of average use, so green coolant needs to be changed periodically to minimize the risk of corrosion damage.
• "OAT" (Organic Acid Technology) is usually dyed orange to distinguish it from other types of antifreeze. In 1996, GM began using a new extended-life antifreeze, called "Dex-Cool." The coolant contains a totally different kind of additive package called Organic Acid Technology (OAT). The OAT corrosion inhibitors are slower acting and provide protection over a longer period of time. OAT coolants typically have a service life of up to five years or 150,000 miles, making coolant changes less frequent - but still necessary (a fact that many motorists seem to forget).
  
  Other applications that currently use OAT antifreeze as the factory-fill coolant include 1996-and-newer Audi, Jaguar, Porsche, Volkswagen and Land Rover, 2001-and-newer Saab, and 1996-and-newer Toyota, Nissan, Honda, Mazda and other Asian makes.
  
  Though OAT provides good protection for aluminum, it may not be the best choice for older vehicles with copper/brass radiators because of the lead-based solder used in the radiator. Some say OAT-based coolants may also provide little protection against cavitation erosion in water pumps with aluminum housings (unless the pump impeller is carefully designed to minimize cavitation).
• "HOAT" (Hybrid Organic Acid Technology) antifreeze is usually dyed yellow but may also be dyed orange or green. HOAT coolants are currently used by Ford, Chrysler, Mercedes, BMW and Volvo. The additive package in a HOAT formula coolant also contains silicates for added aluminum protection. Most of the antifreezes in this category also meet the European "G-O5" specification for hybrid extended life coolant. The service life for HOAT is also five years or 150,000 miles.

UNIVERSAL COOLANTS?
Why not just have one "universal" coolant for all makes and models? Some antifreeze suppliers do sell a universal product. They claim it is fully compatible with all types of coolants and is safe to use in virtually any American, Asian or European vehicle application. Universal coolants are typically OAT or HOAT formulas that can go up to five years or 150,000 miles between changes when the coolant is used to replace another coolant or is added to a cooling system that already contains an OAT or HOAT coolant. But if used to top off a cooling system that already contains an IAT green formula coolant, the service life is the same as the original IAT green coolant (two to three years or 36,000 miles).

The issue with universal coolants is that a single formula cannot meet the conflicting OEM specifications for IAT, OAT and HOAT coolants. If a universal coolant contains silicates, it does not meet the OEM OAT specification. If it contains no silicates, it can't meet the OEM HOAT specification. And if it contains phosphates or inorganic acid technology ingredients, it can't meet the OEM OAT or HOAT specifications. Consequently, some antifreeze suppliers argue there is no such thing as a universal coolant because one formula cannot meet all the conflicting OEM specifications. This means distributors must offer three different coolants to meet the IAT, OAT and HOAT specifications - otherwise the coolant may not satisfy the OEM warranty requirements. That's why the safest recommendation is to use the type of coolant specified by the vehicle manufacturer.

Of course, once a vehicle is out of warranty, motorists can use any type of coolant they want - and many do. Many people still prefer the traditional green formula coolant because it's the least expensive, even if it requires a little more maintenance. Others may want to switch from a green coolant to a longer-life OAT or HOAT coolant to reduce the need for maintenance. The aftermarket gives motorists a choice so they can choose a product that best suits their needs.

COOLANT MARKET
According to market research, there are approximately 224 million registered cars and light trucks on the road today. Of these, 56 percent or 125 million were originally equipped with IAT (green formula) antifreeze, 34 percent or 76 million were factory filled with OAT (orange) antifreeze and 10 percent or 23 million were factory filled with HOAT (yellow or G-05) antifreeze.

If a customer chooses a different type of coolant than that which was originally in the vehicle's cooling system, the cooling system should be flushed to remove all of the old coolant. This will avoid any potential incompatibility issues between IAT, OAT and HOAT coolants. We have not heard of any horror stories of bad things happening inside a cooling system when different types of coolants are intermixed. But antifreeze suppliers caution against mixing different types. Their advice is to use "same with same."

Regardless of which type of coolant is used, there are several points worth noting. If the cooling system contains visible sediment or has had a part fail due to corrosion, it needs to be thoroughly cleaned and flushed before the system is refilled. This can be done with a coolant flushing-cleaning machine, or by adding a can of cooling system cleaner to the radiator, driving the vehicle for several days (follow the directions on the product) and then draining and flushing the cooling system. Accumulated rust and scale can reduce heat transfer and may make the engine run hot or overheat. Any crud inside the cooling system will also react with the corrosion inhibitors in the fresh antifreeze, reducing the potential service life of the coolant.

ADDING ANTIFREEZE
Unless a customer is buying premixed antifreeze that requires no water (because it already contains water), antifreeze should always be mixed in equal parts with clean, distilled or deionized water when the cooling system is refilled. Avoid using ordinary tap water or even bottled drinking water because it usually contains dissolved minerals and salts that can shorten the life of the corrosion inhibitors in the antifreeze.

A 50/50 mixture of water and ethylene glycol antifreeze (any type) will protect against freezing down to -34 degrees F and boilover up to 263 degrees F with a 14 psi radiator cap. Never use straight water or straight antifreeze in the cooling system! Straight water provides no corrosion protection whatsoever. It will also freeze at 32 degrees F and boil at 212 degrees F (unpressurized). Straight antifreeze is also a poor choice because it freezes at 10 degrees F and does not conduct heat as efficiently as water (which may cause the engine to run hot and overheat). The best way to change the coolant is with a coolant exchange machine that replaces all of the cold coolant with new coolant. Just draining the radiator may leave up to a gallon of old coolant inside the engine block.

WATER PUMPS
The water pump is the heart of the cooling system because it circulates the coolant between the engine and radiator. The radiator is a heat exchanger and allows air flow to carry heat away from the coolant so the engine doesn't run too hot. A pump failure or a plugged radiator will usually make the engine overheat.

Water pumps work hard, typically pumping several hundred gallons of coolant per hour at highway speeds. Because of the continuous load, pump failures are not uncommon. The first symptom is usually leakage at the pump weep hole. Other symptoms include bearing noise (rumbling, chirping or growling), loss of coolant (through the leaky shaft seal), overheating (from coolant loss or separation of the impeller from its shaft) and fan wobble.

One way to spot a water pump with bad shaft bearings is to check pulley or fan play with the engine off. The pulley or fan should not wobble or show any visible play when it is tugged by hand. Suspected seal leaks can also be diagnosed by pressure testing the cooling system.

If a water pump needs to be replaced, the choices are a new pump or remanufactured pump. Reman pumps reuse the housing, impeller and shaft, and typically include a new bearing and seal assembly. Because of the variety in OEM pump configurations, make sure the replacement pump has the same mounting configuration, bolt locations and hose connections as the original. Also compare pump heights as these may also vary depending on the dimensions of the timing cover or other belt-driven engine accessories.

When a water pump is replaced, the cooling system should always be drained, flushed and refilled with a fresh mixture of antifreeze and water to restore proper cooling performance and corrosion protection. Belts and hoses should also be carefully inspected, and replaced if any are found to be in less-than-perfect condition. Hoses that are brittle, aged cracked, bulging or chaffed must be replaced. New clamps should also be recommended. Belts that are frayed, cracked or glazed also need to be replaced.

On overhead cam (OHC) engines that use the timing belt to drive the water pump, check the mileage on the original timing belt. If the belt has been in service more than the OEM recommended replacement interval (60,000 miles on older vehicles, and up to 100,000 miles on newer ones), the timing belt should also be replaced. This will save the labor of having to do the job twice because the timing belt usually has to be removed to install the pump.

If the engine has overheated because of a pump failure or loss of coolant, the thermostat should be replaced, too.

Another item that may also need replacing is the fan clutch (if the vehicle has one). The lifespan of the fan clutch and water pump are about the same. Because of this, many experts recommend replacing both components at the same time to reduce the risk of future cooling problems.

RADIATORS
A radiator may have to be replaced if it is leaking or is clogged internally and cannot be cleaned. Leaks can be caused by external or internal corrosion, vibration damage (hairline fatigue cracks) or by punctures (rocks or collision damage). A radiator can also be damaged if the coolant freezes during cold weather (not enough antifreeze in the coolant), or if the engine overheats or runs hot creating steam erosion inside plastic end tanks.

A can of cooling system sealer can often provide a temporary fix for a leaky radiator, and there are special epoxy glues available for patching small leaks in aluminum radiators. But the only permanent fix for leak is to repair or replace the radiator. A new radiator costs about the same as having an old radiator repaired, and it usually comes with a better warranty. Plus, there's no waiting for the radiator shop to fix the old radiator - which may take several days depending on the work load.

Radiators on most late-model vehicles are aluminum with either aluminum or plastic end tanks. Most radiators on older vehicles have a copper/brass core with metal end tanks. Aluminum is lighter weight and lasts longer because it has no lead soldered joints. Either type of radiator can be used in a vehicle, but the best advice is to replace same with same. For high-performance applications or hard-working vehicles, a stock copper-brass radiator or aluminum radiator can be replaced with a larger, thicker or more efficient aluminum radiator to improve cooling performance.

To consolidate applications, many replacement radiators are designed to fit as many vehicles as possible. The radiator may have extra fittings (which can be plugged if they are not needed). But as long as the radiator fits the engine compartment and has the necessary hose connections, it should work as long as it has a cooling capacity that is equal to or greater than the original radiator.

Before installing a new radiator, the cooling system should be cleaned and flushed. Other items that should also be replaced include the radiator cap (make sure the pressure rating is the same as the original), radiator hoses (upper and lower), heater hoses and hose clamps. The cooling system should then be refilled with fresh antifreeze and clean distilled or deionized water (not tap water).

 ELECTRICAL SYSTEMS

Electrical system parts include large rotating electrical components such as alternators and starters, solenoids, relays, batteries, battery cables, wiring, fuses, bulbs and small electrical motors for things such as power windows and seats.

Of all these parts, batteries are probably the most often replaced component. The average battery life is about four to five years in most areas of the country, and only about three years in really hot climates. Heat is hard on batteries because it increases evaporation of water from the cells. Maintenance-free batteries normally do not require make-up water, but in hot climates water loss can be a problem. Most people don't check their batteries anymore, and on sealed-top batteries there are no caps to remove. Consequently, battery life suffers.

If a customer needs a new battery, you might recommend upgrading to a gel-type battery that contains no liquid water. The electrolyte is a gel-like substance sandwiched between absorbent fiberglass mats in the cells. This makes the batteries spill-proof and much more resistant to heat damage. High-density, spiral-wound batteries also use this same type of gel electrolyte.

Each cell inside a battery produces a little over two volts of electricity. Automotive batteries contain a total of six cells, so the total voltage is about 12 volts. A fully charged battery will actually read about 12.65 volts if you place a voltmeter across the terminals.

Post configurations will vary depending on the application. Most General Motors batteries have flat terminals on the side to which the positive and negative cables are attached with bolts. Everybody else uses batteries with two top posts (one negative and one positive). Some aftermarket "universal" replacement batteries have both types of posts (top and side) to reduce the number of different batteries needed to cover vehicle applications.

Batteries come in different lengths, heights, widths and post configurations, which are classified according to group sizes. A replacement battery must be a compatible group size to physically fit the battery tray and cable locations on the vehicle.

Another difference in batteries is their power ratings. The most commonly used number is the "Cold Cranking Amp" (CCA) capacity, which is the maximum number of amps the battery can deliver when cranking the engine. The higher the number, the more amps the battery can provide for reliable cold starting.

Another power rating number that's important, but may be less familiar, is the battery "Reserve Capacity" (RC) rating. This is how many amp hours of current the battery can provide should the charging system fail. A replacement battery should have CCA and RC that meet or exceed the OEM battery requirements.

Battery date codes are also important. Batteries age on the shelf, so the oldest ones should always be sold first to keep the stock fresh. The number/letter date code on the battery reveals when it was manufactured. The number indicates the year, and the letter corresponds to the month (A = January, B = February, C = March, etc.)

To prevent comebacks, new and used batteries should be tested to confirm their state of charge and condition. A low battery that still has good cell plates can be recharged and returned to service. But if a battery fails a "load test" or will no longer accept a charge, it must be replaced. Batteries contain lead and should always be recycled. Handle all batteries with care because they contain acid.

ROTATING ELECTRICAL

The alternator is part of the charging system and generates voltage to keep the battery charged and to operate the ignition system, computer and other electrical accessories on the vehicle. The alternator is belt driven and produces an alternating current (AC) that is converted into 12 volts of direct current (DC) by diodes (the rectifier assembly) on the back of the alternator. The output voltage is controlled by the engine computer or an internal or external voltage regulator according to demand. The higher the load on the electrical system and battery, the higher the charging output of the alternator. Most charging systems that are working properly produce a charging voltage of about 13.8 to 14.2 volts at idle with the lights and accessories off.

Alternator output can be tested with a voltmeter, ammeter or special test equipment. Noisy shaft bearings may also require alternator replacement. Alternators often fail because of excessive heat and electrical overloads. The higher electrical loads that are common in many vehicles today can tax many OEM alternators to the limit, especially those that are equipped with high-output aftermarket audio systems, auxiliary lighting and other electrical accessories that increase the amp load on the charging system.

Replacement alternators should always have the same or higher amp rating as the original. Some of these premium-priced units have twice the output of a stock alternator and can greatly extend alternator life in demanding applications. Most are bolt-on replacements for the stock alternator, but heavier gauge cables are also required to handle the higher output.

Starters are replaced less often than alternators because they are only used to start the engine. Fuel-injected engines usually require little cranking to start, so the starter doesn't have to work very hard, except during cold weather when the oil in the engine thickens and makes it harder to crank. Prolonged cranking is what kills many starters because heavy cranking causes the starter to overheat.

The starter is mounted on the engine or transmission bellhousing and engages teeth on the flywheel to crank the engine. A one-way, over-running clutch is used to protect most starters against damage should the starter remain engaged after the engine starts.

There are several different types: direct-drive starter motors, gear-reduction starter motors and permanent-magnet starter motors (reduced-size starters with permanent magnets inside instead of wire coils). It's important to handle permanent-magnet starters with care because banging them on the counter or floor may break the magnets inside.

Because of the high load on the starter, good electrical connections are extremely important. Loose, corroded or undersized battery cables may not deliver enough amperage to crank the engine at normal speed, causing hard starting. Starter drives (which can be replaced separately on many starters) can also fail, preventing the motor from engaging the flywheel. A bad solenoid or relay will prevent the starter motor from cranking at all. Accurate diagnosis of a starter problem is important to prevent unnecessary parts replacements and returns.

OTHER ELECTRICAL

Battery cables should be replaced if loose, damaged or too small for the application. Engine ground straps are equally important and are an often-overlooked cause of charging and starting problems.
Fuses protect against electrical overloads and are designed to blow if there's too much current in a circuit. Overloads should not normally occur, so if a fuse has failed, there may be a short in the circuit. Replacement fuses must have the same amp rating as the originals. Relays are switching devices used to route power to other components such as the fuel pump, ABS system, lights and so on. Relays may be located in the engine compartment or almost anywhere in the vehicle (locating a particular relay often requires referring to the vehicle's wiring diagram).

Lamps and bulbs for interior and exterior illumination come in various sizes and styles. Replacement lamps and bulbs must have the same mounting and electrical connections as the original (compare the old and new bulbs or refer to an application chart), but headlamps can often be upgraded for more light output with higher output bulbs.

 EMISSION CONTROL SYSTEM

All 1996-and-newer vehicles have Onboard Diagnostics II (OBD II) as part of their programming to detect and identify emission faults. OBD II monitors ignition misfires, the efficiency of the catalytic converter, the operation of the engine's sensors and feedback fuel control loop and many other functions.

One way the engine control system minimizes pollution is by carefully controlling the air/fuel mixture. The engine computer does this by monitoring the amount of unburned oxygen (O2) in the exhaust with an oxygen sensor mounted in the exhaust manifold. A voltage signal produced by the O2 sensor tells the computer if the mixture is rich or lean. The computer then compensates by decreasing or increasing the on-time of the injectors.

In addition to the Powertrain Control Module (PCM), key engine sensors and various subsystems are used to control specific pollutants. These include the Positive Crankcase Ventilation (PCV) system, the Exhaust Gas Recirculation (EGR) system, the Evaporative Emission Control System (EVAP), the Air Injection Reaction (AIR) system and the catalytic converter.

The PCV system recirculates blowby vapors from the crankcase back into the intake manifold so the vapors can be reburned inside the engine. This prevents the escape of blowby vapors into the atmosphere, and also helps remove moisture from the crankcase to prolong the life of the oil and prevent the formation of engine-damaging sludge. The PCV valve is usually located in a valve cover, and is attached to the intake manifold by a hose.

The EGR system reduces the formation of oxides of nitrogen (NOx) in the engine by reducing peak combustion temperatures when the engine is under load. When the engine is under load, intake vacuum drops. This causes the EGR valve on the intake manifold to open a port between the intake and exhaust manifolds. Loss of EGR can result in increased emissions and engine-damaging detonation. The EGR valve is calibrated to the engine application, so a replacement valve must have the same flow characteristics. Some replacement EGR valves have various adapters to modify the flow rate.

The EVAP system controls evaporative emissions from the fuel system and fuel tank. Fuel vapors are trapped in a charcoal-filled canister, then vented into the engine through a purge valve to be reburned. A leaky fuel filler cap can allow fuel vapors to escape into the atmosphere. Replacement gas caps must be the same type as the original (caps for some older vehicles may be vented, but newer ones are sealed). Fuel vapor leaks are monitored by the OBD II system.

The Air Injection Reaction (AIR) system on older vehicles uses an air pump to pump extra oxygen into the exhaust system to reduce pollution. A diverter valve assembly on the air pump controls the flow of air into the exhaust manifold. On some vehicles, air is also routed to the catalytic converter. An anti-backfire valve prevents hot exhaust gases from flowing backwards into the system and damaging the diverter valve or air pump. Problems in this system can cause an increase in emissions.

The catalytic converter is an "afterburner" in the exhaust system that helps reburn unburned pollutants. The catalyst inside the converter can be contaminated by lead (leaded gasoline), silicon (coolant leaks) or phosphorus (burning oil), reducing its efficiency. On 1996-and-newer vehicles with OBD II, a second oxygen sensor is mounted behind the converter to monitor its operation. If converter efficiency drops below a certain level, it will turn on the "Check Engine" light.

Converters can also be damaged by overheating if an engine is misfiring, leaking compression or burning oil. If the converter gets too hot, the ceramic honeycomb inside may melt causing an obstruction in the exhaust. A plugged converter will create backpressure that reduces engine performance and may even cause the engine to stall. There is no way to clean a plugged or contaminated converter, so replacement is the only option. Replacement converters must be the same as the original.

 FUEL SYSTEMS

Electronic fuel injection systems come in several versions. Throttle Body Injection (TBI) was used in the 1980s on many vehicles as an intermediate step from electronic carburetion to multiport fuel injection. TBI uses one or two fuel injectors mounted in a throttle body to fuel the engine. Multiport fuel injection (MFI), which is used on almost all late-model engines, has a separate fuel injector for each cylinder. The injectors are usually mounted in the intake manifold and spray fuel into the intake ports, but on some of VW's "direct injection" applications, the injectors spray directly into the combustion chamber. First-generation MFI systems fire all the injectors simultaneously or each bank separately, but most of the newer MFI systems are "sequential" fuel injection (SFI) systems that fire each injector separately just as the intake valve is about to open.

Another variation is General Motor's "Central Point Injection" (CPI) system. Here, a centrally-located "Maxi" injector routes fuel to mechanical poppet valve injectors at each cylinder. The CPI system works much like an old Bosch K-Jetronic fuel injection system, except that the electronic Maxi injector replaces the complicated mechanical K-Jetronic fuel distributor.

Most injectors are electronic and have a solenoid valve at the top to open the nozzle. The Powertrain Control Module (PCM) determines the on-time of each injector pulse to regulate fuel delivery (a longer on-time means more fuel and a richer mixture). The PCM uses inputs from the oxygen sensor in the exhaust, as well as throttle position, engine speed, load and airflow to control the injectors.

Dirty injectors are a common problem. Injector nozzles can become clogged with fuel varnish over time, causing a loss of engine performance and misfiring. Injectors can also leak fuel, causing an increase in fuel consumption and emissions. An injector failure will result in a dead cylinder and power loss. A shorted injector may rob voltage from the other injectors and cause the engine to stall.

Fuel pump pressure ratings vary depending on the application, but typically range from 35 to 85 psi. Pump designs also vary and include single- or double-vane, roller vane, turbine or gerotor style pumps. Most have a one-way check valve to maintain pressure in the fuel system when the engine is shut off.

Fuel pumps can fail for a variety of reasons: old age, loss of voltage, ground at the power relay, wiring connections and pump motor or bearing damage. Running the fuel tank empty may damage the pump because it relies on fuel for lubrication.

Replacement fuel pumps must have the same pressure rating and flow characteristics of the original, but do not have to be the same type as the original. The pump is usually part of the fuel sending unit and may be replaced separately or as a complete assembly. The fuel inlet strainer sock should also be replaced when the pump is changed.

When fuel reaches the engine, it enters a fuel rail and goes to the injectors. A "fuel pressure regulator" on the fuel rail maintains a certain operating pressure. Inside is a spring-loaded diaphragm attached to a source of intake vacuum. As engine load (vacuum) changes, pressure is adjusted as needed to maintain proper fuel delivery. Excess fuel is routed back to the fuel tank through a return line. Many newer vehicles have "returnless" systems that do not have a regulator on the engine fuel rail. The regulator is in the fuel tank with the pump. Regulator problems that alter fuel pressure can hurt engine performance and emissions.

Airflow into the engine is regulated by a "throttle body" attached to the intake manifold. Air first flows through an air filter, then through the throttle body before passing through the manifold and into the engine. The PCM must monitor the amount of air entering the engine, so some fuel injected systems have a vane or mass airflow sensor ahead of the throttle body. Other systems estimate airflow based on throttle position, RPM, temperature and engine load.

HIGH- TECH SUSPENSION AND RIDE CONTROL

The performance shock market includes many different segments: trucks and SUVs, sport compact cars (Honda, Dodge Neon, Mitsubishi, etc.), sports sedans (BMW, Audi, etc.), sports cars (Porsche, Corvette, Mazda Miata, etc.), street performance cars (late-model Mustang, Camaro), vintage muscle cars (older Mustang, Camaro, Dodge Chargers, etc.), hot rods ('32 Fords, etc.) and kit cars (replica Cobras and others).

Nobody knows for sure how many performance shocks and struts are sold annually in the aftermarket, but the overall replacement aftermarket for all kinds of shocks (stock and performance) is estimated to be about 22 million units a year in the U.S.

The problem with stock replacement shocks and struts is that customers don't buy them until their originals have worn out. Even then, they may not realize how much the ride control characteristics of their original equipment dampers have faded over the years. The piston seals inside shocks and struts wear as the miles accumulate, and eventually they lose the ability to maintain a leak-free seal. The valves that control compression and rebound inside a shock also wear and provide less resistance than when they were new. So the question is, at what point are the original equipment shocks and struts no longer providing adequate ride control?

At least one of the leading shock manufacturers now recommends replacing shocks and struts every 50,000 miles. Its reasoning is that the average shock loses a significant portion of its original dampening ability by the time it has experienced 50,000 miles of everyday driving. Shocks contribute to driving safety by maximizing wheel contact with the road. They also minimize body roll, which reduces the risk of rollover in SUVs with a high center of gravity.

With performance shocks, it's a different story. You don't have to wait for the original equipment units to wear out. You can make a sale at virtually any point in a vehicle's life, whether it is 20 years old or brand new off the showroom floor. People buy performance shocks when they want to upgrade their vehicle's suspension. So the key to selling performance shocks is (1) knowing what's available from the various shock suppliers and (2) being able to recommend specific types of shocks for specific types of vehicle applications and driving situations.

For example, a customer walks into your store and asks if you have "coil-overs" to fit his Honda. If you don't know what a coil-over is, you're going to have a hard time looking up a product to fit his car let alone make the sale. You need to be product savvy and speak the same lingo as your performance customers.

Coil-overs are one of the hottest upgrade options today for sport compact cars. Essentially, a coil-over is nothing more than an adjustable bolt-in MacPherson strut. It's a shock absorber with a coil spring wrapped around it, and it has an adjustable spring seat plate that can be turned to raise or lower the position of the seat. This changes the spring rate of the coil-over for a softer or stiffer ride. It can also be used to raise or lower ride height depending on the application. There are also "air spring" versions of coil-overs that use a rubber air spring instead of a coil spring to alter spring rate and/or ride height. The shock in a coil-over may be a conventional gas-charged twin-tube design, or a high-pressure monotube design with or without adjustable valving. Some suppliers even sell electronic versions of their coil-overs that allow the driver to adjust the suspension on the fly from a control box mounted inside the car.

IT'S A GAS
To sell performance shocks, you also have to know something about gas pressurizing. The basic idea behind pressurizing a shock is to keep the hydraulic fluid under pressure so it won't cavitate and foam, which leads to "shock fade" (a loss of ride control caused by uneven or decreased fluid resistance). In a nonpressurized shock, the hydraulic fluid can't keep pace with the rapid up and down motions of the piston and is quickly churned into foam. This reduces resistance and the shock's ability to dampen suspension motions. As a result, dampening control lags behind suspension movement and both handling and traction suffer.

By pressurizing the fluid reservoir with nitrogen gas, the formation of bubbles is minimized. The dampening characteristics of the fluid remain constant and shock fade is eliminated. Pressurization also helps to extend the shock more quickly after it's been compressed and at the same time increases its resistance to compression (making it slightly stiffer). Gas charging provides more consistent control, reduces lag, lessens body roll and improves traction recovery on irregular surfaces. That's why gas shocks and struts should always be recommended for upgrading suspension performance.

Most new vehicles come factory-equipped with gas shocks or struts, but these are not necessarily performance dampers. The gas helps, but by itself doesn't create a true performance shock. Generally speaking, the higher the pressure, the greater the resistance to foaming and cavitation. But you can't really compare shocks by pressure ratings alone because performance depends on the design of the shock and its internal valving. There are a lot of differences between shocks, and pressure ratings is only one of them.

Gas shocks come in one of two basic varieties: single tube (monotube) and double or twin tube. The single tube variety has all the major components contained within a single large tube (thus the name) and typically uses a very high-pressure charge (280 to 360 psi). The gas charge is separated from the hydraulic fluid by means of a floating piston in the top or bottom of the tube. This type of shock must be manufactured with a heavier gauge cylinder and a highly polished internal surface (some are Teflon lined).

A less expensive alternative for upgrading ride control performance is the double or twin tube gas shock. Available from many of the same suppliers of single tube shocks, the double tube design is essentially a gas-pressurized conventional shock with lower pressure. Some are in the 70-130 psi range, while others are 112 to 130 psi or higher.

PERFORMANCE VALVING
Many OEM dampers are valved more for ride comfort than ride control. Soft valving provides a nice boulevard ride, but the trade-off is often reduced body control and more roll - exactly what you don't want on a sport compact car, sports sedan, sports car or SUV with a high center of gravity.Valving is what makes a shock a performance shock or a standard shock. It defines the compression and rebound characteristics of the damper as well as its ride control envelope. Some performance shocks are designed primarily for the track and are probably too harsh for a daily driver.

Likewise, some street performance shocks provide significantly better handling than stock shocks, but are not the best choice for an all-out racing application. As we mentioned earlier, you have to match the product to the application - and your best sources of guidance on this subject are your shock suppliers. They design and build the shocks, and they know which products work best in which kinds of driving situations. If they recommend a particular shock for the street or the track, follow their advice.

ADJUSTABLE SHOCKS
One thing to keep in mind about performance shocks and struts is that stiffer isn't always better. What works great on the race track may be too harsh on the kidneys for normal driving on the street. That's where adjustable shocks and struts come in handy.

Adjustable shocks are a good choice for "dual-purpose" vehicles that are driven mostly on the street but used occasionally off-road or for racing or towing. Adjustable shocks allow the driver to dial-in the desired amount of stiffness. By turning an adjustment screw or dial, or rotating the position of the piston rod in the damper, the internal valving is changed to increase or decrease the resistance offered by the shock. The shocks can be set at "normal" for everyday driving, or "firm" or "extra firm" when things get serious. Some aftermarket shocks offer up to eight or more different settings. And some are available with electric motors or solenoids that allow the driver to change settings from the driver's seat.

TRUCKS & SUVS
Owners of trucks and SUVs are typically interested in a different kind of performance shock. They are usually interested in better handling stability rather than cornering agility. When their vehicle is used to pull a trailer, it should maintain a sure-footed track without tail wagging or whipping. The steering feel shouldn't change drastically as the load increases, and the headlights should remain well focused on the road ahead. There should be no bumper dragging, no suspension bottoming, and no steering wander or instability.

Firming up the suspension with a set of performance shocks will help keep the body flat and reduce roll and sway. Excessive body roll when cornering is bad because it unloads the wheels on the inside while causing excessive camber changes on both sides of the suspension (camber is the inward or outward tilt of the wheel that affects tread contact with the road). A more stable vehicle is also a safer vehicle because it allows the driver to make sudden lane changes or take evasive maneuvers with greater confidence.

For those who love to take their vehicles off-road, shocks with increased suspension travel may be required. Off-road vehicles are often raised to increase ground clearance and tire clearance. If the original shocks aren't long enough, they may bottom out and limit suspension travel.

 IGNITION SYSTEM

All gasoline-fueled engines have a spark ignition system to ignite the air/fuel mixture in the cylinders. The spark is created by a high-voltage surge from an ignition coil. The coil is triggered by an ignition module and/or the PCM using a signal from a distributor pickup or crankshaft position sensor. High-voltage from the coil travels though a thickly insulated cable to the spark plug where it jumps across the plug's electrodes, creating a spark.

If the engine has a distributor, a single coil is used on most engines to supply high-voltage to all the spark plugs (a few Japanese applications use two coils). If the engine has a "distributorless ignition system " (DIS), each spark plug has its own separate coil. On General Motors "waste spark" DIS systems, two spark plugs share the same coil. Many newer vehicles have coils that are mounted directly over the spark plug and use no plug wires. These are called "coil-on-plug" (COP) ignition systems. Another variation is "coil-near-plug" (CNP) systems that mount individual coils near the spark plugs and connect the coils to the plugs with short wires.

The most commonly replaced ignition parts are the spark plugs and plug wires. On older vehicles with distributors, the distributor cap and rotor are also service items. Ignition coils, modules and sensors are only replaced if they have failed.

Ignition coils come in various shapes and sizes, but all do essentially the same thing: They are step-up transformers that convert 12 volts DC into 7,000 to 40,000 or more volts DC. The actual voltage required to fire a spark plug will vary depending on engine speed, load, temperature, resistance in the plug and wires, and the distance across the spark plug electrodes.

Inside the coil are two sets of copper wire windings, one inside the other. The primary windings are made up of several hundred loops of heavy wire around the iron core of the coil. The secondary windings consist of several thousand turns of very fine wire inside the primary windings. When the primary current is switched on, the coil charges up and creates a powerful magnetic field. When the primary current is switched off, the collapse of the magnetic field induces a high-voltage surge in the secondary windings that creates the spark.

If the coil windings short out or break, the coil may not produce enough voltage to fire the spark plugs causing the engine to run rough or die. Hairline cracks in the coil housing or insulation can also weaken or kill the spark. Coils can be tested by measuring their primary and secondary resistance with an ohmmeter or a spark tester. Replacement coils must be the same type as the original to match the engine's voltage requirements.

Electronic ignition systems all use some type of transistorized switching module to turn the coil(s) on and off. On some vehicles (GM and Ford), the module may be mounted on or in the distributor. On DIS systems, it is often part of the coil pack assembly. Modules can be damaged by heat and vibration. A module failure will usually cause a no-spark, no-start condition. GM High Energy Ignition (HEI) modules in older vehicles require a thin layer of dielectric grease underneath to conduct heat away from the module. If your customer forgets the grease, the module may not live very long.

Ignition modules may receive a trigger signal directly from a distributor pickup (magnetic, Hall effect or optical), a crankshaft position sensor or the PCM. A fault in any of these other components or the wiring can prevent the ignition system from firing. Accurate diagnosis is essential to prevent unnecessary parts replacements and returns.

If a vehicle has a distributor, the cap and rotor may develop carbon tracks and cracks over time. This can lead to ignition misfire and hard starting. Replacing the cap and rotor when the spark plugs are changed is often necessary to restore "like-new" ignition performance.

Plug wires connect the distributor or individual coils to the spark plugs. Also called ignition cables, they come in various types (suppression and solid core - also called "mag" wires) and with various types and grades of insulation and jacketing (silicone, EPDM and other materials). The higher the temperature resistance of the insulation and jacketing, the better. Cable diameters are usually 7 or 8 mm, and each cable is a different length to fit specific spark plugs. Replacement cables must be the same size and length as the original. Plug wires may be replaced individually or in complete sets (wires should be changed one at a time to avoid mixing up the firing order). Replacement is needed if internal resistance in the wires exceeds specifications, the wiring is damaged, or the plug boots or terminals fit poorly or are loose.

Finally, we come to the business end of the ignition system: the spark plugs. Spark plugs come in different sizes, lengths, threads and electrode configurations, but all have some type of center electrode surrounded by a ceramic insulator in a threaded steel shell. The "heat range" (operating temperature) of a spark plug depends on the length and shape of the ceramic insulator. The spark plug has to run hot enough so fuel deposits don't build up on the tip, foul the electrode and cause it to misfire. But it also has to conduct enough heat away from the tip so the tip doesn't get too hot when the engine is under load and cause pre-ignition. Many spark plugs have a "copper core" center electrode that improves heat conduction and gives the plug a broader operating range.

Spark plugs are designed for specific engines. The diameter, length and pitch of the threads that screw into the cylinder head must match the application. How far the tip of the spark plug extends into the combustion chamber (called "reach") must also be correct for the application, otherwise the tip of the plug may hit the piston or valves. Always follow the spark plug listings in your plug supplier's catalog.

The distance across the electrode gap at the end of the spark plug must also be set to certain specifications for the engine to run properly. If the gap is too narrow, the spark may not be long enough to ignite the fuel mixture reliably, resulting in ignition misfire. If the gap is too wide, there may not be enough available voltage to create a spark, also causing ignition misfire.

STEERING AND SUSPENSION

The steering system gives the driver control over the vehicle's direction while the suspension allows the tires to roll over bumps and dips in the road without losing their grip. Except for periodic checks of the power steering fluid level, the steering and suspension systems on most vehicles today do not require any maintenance. Parts are not replaced until they wear out or are damaged - unless the vehicle's owner is upgrading the suspension for a specific purpose such as increased load carrying capacity, towing or handling.

STEERING PARTS

The major components in the steering system include the steering box or rack, inner and outer tie rod ends, idler arms, center links, power steering pump and hoses.Most vehicles today have "rack and pinion" steering that uses a pinion gear on the end of the steering input shaft to move a horizontal bar (rack) sideways. The rack is connected to the tie rods with sockets, which are enclosed in rubber bellows. The linkage has outer tie rod ends only. On some GM applications, a "center-mount" rack is used where the tie rods bolt to the center of the rack rather than the ends. Worn inner sockets can cause steering looseness and tire wear. Wear in a power rack control valve housing can increase steering effort. Fluid inside the bellows indicates leaky seals and a need to replace the rack.

The other type of steering system is the "recirculating ball" steering gear in which ball bearings turn against the worn gear to move the steering linkage. The steering box is connected to the steering linkage with a pitman arm. An idler arm supports the other side of the linkage, which includes a center link, inner and outer tie rod ends and tie rods. Steering wander and looseness can result if the idler arm bushing is worn.

Most vehicles have power steering and use a belt-driven pump to reduce the effort required to steer the wheels. Some power steering pumps also provide hydraulic assist for power brakes (Hydroboost systems). A worn pump will usually make noise and/or leak fluid. If a replacement pump does not come with a pulley, the pulley from the old pump will have to be removed and installed on the new pump. Changing the power steering fluid is also recommended (use the type specified by the vehicle manufacturer).

Power steering systems have two hoses, a high-pressure hose to carry pressure from the pump to the steering gear or power cylinder, and a return hose back to the pump reservoir. Leaks can cause steering problems. Replacement hoses may be preformed or made up using various end fittings and a crimping press.

SUSPENSION PARTS

There are two basic types of front suspensions: short-long arm (SLA) and strut. An SLA suspension uses upper and lower "control arms" of unequal length to support the steering knuckle. Each arm is connected to the knuckle by a "ball joint" (one upper, one lower). A strut suspension typically uses a MacPherson strut in place of the upper control arm and ball joint. The strut combines a shock absorber and spring into one assembly, and serves as the steering pivot for the knuckle. A bearing plate at the top of the strut supports the weight of the vehicle.

On some vehicles, a modified strut configuration is used when the spring is not around the strut, but is located between the lower control arm and subframe. On "wishbone" strut suspensions, the strut supports the weight, but an upper control arm is also used to locate the steering knuckle. Almost all front-wheel drive vehicles, as well as many rear-wheel drive vehicles, have strut suspensions. SLA suspensions are used on most rear-wheel drive cars and light trucks.

Suspension parts that may need to be replaced include ball joints, shocks, struts, control arm bushings and springs. Most people don't think springs ever wear out, but over time the constant force of gravity can cause springs to weaken and sag. This changes ride height and alignment, which can increase tire wear and cause handling and ride problems.

Shocks and struts are the most commonly replaced items because they suffer the most wear. Inside is a piston that pumps back and forth through an oil-filled tube. This creates friction that dampens the motions of the suspension and keeps the vehicle stable and the tires in contact with the road. Control valves in the piston and the bottom of the shock or strut vary the resistance by venting fluid as the velocity of the piston changes. The piston rod has a seal that keeps the oil inside and prevents outside contaminants from entering the shock or strut. Over time, this seal wears out and allows the precious fluid inside to leak out. As a result, the shock or strut loses its ability to control the suspension, ride quality goes out the window and the suspension becomes bouncy and rough.

There are two basic types of shocks and struts: twin-tube and monotube. Twin-tube shocks have an oil reservoir around the outside of the piston chamber. Oil moves back and forth from the chamber through the valves in the end of the shock. Monotube shocks are only a single tube with no outer chamber. One end of the shock is filled with pressurized gas with a floating piston seal separating the gas charge from the oil. Twin-tube shocks may also be pressurized with nitrogen gas because gas-charging reduces cavitation, foaming and shock fade. Monotube shocks are typically charged at a much higher pressure (up to 360 psi) and are used more on performance applications because of their quick-acting and firmer ride characteristics.

Shocks and struts are usually replaced in pairs or sets. Replacement shocks and struts with larger piston bores, increased gas pressure or other special features like adjustable valving can be installed to upgrade ride control performance. Likewise, monotube shocks are often used to replace twin-tube shocks to increase suspension stiffness and cornering agility.

One item that may also need to be replaced when replacing struts are the "bearing plates" atop the strut that allow the strut housing to pivot when the wheels are steered. Looseness in the bearing plate can cause steering noise. Binding may increase steering effort and prevent the wheels from recentering following a turn.

MacPherson strut assemblies usually require a spring compressor to disassemble and reassemble, but preassembled replacement struts with new springs and bearing plates are also available to make installation easier. Wheel alignment is usually necessary after replacing struts.

Ball joints are another part that may need to be replaced at some point in the vehicle's life. The joints connect the steering knuckle to the control arms. They may also be used on rear control arms. Joints that carry weight are called "loaded joints" while those that do not are called "follower" joints. SLA suspensions have two upper and two lower ball joints. MacPherson strut suspensions have only two lower ball joints.

A ball joint is so named because of its ball-and-socket construction. The ball stud may ride against a metal gusher bearing, or it may be highly polished to reduce friction and be enclosed in a polymer bearing. Most low-friction ball joints are sealed and do not have a grease fitting for lubrication. Older, gusher style joints have grease fittings so they can be lubricated periodically.

When ball joints become worn, they can make suspension noise and upset wheel alignment. There is also a danger the joint may separate, allowing the suspension to collapse. Replacing a ball joint requires a separator tool or fork to separate the stud from the knuckle once the stud nut has been removed. Some joints are bolted or riveted to the control arm, others are screwed in and others are press-fit into the arm.

 TECH FORUM: ALTERNATORS

QUESTION: Why do alternators have such a high return rate?

ANSWER: Alternators have such a high return rate because they are often misdiagnosed as the cause of a charging problem or low battery.

The alternator is the heart of the charging system. It generates the amps needed to keep the battery fully charged and to supply the demands of the ignition system, electrical system and onboard electronics. The alternator is so named because it converts alternating current (AC) into direct current (DC).

An alternator produces current by rotating a magnetic field inside a stationary conductor. The rotor inside the alternator produces the magnetic field. As the magnetic poles of the rotor pass beneath the three stationary stator windings in the alternator housing, a three-phased alternating current (AC) is induced in the stator windings. The AC current is then "rectified" (converted) to direct current (DC) by a diode trio (three sets of paired diodes) on the back of the alternator.

The alternator's output is controlled by switching its field current on and off. A "voltage regulator," located inside the alternator, on the back of the alternator or mounted elsewhere in the engine compartment, switches the field current on and off to control current output. The higher the load on the electrical system, the higher the current output of the alternator, if everything is working right.
On many newer vehicles, the powertrain control module (PCM) controls the duty cycle of the alternator to regulate voltage output. There is no voltage regulator on the alternator.

When a vehicle has a charging problem, the fault might be in the alternator, the voltage regulator (if it has one), the PCM or something else such as a wiring problem, loose or corroded battery cables or even a bad battery.

The days of DIY alternator rebuilding are history, so if anything inside the alternator has gone bad (stator or rotor windings, brushes, diodes or internal regulator), the whole unit has to be replaced with a new or remanufactured alternator.

If the fault is not diagnosed correctly, and the real problem is not the alternator, then obviously replacing the alternator won't help. The problem will still be there, and the alternator will likely comeback as being "faulty."

Most alternators that are returned under warranty have nothing wrong with them. If a new or reman alternator does not seem to be working correctly after it is installed, chances are the real problem is something else (such as a bad connector, wiring problem, bad battery cables, missing or loose ground strap, bad battery, etc.).

When an alternator fails, the first indication of trouble may be a glowing alternator warning light, a low voltage gauge, dim headlights or a battery that keeps running down. Sometimes, there may be only a partial failure. The alternator will still produce current, but not enough current to keep the battery fully charged. This kind of failure can be caused by one or more bad diodes in the diode trio (rectifier). The leading cause of alternator failure is overheating - which, in turn, is often due to overworking the charging system (extended idling with lights, air conditioning, sound system and other accessories all on, or trying to keep a bad battery charged up).

One of the best ways to make sure an alternator has really failed and needs to be replaced is to bench test the customer's old unit in the store. This requires an alternator tester and some know-how about hooking up the test connections and running the test.

The tester will spin the alternator and measure its voltage and current output. As a rule, a good alternator will read within the specified voltage range (usually 14 to 16.2 volts), and 90 percent of its rated amp output. If the output is less than specifications, the alternator has reached the end of the road and needs to be replaced.

If an externally regulated alternator tests well but does not work when installed on the vehicle, the regulator may be faulty or have a poor ground connection. Or, there may be a wiring problem.
It's also a good idea to test the new and/or remanufactured alternator the customer is buying before it leaves the store. Why? To verify the unit is working - so if it doesn't work after he installs it, he'll know the problem wasn't the alternator, but something else.

Replacement alternators should have the same or higher amp rating as the original.

 TECH FORUM: CERAMIC BRAKE PADS

QUESTION: What's so hot about ceramic brake pads?

ANSWER: In a nutshell, ceramic pads are quieter than semi-metallic pads. Ceramic pads are also less harsh on rotors than their semi-metallic cousins, tend to wear better, and unlike many European formula non-asbestos organic (NAO) pads, don't produce a lot of ugly black brake dust that sticks to the wheels.

"Ceramic" has been a buzzword for a number of years in the brake pad business. Ceramic brake pads were first installed as original equipment on a few import cars back more than two decades ago.

Over the years, the use of ceramic linings has steadily grown, and today nearly 75 percent of all new vehicles come factory-equipped with some type of ceramic linings. This has created a growing demand for ceramic replacement linings in the aftermarket, to which brake suppliers have responded by introducing their own ceramic brake pad product lines.

The trouble is that "ceramic" is a very vague word that some say has been misused and abused. There is no standard industry definition of what a ceramic brake pad is supposed to be. It's a lot like trying to describe pizza. Everybody has his or her own recipe, and some are better than others.
The only thing we can say about ceramic brake pads, in general, is that they do contain some type of ceramic fibers or particles. But how much ceramic and what type of ceramic is anybody's guess. As a rule, most ceramic pads do not contain any chopped iron, although some do. As for the ceramics themselves, the fibers or particles may range in size from 0.4 to as much as 80 microns. Smaller is supposed to be better, but suppliers disagree on that point as well.

The ceramic content and size of the fibers or particles is obviously an important factor, but is only part of the total friction package. The wear and friction characteristics of any given ceramic lining material also depend on other things, such as the fillers, binders, resins and other ingredients that go into the mix. In fact, up to 20 different ingredients may be used to achieve the desired properties. Increasing the ceramic content improves overall braking performance up to a point, but beyond a certain level it may actually reduce braking effectiveness. The trick is to find the best balance of ingredients that deliver the best results. Because of this, friction formulas are a closely guarded secret among brake suppliers. It takes a lot of money to engineer, test and perfect new friction products, so the last thing a brake supplier want to see is a competitor copying its formula.

Some of the companies that sell ceramic brake pads actually use a number of different ceramic formulas within their product lines to broaden their coverage. Other companies may use the same friction material for almost everything.

Compared to other types of friction materials, pads with a high ceramic content generally perform best at brake temperatures of less than 500 degrees F, which is well within the temperature range of most normal driving. On these kinds of application, high ceramic content pads will generally show significantly less wear (50 percent or better) than most non-asbestos organic or semi-metallic linings, and be noticeably quieter and produce almost no visible dust (which is a problem on many European cars). Because of these advantages, ceramic linings are typically marketed as a premium upgrade for cars that were originally equipped with NAO or semi-metallic linings.

Does that mean ceramic linings are the best for all applications? No. For higher temperature applications such as larger, heavier trucks and SUVs, semi-metallic linings generally perform better at temperatures above 500 degrees F. Because of this, some brake suppliers do not offer ceramic linings for certain trucks and SUVs that are originally equipped with semi-metallic linings.

Q: What qualities are most important in replacement brake linings?

A: That's a difficult question to answer since different customers want different performance characteristics, and different applications have different requirements. In order to provide an answer, we looked to a recent JD Powers survey, which revealed the most important things brake customers want. They are:
1. Stopping power;
2. Good pedal feel (no soft or mushy pedal);
3. Quiet operation (no squeals or other objectionable noise);
4. No brake pulsation (which is a function of rotor wear and runout);
5. Durability

To satisfy these expectations, brake suppliers use a variety of friction materials ranging from high-content ceramics to low-metallics to semi-metallics to non-asbestos organics.

Ceramics provide a consistent pedal feel that is the same whether the pads are hot or cold because the coefficient of friction doesn't drop off as quickly as semi-metallics. NVH (noise, vibration and harshness) is also less with ceramics, so that's another reason to recommend ceramic linings.

The survey didn't ask motorists about brake dust, but if it had it's almost certain that owners of most BMWs and other European luxury cars would say they're not overjoyed with the sticky black dust that their OEM brake pads leave all over their wheels. The low dust characteristics of ceramic linings would certainly appeal to this type of customer, so it's important to let them know that low dust is one of the selling points of ceramic linings.

The survey also didn't ask about cost, which can be a concern with some customers. Many brake suppliers offer value-line products as well as standard and premium products to appeal to the budgets of a wide range of customers. All are safe, but as a rule, value-priced linings use lower-grade materials that won't wear as well as standard or premium linings. The pads may also lack design features such as slots and chamfers that help reduce noise.

Q: Are there any tricks for quieting brake squeal?

A: Brake noise is usually the result of high-frequency vibrations between the pads and rotors when the brakes are applied.

There are many causes of noisy pads. The underlying cause may be pads that are inherently noisy and fail to dampen noise, rough worn rotors, missing shims, springs or anti-rattle clips, excessive clearances between the pads and calipers, or the calipers and their mounts.

The most common fix is to resurface the rotors and replace the pads. A smooth finish on the rotors allows the pads to glide over the surface rather than bump and shake.

Most new OEM and aftermarket rotors come with a surface finish between 30 and 60 inches RA (roughness average), with many falling in the 40 to 50 RA range. Some OEM specifications say any finish less than 80 RA is acceptable.

A brake lathe with sharp tool bits should produce a rotor finish that meets these specifications. The most common mistakes that are made when turning rotors is using dull bits, not mounting the rotors squarely on the lathe arbor (too much runout) and turning the rotors at too high a feed rate. Some technicians and machinists will also sand the rotors after they have been turned to smooth the surface even more, though this really shouldn't be necessary.

As for replacement pads, recommend premium pads that have the same noise-suppressing slots and chamfers as many OEM pads. High content ceramic pads are generally the quietest material (semi-mets are the noisiest).

The pads should be installed with the noise-dampening shims on the back, unless the shims are integrally molded into the pads. Another trick is to apply a thin layer of high-temperature brake lubricant to the back of the pads so the pads won't vibrate against the calipers or caliper pistons.

There are also aerosol products that can be sprayed on the rotors to dampen noise during the initial pad break-in period. These products help fill voids on the surface and act like a lubricant to help the pads glide quietly across the rotors when the brakes are applied. The coating eventually wears off, but usually after the critical break-in period has passed.

 TECH FORUM: ENGINE MANAGEMENT

QUESTION: Why does the "Check Engine" Light Come On?

ANSWER: That's a question that many people ask. Most people think the Check Engine Light comes on if an engine has any kind of problem whatsoever. But guess what? They're wrong!
The Check Engine Light only comes on if a fault is emissions-related. The fault does not actually have to cause a measurable increase in emissions, but if the computer thinks the fault might cause emissions to increase, it will log a diagnostic trouble code (DTC) and turn on the light.

The Check Engine Light, which is officially called the Malfunction Indicator Lamp (MIL), is essentially an emissions warning light and nothing more. It does not come on when the oil needs to be changed, if the oil level is low or if the engine is overheating. There are other warning lights for these kind of problems.

Many problems that trigger the light are relatively minor and have little or no effect on engine performance. But because these problems might cause an increase in emissions, the onboard diagnostic (OBD II) system is required by law to monitor the fault, log a code and turn on the MIL anyway. The reason for doing this is because a fault has the potential for increasing emissions beyond federal limits. On late-model cars, the tailpipe and evaporative emission standards are very strict. Consequently, it doesn't take much to push emissions over the limit.

A loose gas cap can trigger the light. Why? Because fuel systems on late-model vehicles are sealed to prevent the escape of fuel vapors into the atmosphere. If the gas cap isn't tightened all the way, or the cap leaks because of a damaged seal, OBD II will catch the fault, set a code and turn on the MIL.

Misfires can cause a huge increase in hydrocarbon emissions as well a loss of performance, rough idle and other problems such as converter damage if the misfire is steady rather than intermittent. Misfires can be caused by a lot of things including worn or fouled spark plugs, bad plug wires or ignition coils, dirty fuel injectors, air leaks or other conditions that cause the air/fuel mixture to run leaner than normal, or even a loss of compression in one or more cylinders because of a burned valve or blown head gasket. All engines will experience an occasional misfire, but if the misfire rate exceeds a certain threshold, OBD II sees it as a problem, sets a code and turns on the light. Sometimes, the MIL will flash while the vehicle is being driven. This is a warning to the driver that the engine is misfiring under load.

Sensor failures or electronic faults within the powertrain control module (PCM) itself can also turn on the MIL. If a sensor is not generating a good signal, or the signal does not make sense for the operating conditions at the time (a wide open signal from the throttle position sensor while the engine is idling, for example), it is recognized as a fault.

In some cases, loss of a key sensor signal or bogus information from the sensor will wreak havoc on the operating logic of the engine management system. On some systems, the PCM can substitute a "simulated" value for a missing sensor signal, but it depends on the type of fault and the sensor that is involved.

A coolant sensor that always reads cold, for example, will prevent the engine management system from going into a "closed loop" mode of operation. This means the PCM ignores the signal from the oxygen sensor and keeps the fuel mixture rich because that's what a cold engine likes. If the engine is warm, though, staying in open loop too long increases emissions and fuel consumption.

A dirty mass airflow sensor, on the other hand, may mislead the PCM into thinking the engine is using less air than it actually is. This can cause a lean fuel condition, but the PCM may be able to compensate by using other sensor inputs to check the accuracy of the mass airflow sensor's input.
One very important point to keep in mind with respect to the Check Engine Light on OBD II vehicles is that you must use a code reader, scan tool or scanner software to read and clear codes. There are no manual flash codes on OBD II vehicles. Disconnecting the battery in an attempt to erase a code so the MIL will go off is also a very bad idea because it can cause the PCM to forget learned settings. This may cause additional driveability problems, and may even require using a scan tool to "reset" lost information.

 TECH FORUM: FUEL INJECTION

QUESTION: What happens when fuel injectors get dirty?

ANSWER: What will happen is that the engine may experience a variety of driveability and performance problems such as hesitation or stumble when accelerating, loss of power, rough idle, reduced fuel economy and increased emissions.

With electronic fuel injection systems, the air/fuel ratio is controlled by the powertrain control module (PCM). The PCM monitors engine speed, load and other operating conditions through its various sensors. Using these inputs, the PCM calculates how much fuel the engine needs and commands the injectors to spray a certain amount of fuel into the engine. The PCM does this by grounding each injector circuit for a predetermined length of time. The longer the length or duration of each injector pulse, the more fuel that injector sprays into the engine.

The injector pulses are also timed to coincide with the rotation of the crankshaft. Each injector may squirt once every revolution of the crank, or every other revolution of the crank depending on the application. On older EFI systems, all the injectors are usually triggered simultaneously. But on most newer "sequential" EFI systems, each injector is pulsed individually just as the intake valve is about to open. The duration of each injector pulse can also be varied between cylinder firings to adjust the air/fuel mixture.

All this, of course, is based on the assumption that all the injectors are clean and deliver their normal doses of fuel with each pulse.

The actual volume of fuel that each injector delivers depends on four things: 1) fuel pressure (which is maintained within a certain range by the fuel pressure regulator), 2) intake vacuum (which changes with engine load), 3) the on-time of each injector pulse (which the PCM varies depending on engine speed and load), and 4) the size of the orifice in the injector spray nozzle (which is fixed).

When dirt or fuel varnish deposits build up in the injector orifice, it creates a restriction that reduces the amount of fuel delivered with each squirt. Dirty injectors, therefore, run lean and don't deliver as much fuel as the engine needs. This creates the lean fuel condition that can cause misfires, loss of performance and increased emissions.

What's more, the deposits in the nozzle disrupt the injector's normal spray pattern, which is very important for proper fuel atomization and clean combustion.

If the situation is bad enough to cause a significant number of misfires or loss of power, the onboard diagnostics in the PCM may set one or more diagnostic trouble codes and turn on the Check Engine Light (Malfunction Indicator Lamp or MIL). Misfires may also cause the MIL to flash or come on momentarily while the vehicle is being driven.

A P0300 random misfire code is typically an indication that the engine is running lean. A P0171 lean code is another telltale sign that the engine is not getting enough fuel. The cause may be dirty fuel injectors, or something else such as a vacuum leak, low fuel pressure or a dirty mass airflow sensor. In any event, the problem needs to be diagnosed and repaired to restore normal performance and fuel economy. And if the vehicle has to take an emissions test, it won't pass if there are any codes present regardless of cause or effect.

It doesn't take much of a restriction in an injector to lean out the fuel mixture. Only an 8 to 10 percent restriction in a single fuel injector can be enough to upset the air/fuel mixture and cause misfires.

In turbocharged engines, dirty injectors can have a dangerous leaning effect that may lead to engine-damaging detonation and/or preignition.

By Counterman

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