AUTOMOTIVE TECHNICAL TERMS B
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BACKFIRE
This is the popping or banging sound sometimes heard in the exhaust when decelerating. It can indicate a problem such as over-rich carburetion, a bad exhaust valve or an ignition problem (retarded timing or a cracked distributor cap). If the backfiring occurs through the carburetor, it may mean over-advanced timing, a bad intake valve or a cracked distributor cap.
BACK PRESSURE
This is the pressure that backs up in the exhaust system as a result of the restriction caused by the muffler, catalytic converter and tailpipe. The faster you drive and/or the greater the load on the engine, the higher the back pressure in the exhaust system. Back pressure inhibits the exit of exhaust gases so the engine has to work harder to push the exhaust out. This cuts down on engine power and fuel economy. Some of the causes of high back pressure include a clogged converter, a damaged or collapsed exhaust pipe or a restrictive muffler.
BACK PRESSURE EGR
Some emissions control systems use a back-pressure sensor or diaphragm to monitor backpressure so that exhaust gas recirculation (EGR) flow can be increased when the engine is under maximum load (and producing maximum back pressure). See EGR.
BACKSPACING
The distance from the back edge of a wheel rim to the back of the center section.
BALL JOINT
A flexible coupling in a vehicle's suspension that connects the control arm to the steering knuckle. A ball joint is so named because of its ball-and-socket construction. Some are designed to never require grease while others should be lubed every six months. As the joint wears, it becomes loose. The result is suspension noise and wheel misalignment. See Ball Joints.
BAROMETRIC PRESSURE
The pressure exerted by the weight of the earth's atmosphere, equal to one bar, 100 kilopascals, or 14.7 psi (often rounded off to 15 psi) at sea level. Barometric pressure changes with the weather and with altitude. Since it affects the density of the air entering the engine and ultimately the air/fuel ratio, some computerized emissions control systems use a barometric pressure sensor so that the spark advance and EGR flow can be regulated to control emissions more precisely.
BAROMETRIC PRESSURE SENSOR
A device that senses barometric pressure for the engine control system. May be combined with a Manifold Absolute Pressure (MAP) sensor.
BATTERY
The battery is a storehouse of electrical energy for starting the engine. All cars and light trucks today have a 12-volt battery. Most are also maintenance-free, meaning you do not have to add water to them periodically. Some even have built-in charge indicators to tell you if they need charging. A green dot in the window means the battery is at least 75% charged, no dot means it needs recharging, and a clear or yellow window means you need a new battery because the water level inside is low. Do NOT try to jump start or charge such a battery. You might be able to salvage the battery if you can pry the sealed caps open and add water, but usually the battery must be replaced. Batteries are rated according to their Cold Cranking Amp (CCA) capacity. As a rule of thumb, an engine needs a minimum of one CCA for every cubic inch of displacement, and preferably two. The higher the CCA rating of the battery, the better. A typical passenger car battery might be rated at 500 CCA or higher. See Battery Testing, and Battery Jump Starting.
BLOWBY
A condition where combustion gases literally blow around the piston rings. When air and fuel are ignited inside the combustion chamber, the resulting explosion creates tremendous heat and pressure. The piston rings are supposed to seal against the cylinder walls to prevent the hot gases from escaping. But every engine suffers a small amount of blowby anyway. If the rings and cylinders are worn, blowby can be a real problem. The gases are mostly water vapor and unburned fuel, so when they enter the crankcase they contaminate the oil. Most of the gases are sucked out through the crankcase ventilation system (See PCV Valve) before they can do much damage. But in an engine with a lot of wear, excessive blowby can lead to rapid sludge buildup.
BODY ROLL
The leaning or tipping of a vehicle's body to one side when turning sharply. This reduces traction and increases tire scuff due to undesirable alignment changes. Body roll is controlled primarily by a sway bar, but the stiffness of the springs and shocks also play a role.
BOOTS
Also called bellows, these are the protective rubber (synthetic or natural) or hard plastic (usually Hytrel) covers that surround CV joints. The boot's job is to keep grease in and dirt and water out. Split, torn or otherwise damaged boots should be replaced immediately. Old boots should never be reused when servicing a joint. Always install new boots.
BRAKE BLEEDING
This is the process of removing air bubbles from the brake system by pumping fluid through the lines. Air bubbles are bad because they compress when pressure is applied resulting in a low or spongy feeling pedal. The correct procedure for bleeding the brakes on most RWD vehicles is to start at the furthest wheel. Do the right rear then left rear brake, followed by the right front and left front brakes. On a FWD vehicle with a diagonally-split brake system, do the right rear then left front brake, followed by the left rear and right front brake. See Bleeding Brakes.
BRAKE CALIPERS
The part of the disc brake that squeezes a pair of brake pads against the rotor. A caliper is nothing more than a casting with a piston inside. When hydraulic pressure pushes the piston out, it forces the brake pads against both sides of the rotor. Some calipers are "floating" in that they slide back and forth and self-center over the rotor. Others are said to be "fixed" because they do not move in and out. See Loaded Brake Calipers.
BRAKE DRUMS
The cast iron housing and friction surface around a drum brake. The brake shoes expand outward and rub against the inside surface of the drums when the brakes are applied. Worn drums often take on a grooved appearance. The inner surface should be turned smooth on a brake lathe when the shoes are replaced. If the drum has worn too thin, is cracked, warped or has taken on a bell-mouthed shape, it must be replaced. The spring around the outside of the drum on some vehicles is there to soak up vibrations and noise. See Drum Brake Service.
BRAKE FLUID
The brake system uses a glycol-based hydraulic fluid. The fluid is "hygroscopic," which means it tends to absorb moisture over time (never leave a can of brake fluid open for this reason). Moisture lowers the boiling point of the fluid and causes internal corrosion in the brake system. That is why the fluid should be replaced when brake repairs are made or every two years for preventive maintenance. There are several different types, based on the boiling temperature and other characteristics of the fluid. DOT 3 or DOT 4 are used in most passenger cars and light trucks. Use only the type of fluid specified by the vehicle manufacturer. Using DOT 3 in an application that calls for DOT 4 might create a safety hazard. DOT 5 brake fluid is different from DOT 3 and DOT 4 in that it is silicone-based. DOT 5 is NOT recommended for any vehicle equipped with antilock brakes - but it can provide long-lasting protection against corrosion for vehicles that are stored for long periods of time or are driven in wet environments. See Brake Fluid.
BRAKE JOB
A typical brake job includes replacing the brake linings (new disc brake pads and shoes), resurfacing the rotors and drums, adding fresh brake fluid and bleeding the system, and inspecting/replacing any other worn components (usually at extra cost). If rotors or drums are worn beyond safe limits, they can't be resurfaced and must be replaced. Leaky disc brake calipers, drum brake wheel cylinders or the master cylinder should be rebuilt or replaced. See Brake Job.
BRAKE LININGS
The friction material on disc brake pads or drum shoes. A variety of materials are used including asbestos, semi-metallic fibers, Fiberglass and Kevlar. Asbestos linings are used on most older vehicles and on the rear drum brakes. Semi-metallic linings are used on the front brakes of many front-wheel drive applications. Others may be factory equipped with ceramic-based linings. Never substitute one type of brake lining material for another. The linings rub against the rotors or drums to create friction. This produces a tremendous amount of heat. If the heat builds up faster than it can be shed, the brakes can fade (See Brake Fade). The linings are a high wear item. Front brakes, especially those on FWD vehicles, receive the most wear. Average life for front brakes ranges from 30,000 to 60,000 miles. For rear brakes, 60,000 to 100,000 miles is the norm. Linings should be replaced when worn down to the lining rivet heads, or when lining thickness is less than 1/8th inch or minimum service specifications.
BRAKE PADS
These are the linings used in the front disc brakes. They are called pads because of their flat pad-like shape. Each brake uses a pair of pads (one inner, one outer). Replacement pads are sold in two-pair sets, and are fairly easy to change (See Brake Squeal). Calipers should be inspected for leaks (See Calipers), and the rotors resurfaced to restore a smooth surface (See Brake Rotors). See Brake Pads.
BRAKE ROTORS
The flat disk-like plates that provide the friction surface in a disc brake. When hydraulic pressure is applied to the caliper, the brake pads are squeezed against both sides of the rotor producing friction and heat. Some rotors have cooling fins between both faces and are called "vented" rotors. The rotors should always be resurfaced when new pads are installed. If worn beyond safe limits, cracked or severely warped, the rotor must be replaced. See Brake Rotors.
BRAKES
The brake system uses hydraulic pressure to stop the vehicle when you step on the brake pedal. Pushing the pedal down pumps fluid from the master cylinder to the brakes at each wheel. This squeezes the brake linings against the rotors and drums, creating friction which brings the vehicle to a halt. The only maintenance the system requires is to check the fluid level periodically, and to replace the fluid every couple of years, or when brake repairs are performed. See Brake Job.
BRAKE SHOES
The brake linings used in drum brakes (the rear brakes on most cars). Each drum contains two shoes (a primary or leading shoe, and a secondary or trailing shoe). Replacement shoes are sold in sets of four, one pair for each brake. When shoes are replaced, the condition of the mounting hardware and return springs should be carefully inspected. Replace any worn, damaged or stretched components. Drums should also be turned on a lathe to restore a smooth surface. See Drum Brake Service.
BRAKE SQUEAL
The annoying high pitched screech that�s sometimes heard when braking. A common ailment on many disc brake-equipped cars, it is caused by vibration between the brake pad and rotor. It causes no harm, but metallic scraping sounds should be investigated because it usually means the brake linings are worn down to their metal backing plates. If not replaced, the metal-to-metal contact can ruin the rotors or drums. Brake squeal can be eliminated by installing shims on the backs of brake pads, by applying anti-squeal compound or a moly-based brake grease never ordinary chassis grease) to the backs of the pads, and/or resurfacing the rotors. Applying a nondirectional swirl finish on the rotors can provided added noise suppression. See Brake Noise.
BTU
Abbreviation for British Thermal Units. One BTU is the amount of heat it takes to heat one pound of water one degree Fahrenheit. The energy value of various fuels is often expressed in so many BTUs per gallon. Gasoline, for example, has around 120,000 BTUs per gallon.
BUMP STEER
The tendency of a vehicle to suddenly veer or swerve to one side when hitting a bump or dip in the road. The condition is caused by uneven toe changes that occur as a result of the steering linkage or rack not being parallel with the road surface. This causes the wheels to change toe unevenly as the suspension undergoes jounce and rebound.
BUMP STOPS
Rubber bumpers (often cone or wedge shaped) on the chassis that limit suspension travel. "Bottoming out" the suspension means hitting the bump stops.
BUSHINGS
A liner, grommet or sleeve made of rubber, plastic or metal that fits around a bolt or bar to support, position and in some instances cushion the part. Bushings are used around the pivot bolts that attach the control arms to the chassis. They are also used around sway bars, the links that connect the ends of the sway bar to the control arms, and on the ends of strut rods. Rubber or soft elastomer bushings provide "compliance" in the suspension to help dampen road noise, vibrations and feedback. Hard plastic (usually polyurethane) bushings "firm" up the suspension for improved handling but also increase ride harshness.
This is the popping or banging sound sometimes heard in the exhaust when decelerating. It can indicate a problem such as over-rich carburetion, a bad exhaust valve or an ignition problem (retarded timing or a cracked distributor cap). If the backfiring occurs through the carburetor, it may mean over-advanced timing, a bad intake valve or a cracked distributor cap.
BACK PRESSURE
This is the pressure that backs up in the exhaust system as a result of the restriction caused by the muffler, catalytic converter and tailpipe. The faster you drive and/or the greater the load on the engine, the higher the back pressure in the exhaust system. Back pressure inhibits the exit of exhaust gases so the engine has to work harder to push the exhaust out. This cuts down on engine power and fuel economy. Some of the causes of high back pressure include a clogged converter, a damaged or collapsed exhaust pipe or a restrictive muffler.
BACK PRESSURE EGR
Some emissions control systems use a back-pressure sensor or diaphragm to monitor backpressure so that exhaust gas recirculation (EGR) flow can be increased when the engine is under maximum load (and producing maximum back pressure). See EGR.
BACKSPACING
The distance from the back edge of a wheel rim to the back of the center section.
BALL JOINT
A flexible coupling in a vehicle's suspension that connects the control arm to the steering knuckle. A ball joint is so named because of its ball-and-socket construction. Some are designed to never require grease while others should be lubed every six months. As the joint wears, it becomes loose. The result is suspension noise and wheel misalignment. See Ball Joints.
BAROMETRIC PRESSURE
The pressure exerted by the weight of the earth's atmosphere, equal to one bar, 100 kilopascals, or 14.7 psi (often rounded off to 15 psi) at sea level. Barometric pressure changes with the weather and with altitude. Since it affects the density of the air entering the engine and ultimately the air/fuel ratio, some computerized emissions control systems use a barometric pressure sensor so that the spark advance and EGR flow can be regulated to control emissions more precisely.
BAROMETRIC PRESSURE SENSOR
A device that senses barometric pressure for the engine control system. May be combined with a Manifold Absolute Pressure (MAP) sensor.
BATTERY
The battery is a storehouse of electrical energy for starting the engine. All cars and light trucks today have a 12-volt battery. Most are also maintenance-free, meaning you do not have to add water to them periodically. Some even have built-in charge indicators to tell you if they need charging. A green dot in the window means the battery is at least 75% charged, no dot means it needs recharging, and a clear or yellow window means you need a new battery because the water level inside is low. Do NOT try to jump start or charge such a battery. You might be able to salvage the battery if you can pry the sealed caps open and add water, but usually the battery must be replaced. Batteries are rated according to their Cold Cranking Amp (CCA) capacity. As a rule of thumb, an engine needs a minimum of one CCA for every cubic inch of displacement, and preferably two. The higher the CCA rating of the battery, the better. A typical passenger car battery might be rated at 500 CCA or higher. See Battery Testing, and Battery Jump Starting.
BLOWBY
A condition where combustion gases literally blow around the piston rings. When air and fuel are ignited inside the combustion chamber, the resulting explosion creates tremendous heat and pressure. The piston rings are supposed to seal against the cylinder walls to prevent the hot gases from escaping. But every engine suffers a small amount of blowby anyway. If the rings and cylinders are worn, blowby can be a real problem. The gases are mostly water vapor and unburned fuel, so when they enter the crankcase they contaminate the oil. Most of the gases are sucked out through the crankcase ventilation system (See PCV Valve) before they can do much damage. But in an engine with a lot of wear, excessive blowby can lead to rapid sludge buildup.
BODY ROLL
The leaning or tipping of a vehicle's body to one side when turning sharply. This reduces traction and increases tire scuff due to undesirable alignment changes. Body roll is controlled primarily by a sway bar, but the stiffness of the springs and shocks also play a role.
BOOTS
Also called bellows, these are the protective rubber (synthetic or natural) or hard plastic (usually Hytrel) covers that surround CV joints. The boot's job is to keep grease in and dirt and water out. Split, torn or otherwise damaged boots should be replaced immediately. Old boots should never be reused when servicing a joint. Always install new boots.
BRAKE BLEEDING
This is the process of removing air bubbles from the brake system by pumping fluid through the lines. Air bubbles are bad because they compress when pressure is applied resulting in a low or spongy feeling pedal. The correct procedure for bleeding the brakes on most RWD vehicles is to start at the furthest wheel. Do the right rear then left rear brake, followed by the right front and left front brakes. On a FWD vehicle with a diagonally-split brake system, do the right rear then left front brake, followed by the left rear and right front brake. See Bleeding Brakes.
BRAKE CALIPERS
The part of the disc brake that squeezes a pair of brake pads against the rotor. A caliper is nothing more than a casting with a piston inside. When hydraulic pressure pushes the piston out, it forces the brake pads against both sides of the rotor. Some calipers are "floating" in that they slide back and forth and self-center over the rotor. Others are said to be "fixed" because they do not move in and out. See Loaded Brake Calipers.
BRAKE DRUMS
The cast iron housing and friction surface around a drum brake. The brake shoes expand outward and rub against the inside surface of the drums when the brakes are applied. Worn drums often take on a grooved appearance. The inner surface should be turned smooth on a brake lathe when the shoes are replaced. If the drum has worn too thin, is cracked, warped or has taken on a bell-mouthed shape, it must be replaced. The spring around the outside of the drum on some vehicles is there to soak up vibrations and noise. See Drum Brake Service.
BRAKE FLUID
The brake system uses a glycol-based hydraulic fluid. The fluid is "hygroscopic," which means it tends to absorb moisture over time (never leave a can of brake fluid open for this reason). Moisture lowers the boiling point of the fluid and causes internal corrosion in the brake system. That is why the fluid should be replaced when brake repairs are made or every two years for preventive maintenance. There are several different types, based on the boiling temperature and other characteristics of the fluid. DOT 3 or DOT 4 are used in most passenger cars and light trucks. Use only the type of fluid specified by the vehicle manufacturer. Using DOT 3 in an application that calls for DOT 4 might create a safety hazard. DOT 5 brake fluid is different from DOT 3 and DOT 4 in that it is silicone-based. DOT 5 is NOT recommended for any vehicle equipped with antilock brakes - but it can provide long-lasting protection against corrosion for vehicles that are stored for long periods of time or are driven in wet environments. See Brake Fluid.
BRAKE JOB
A typical brake job includes replacing the brake linings (new disc brake pads and shoes), resurfacing the rotors and drums, adding fresh brake fluid and bleeding the system, and inspecting/replacing any other worn components (usually at extra cost). If rotors or drums are worn beyond safe limits, they can't be resurfaced and must be replaced. Leaky disc brake calipers, drum brake wheel cylinders or the master cylinder should be rebuilt or replaced. See Brake Job.
BRAKE LININGS
The friction material on disc brake pads or drum shoes. A variety of materials are used including asbestos, semi-metallic fibers, Fiberglass and Kevlar. Asbestos linings are used on most older vehicles and on the rear drum brakes. Semi-metallic linings are used on the front brakes of many front-wheel drive applications. Others may be factory equipped with ceramic-based linings. Never substitute one type of brake lining material for another. The linings rub against the rotors or drums to create friction. This produces a tremendous amount of heat. If the heat builds up faster than it can be shed, the brakes can fade (See Brake Fade). The linings are a high wear item. Front brakes, especially those on FWD vehicles, receive the most wear. Average life for front brakes ranges from 30,000 to 60,000 miles. For rear brakes, 60,000 to 100,000 miles is the norm. Linings should be replaced when worn down to the lining rivet heads, or when lining thickness is less than 1/8th inch or minimum service specifications.
BRAKE PADS
These are the linings used in the front disc brakes. They are called pads because of their flat pad-like shape. Each brake uses a pair of pads (one inner, one outer). Replacement pads are sold in two-pair sets, and are fairly easy to change (See Brake Squeal). Calipers should be inspected for leaks (See Calipers), and the rotors resurfaced to restore a smooth surface (See Brake Rotors). See Brake Pads.
BRAKE ROTORS
The flat disk-like plates that provide the friction surface in a disc brake. When hydraulic pressure is applied to the caliper, the brake pads are squeezed against both sides of the rotor producing friction and heat. Some rotors have cooling fins between both faces and are called "vented" rotors. The rotors should always be resurfaced when new pads are installed. If worn beyond safe limits, cracked or severely warped, the rotor must be replaced. See Brake Rotors.
BRAKES
The brake system uses hydraulic pressure to stop the vehicle when you step on the brake pedal. Pushing the pedal down pumps fluid from the master cylinder to the brakes at each wheel. This squeezes the brake linings against the rotors and drums, creating friction which brings the vehicle to a halt. The only maintenance the system requires is to check the fluid level periodically, and to replace the fluid every couple of years, or when brake repairs are performed. See Brake Job.
BRAKE SHOES
The brake linings used in drum brakes (the rear brakes on most cars). Each drum contains two shoes (a primary or leading shoe, and a secondary or trailing shoe). Replacement shoes are sold in sets of four, one pair for each brake. When shoes are replaced, the condition of the mounting hardware and return springs should be carefully inspected. Replace any worn, damaged or stretched components. Drums should also be turned on a lathe to restore a smooth surface. See Drum Brake Service.
BRAKE SQUEAL
The annoying high pitched screech that�s sometimes heard when braking. A common ailment on many disc brake-equipped cars, it is caused by vibration between the brake pad and rotor. It causes no harm, but metallic scraping sounds should be investigated because it usually means the brake linings are worn down to their metal backing plates. If not replaced, the metal-to-metal contact can ruin the rotors or drums. Brake squeal can be eliminated by installing shims on the backs of brake pads, by applying anti-squeal compound or a moly-based brake grease never ordinary chassis grease) to the backs of the pads, and/or resurfacing the rotors. Applying a nondirectional swirl finish on the rotors can provided added noise suppression. See Brake Noise.
BTU
Abbreviation for British Thermal Units. One BTU is the amount of heat it takes to heat one pound of water one degree Fahrenheit. The energy value of various fuels is often expressed in so many BTUs per gallon. Gasoline, for example, has around 120,000 BTUs per gallon.
BUMP STEER
The tendency of a vehicle to suddenly veer or swerve to one side when hitting a bump or dip in the road. The condition is caused by uneven toe changes that occur as a result of the steering linkage or rack not being parallel with the road surface. This causes the wheels to change toe unevenly as the suspension undergoes jounce and rebound.
BUMP STOPS
Rubber bumpers (often cone or wedge shaped) on the chassis that limit suspension travel. "Bottoming out" the suspension means hitting the bump stops.
BUSHINGS
A liner, grommet or sleeve made of rubber, plastic or metal that fits around a bolt or bar to support, position and in some instances cushion the part. Bushings are used around the pivot bolts that attach the control arms to the chassis. They are also used around sway bars, the links that connect the ends of the sway bar to the control arms, and on the ends of strut rods. Rubber or soft elastomer bushings provide "compliance" in the suspension to help dampen road noise, vibrations and feedback. Hard plastic (usually polyurethane) bushings "firm" up the suspension for improved handling but also increase ride harshness.
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