Car dictionary: automotive terms

IAC valve

Idle Air Control Valve
Idle Air Control (IAC) valve

The engine computer (ECM) constantly adjusts the idle speed according to the conditions. For example, when you start the engine cold, the ECM increases the idle speed to help warm up the engine faster. In some cars there is a separate idle air control (IAC) valve or solenoid that controls the idle speed. In other cars the electronic throttle is used to control the idle speed.
The idle air control (IAC) valve works by allowing metered amount of air past the closed throttle. The ECM adjusts the engine idle speed by slightly opening or closing the idle air control valve (IAC). For example, when you turn on the air conditioner, the air conditioner compressor adds more load to the engine. To keep the idle speed steady, the ECM slightly opens the idle air control (IAC) valve to add more air to the engine.
Typical problems with the idle air control (IAC) are often caused by carbon build-up inside the valve. Sometimes this could cause the valve to stick, other times, the carbon build-up blocks the IAC valve passages. In either case the engine may have rough or unstable idle. It also could be too high or too low. The carbon build-up can be cleaned with a carb cleaner or another similar product. If the cleaning doesn't help or the idle air control valve has an internal problem, it may need to be replaced.


Volkswagen 2.0 TSI engine Alternator
Two intercoolers of the Nissan GT-R. Photo: Nissan

Many turbocharged and supercharged engines have an intercooler, installed between the turbo- or supercharger and the engine intake.
The purpose of the turbocharger or supercharger is to boost the intake air pressure. Boosting the pressure increases the temperature of the intake air. To improve the engine performance and prevent the engine knock (detonation), the intake air temperature must be reduced. This is the job of the intercooler. The intercooler is essentially a radiator for the intake air. An intercooler is installed in front or on top of the engine. In either setup, the passing air is directed through the intercooler fins, dissipating the heat and reducing the intake air temperature.


Misfiring engine
Cylinder 1 misfires

The term 'misfiring' refers to the engine with one or more cylinders not working properly; meaning there is no full combustion in the affected cylinder. An engine cylinder can misfire for many reasons, including a bad spark plug or ignition wire, faulty fuel injector, vacuum leak, etc. A misfiring engine might shake and exhibit lack of power. When one of the engine cylinders misfires, the unburned fuel entering the exhaust can overheat the catalytic converter. The car computer (ECM) detects that the engine misfires and informs the driver by flashing the Check Engine light on the instrument panel. To find what can cause the engine to misfire, read more »


Generally speaking, overdrive (O/D) is the highest gear in the transmission. On most cars the automatic transmission has three or four speeds and overdrive (which is fourth or fifth speed accordingly). Overdrive allows the engine to have less rpm with higher speed for better fuel efficiency. When you switch it ON, you allow the transmission to shift into overdrive mode after a certain speed has been reached (usually 30-40 mph depending on the load). When it's OFF, you limit transmission shifting to third or fourth speed, not allowing the transmission to shift into overdrive.
In normal driving conditions the overdrive should be always ON. You may need to switch it OFF if you drive in a mountainous area.
[The automatic transmission automatically shifts from O/D to the lower gear when it feels more load. When it feels less load it shifts back to O/D, but under certain conditions, e.g. driving uphill or towing a trailer, the transmission cannot decide whether to stay in O/D or shift into 3rd speed and it starts shifting back and forth. That's the time when you may switch the O/D off and help the transmission to decide].
You also may need to switch the Overdrive OFF when you want to slow down using the engine braking, for example, when driving downhill. For more details, check your owner's manual.

Oxygen Sensor (O2 Sensor)

Oxygen sensor
Heated O2 sensor

An oxygen sensor (O2 sensor), installed in the vehicle's exhaust, measures the amount of the oxygen in the exhaust gases. The engine computer uses the signal from the front and the rear oxygen sensors to adjust the supply of fuel and to monitor the performance of the catalytic converter, which is a major part of the vehicle's emission control system.
A typical modern 4-cylinder car has two oxygen sensors, one in front of the catalytic converter (Front O2 Sensor) and one after it (Rear O2 Sensor). A typical V6 or V8 vehicle has two catalytic converters and four oxygen sensors: one in front and one after each catalytic converter. Read more about Oxygen Sensor

Stabilizer bar bushings

Stabilizer bar link
Stabilizer bar bushing

The stabilizer bar bushings hold the stabilizer bar. Over time, stabilizer bushings wear out, causing thumping or creaking noises when driving over small bumps. New bushings are usually not very expensive and in most cars they are easy to replace.

Stabilizer bar link
Stabilizer bar link

The stabilizer bar link is a part of the car suspension that connects each end of the front or rear stabilizer bar (sway bar) to the control arm or a strut depending on a design. The stabilizer bar link fails quite often and can cause a rattle or knocking noise coming from the suspension on the rough road. It's a fairly inexpensive and easy to replace part.


A car thermostat
A car thermostat

A thermostat is a temperature-operated valve installed within the engine cooling system. A thermostat opens or closes depending on the temperature of the coolant (antifreeze). The thermostat helps warm up the engine faster and maintain the engine temperature within the normal operating range. Thermostat problems are very common. The thermostat could be sticking or not opening or closing fully.

Timing chain

Timing chain
Timing chain in the Ford EcoBoost engine display

A timing chain performs the same function as a timing belt: it drives the engine camshaft(s), at the same time precisely synchronizing the camshaft rotation to the crankshaft. The timing chain is made of metal links connected to each other; very similar to a bicycle chain. The common question is when does the timing chain need to be replaced? The timing chain only needs to be replaced if it's worn or stretched or there is some other problem with it; there are no recommended service intervals. If the timing chain is stretched, it may cause various driveability issues like hard starting, lack of power, misfiring, etc. A stretched timing chain can also produce humming or rattling noises. A timing chain runs inside the engine and is lubricated by engine oil. It will last longer if the engine is well maintained and oil is changed regularly.

Throttle body

Throttle body
Clean throttle body

A throttle body controls the air flow into the engine intake. In a modern car, a throttle body contains a built-in electric motor or actuator that through the gears precisely operates the throttle valve (the gold-colored plate in the photo). There is also a built-in throttle position sensor that monitors the angle of the throttle valve. In older cars, the throttle was controlled by a cable. In late-model cars, the throttle is controlled electronically by the engine computer. When a driver depresses the accelerator pedal, the engine computer receives the signal from the accelerator pedal sensor. Based on this signal the engine computer commands the throttle body motor to open the throttle valve at needed angle. One of the most common problem is when a throttle body gets dirty, as the carbon deposits clog the throttle valve and restrict the air flow. This can cause various problems including rough idle, stalling, as well as the Check Engine light coming on in the instrument panel. A dirty throttle body will need to be cleaned with a special carb spray cleaner, which is not a very expensive service. If any part of the throttle body fails, whether the throttle sensor or the throttle drive motor, it's replaced as an assembly.

Traction Control

Traction Control warning light
Traction Control warning light

The purpose of the Traction Control system is to maintain traction during hard acceleration. Traction Control system monitors the speed of all 4 wheels. If any of the drive wheels begin to slip during acceleration, Traction Control system individually applies brakes to the slipping wheel(s) and reduces engine power. When Traction Control system operates, you might feel that the engine power is reduced and hear some buzzing. The Traction Control warning light may also flash. A Traction Control system cannot help in all possible situations; for example, if a car is stuck in loose, wet snow, it might be easier to free it out by turning the Traction Control OFF and letting the wheels spin freely to cut through the snow for better grip.

Water Pump

Car water pump
Water pump

A water pump is an important part of the engine cooling system. A water pump moves engine coolant (antifreeze) through the engine cooling system, thus providing the coolant circulation between the radiator and the engine (see the diagram in the previous paragraph). A water pump is usually installed on the engine cylinder block. It pulls cooled-down coolant from the radiator and pushes it through the passages (water jackets) in the engine cylinder block and the cylinder head and again out of the engine and into the radiator. This helps keep the engine from overheating; the hot coolant passes through the radiator where it cools down and then returns back to the engine.
A water pump is usually driven by the engine through a drive belt or a timing belt. A water pump consists of the housing with the shaft rotating in a bearing that is pressed into the housing. An outer end of the shaft has a flange that is used to hold a pulley. A serpentine or timing belt runs over the pulley. At the outer side there is a pulley mounted on the shaft. At the inner side there is a seal to keep the coolant from leaking out and the impeller that acts like a centrifugal pump (see the lower picture).
Typical water pump problems include coolant leaks, noisy or defective bearing, damaged or corroded impeller. A faulty water pump might cause the engine to overheat, which can result in serious engine damage. If defective, a water pump is replaced as a unit.
Sometimes you might be suggested to replace the water pump at the same time the timing belt is being replaced. In cases when a water pump is driven by a timing belt it's worth to do because it involves very little extra labor, plus, a water pump is subject to wear anyway. As long as the original parts are being used, I'd recommend to go for it.

Wheel alignment

Wheel Alignment
Checking the wheel alignment

The wheel alignment is the measurement of the angles of the wheels related to each other and to the car's body. In simple words, all wheels of the car must be close to parallel to each other and perpendicular to the road surface for a car to handle properly. Each car has very precise specifications how the wheels should be aligned. If any of the wheels is not aligned properly a car may have some issues with the handling. Improper alignment can also cause increased tire wear. In certain cases a car may even skid because of improper wheel alignment. The wheel alignment can be measured and adjusted on a special alignment rack (see the photo).
Over time, parts of the vehicle's suspension and steering wear out and the wheels become misaligned. The wheel alignment should be checked after the vehicle has been involved in an accident, even a minor one. Even if you hit a curb or a big bump the wheels can get misaligned. The wheel alignment must be re-adjusted if certain parts of the vehicle's suspension or steering are replaced. Also, if you feel that the car doesn't hold the road well or it's difficult to keep it straight, or if the car pulls to one side or if the steering wheel is out of center, the alignment should be checked. Similarly, if you feel that the car jumps sideways or fishtails when driving on a rough road, it's recommended to check the wheel alignment.
Before performing the wheel alignment a mechanic usually checks the suspension and the steering components. If any of the suspension or steering pats is loose or defective, it must be replaced before the wheel alignment. Adjusting the wheel alignment is usually not very expensive, and properly done wheel alignment makes a big difference in the way the car handles.

Wheel bearing

Wheel bearings are installed on or inside the wheel hubs, allowing the vehicle wheels to rotate with very little friction. There are two basic types of wheel bearings used in today's cars and trucks: ball-type and the tapered roller type.

Ball-type wheel bearings

Car Wheel Bearing
Car Wheel Bearing. Click for larger photo

Most cars have one ball-type wheel bearing (in the photo) installed at each wheel. A typical ball-type wheel bearing has two rows of balls inside. The bearing is packed with grease and sealed for life (in the photo). This type of bearing does not need any maintenance and is not repairable; if it wears out or gets noisy, it must be replaced. The most common symptom of a bad wheel bearing is a humming or rumbling noise that gets louder with speed. In some cars, a mechanic can detect a bad or worn-out wheel bearing by checking the wheels for freeplay. Replacing a wheel bearing can cost from $230 to $320 in a repair shop. It's not necessary to replace both wheel bearings if one is bad.

Tapered roller wheel bearings

Tapered Roller Wheel Bearing
Tapered Roller Wheel Bearing

Ttapered roller wheel bearings are used in pickup trucks and truck-based SUVs, some cars as well as in some trailers.
Tapered roller bearings are installed in pairs; two bearings per wheel. This type of wheel bearings need to be serviced in regular intervals. Typically the service includes repacking the bearings with new grease and adjusting the bearing pre-tension. Servicing tapered roller wheel bearings can cost from $120 to $230 per wheel. Both sides of one axle are serviced at the same time.
A bad or worn-out tapered wheel bearing can produce a humming noise that is more pronounced at higher speeds. In some cars, worn-out or out-of-adjustment bearings can cause knocking noise when driving over bumps. Bad tapered roller bearings must be replaced. Tapered roller bearing replacement is a bit more expensive: $240-$320 per wheel.

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