All modern vehicles that are fuel injected have an idle control device. Some are automatic computer throttle control or idle control valves. Both types control the idle when the accelerator pedal is not actuated and the throttle plate is closed. When the throttle plate is closed at an idle, no air can get past the plate for the engine to idle.
On a throttle control device, a motor is actuated, which acts the same as a servo, wherein it will move in and out and open or close the throttle. This is controlled through the computer after it acquires the necessary information from its sensors to control the idle.
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The most common control is the idle air control valve found on most foreign and domestic vehicles. This particular device is always located on or very close to the throttle body on the intake. It is an electrically operated reversible motor about 1 1/2 inches in diameter. It moves a shaft in and out that has a bullet shaped end on it that closes or opens a hole in the idle air control case. There is a hole or hose, depending on vehicle, that picks up air in front of the throttle control plate in the throttle body. This air travels through to the idle air control, and the control either opens or closes off the hole, allowing air to pass to the opposite side of the throttle plate in the throttle body.
There is a predetermined adjusted amount of air necessary to keep the engine idling at a certain rpm. When these begin to malfunction from carbon build-up, the engine has a hard time idling. A common symptom is for the engine rpm to rise and fall continuously or for the car to quit every time the car comes to a stop. The engine can be heard to slow down dramatically and then stall. It will start again easily with no problem, but the moment the foot comes off the accelerator, it tries to die or just dies. They are relatively simple to replace and not very expensive in most cases. It is a common problem when the vehicle gets over 70,000 miles.