An idle air control valve, also called an IAC valve, is found in every vehicle and is located inside of the throttle body. When a car idles at a red light or while parked, the idle air control valve regulates the amount of air that circulates in the engine, which keeps the engine at a steady RPM and prevents rough idling. When an air control valve goes bad due to either carbon build-up or getting stuck, there are a few obvious symptoms.
A bad idle air control valve will prevent adequate air flow to the engine and will result in increasingly lower RPMs when idling. The number of RPMs, while idling, will vary a bit based on the individual vehicle but is generally considered low when below 800 RPM. The RPMs will increase to normal levels when the accelerator is pressed.
As the air control valve becomes increasingly clogged, the engine RPMs, while idling, will continue to drop until the vehicle stalls. This problem can usually be identified by applying the gas. When the vehicle is about to stall, pressing the accelerator will cause the RPMs to increase and will keep the car running. If the pedal is released, the RPMs will quickly drop, and the vehicle will stall. Sometimes, depending on how badly the idle air control valve is clogged, the vehicle may run correctly when the accelerator is pressed, but then sputter or stall when decelerating.
If the idle air control valve has gone bad and no longer regulates the flow of air into the engine, typically due to the IAC being stuck open, the vehicle will idle roughly. A rough idle is characterised by shaking, vibrating and shuddering while the car is on but not moving. A rough idle due to a bad intake air control valve will eventually result in stalling immediately after starting the vehicle.
A bad idle air control valve can cause a car to stall immediately after ignition in warm weather, but the vehicle will start properly when it is cold. When the accelerator is pressed, however, the car will stall. Likewise, if the temperature outside is warm enough, the car may start even though the idle air control valve is bad but will require the driver to immediately press the acceleration pedal to keep it from stalling.