The engine idle speed is monitored and controlled by the Engine Control Module/Powertrain Control Module (ECM/PCM) which uses the Idle Air Control (IAC) Valve to regulate the idle speed. The valve controls the amount of air that is allowed to bypass the intake throttle plate, which enables the engine idle speed to remain constant even though the engine loads change during idle. This is especially true when electrical consumers such as the air conditioner are used.

The minimum idle air speed is set at the factory with a stop screw. This setting allows a certain amount of air to bypass the throttle valves regardless of IAC valve positioning. A combination of this airflow and IAC positioning allows the ECM/PCM to control engine idle speed. During normal engine idle operation, the IAC valve is controlled by the ECM/PCM and properly positioned. No adjustment should be required during routine maintenance. Tampering with the minimum idle speed adjustment may result in premature failure of the IAC valve or improperly controlled engine idle operation.
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