Idle Air Control System

The purpose of the idle air control system is to control engine idle speed while compensating for changes in engine load.

The Idle Air Control (IAC) valve, mounted in the throttle body, controls bypass air around the throttle plate. By moving a conical valve, called a pintle, in towards the seat, airflow will decrease. If the pintle is moved outwards from the seat, a controlled amount of air is allowed to bypass the throttle plate. If the PCM determines that rpm is too low, more air is allowed to bypass the throttle to increase rpm. If rpm is too high, less air is allowed to bypass the throttle plate, decreasing rpm.

During idle, the proper position of the IAC valve is calculated by the PCM, based on battery voltage, coolant temperature, engine load and engine rpm. If the rpm drops below specification and the throttle valve is closed, the PCM moves the IAC valve to control speed.

Once the PCM has sought a desired idle, it can retard or advance spark timing to smooth the idle out or increment/decrement slight changes. This operation keeps the IAC valve from continuously moving in and out.

For more information on the idle air control system, refer to "Fuel Injector" in the Fuel System.

An unstable TP sensor at idle, varying more than four counts (TP counts read on Scan tool), will disrupt closed loop engine idle control.