Idle Air Control

The Idle Air Control (IAC) valve controls engine idle speed, while preventing stalls due to changes in engine load. The IAC valve, mounted in the throttle body, controls by-pass air around the throttle valve. By moving a conical valve in (decreasing airflow), or out (increasing airflow), a controlled amount of air can move around the throttle plate.

If RPM is too low, more air is by-passed around the throttle valve to increase RPM. If RPM is too high, less air is by-passed around the throttle valve to decrease RPM. The IAC valve moves in small steps called counts, which can be measured by test equipment plugged into the ALCL.

During idle, the proper position of the IAC valve is calculated by the ECM based on battery voltage, coolant temperature, engine load and engine RPM. If the RPM drops below a specified value and the throttle plate is closed, the ECM senses a near stall condition. The ECM will then calculate a new IAC valve position to prevent engine stall.

If the IAC valve is disconnected and reconnected with the engine running, the idle RPM may be wrong and must be reset. On vehicles equipped with 3.0L engines, the IAC resets when the ignition switch is turned from the "ON" to the "OFF" position. On all other models, the IAC valve will reset when the vehicle is driven over 35 MPH. When servicing the IAC, it should only be disconnected or connected with the ignition off. This prevents having to reset the IAC.

Different designs are used for the IAC valve. Be sure to use the correct design when replacement is required. The IAC valve affects only the idle characteristics of the vehicle. If it is fully open, too much air will be allowed into the manifold and the idle speed will be high.

If IAC valve is stuck closed, too little air will be allowed in the manifold and idle speed will be too low. If it is stuck part way open, the idle may be rough and will not respond to engine load changes.