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 air flow) or out (increasing air flow), a controlled amount of air can move around the throttle plate.

If RPM is too low, more air is by-passed 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 which plugs 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 RPM, and the throttle plate is closed, the ECM senses a near stall condition. The ECM will then calculate a new valve position to prevent stalls.

If the IAC valve is disconnected and reconnected with the engine running, the idle RPM may be wrong. In this case, the IAC has to be reset. On vehicles equipped with 3.0L engines, the IAC resets when the ignition is turned from the "ON" to "OFF" position. 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 open fully, too much air will be allowed into the manifold and idle speed will be high.

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