Idle Air Control Valve (IAC)

IAC valve controls engine idle speed to prevent stalling during engine load changes. The IAC valve is mounted on throttle body and controls the amount of air by-passed around the throttle plate. The IAC valve controls engine idle speed by moving its pintle in and out in steps referred to as "counts" (zero counts, fully seated; 255 counts, fully retracted). Counts can be measured by observing scan tool display while connected to the DLC.

If engine RPM is too low, pintle is retracted and more air is by-passed around the throttle plate to increase engine RPM. If engine RPM is too high, pintle is extended and less air is by-passed around the throttle plate to decrease engine RPM. Normal counts on an idling engine should be near 18. When engine is idling, PCM determines proper positioning of IAC valve based on battery voltage, coolant temperature, engine load and engine RPM.

If IAC valve is disconnected or reconnected with engine running, IAC loses its reference point and must be reset. On some models, IAC is reset by turning ignition on, then off. Other models require driving vehicle at normal operating temperature over 35 MPH with circuit properly connected. Problems in IAC circuit should set a related DTC.

The IAC valve affects only the idle system. If valve is stuck fully open, excessive airflow into the manifold creates a high idle speed. Valve stuck closed allows insufficient airflow, resulting in low idle speed. For calibration purposes, several different IAC valves are used. Ensure replacement valve is of proper design.