Idle Air Control Valve

The Idle Air Control (IAC) valve controls engine idle speed during engine load changes to prevent stalling. The IAC valve is mounted on throttle body or on upper manifold assembly, and controls the amount of air by-passed around the throttle plate. To control engine idle speed, the IAC valve moves its pintle in and out in steps referred to as "counts" (zero counts, fully seated; 255 counts, fully retracted). Counts can be measured using a scan tool plugged into the Data Link Connector (DLC).

Normal counts on an idling engine is usually about 4-60. When engine is idling, PCM determines proper positioning of IAC valve based on battery voltage, engine coolant temperature, engine load and engine RPM. 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.

If IAC valve is disconnected or connected with engine running, IAC loses its reference point and has to be reset. Resetting of IAC is accomplished on some models by turning ignition on and off. On other models, driving vehicle at normal operating temperature and speed greater than 35 MPH with circuit properly connected may be necessary. Problems in IAC circuit should set a related diagnostic trouble code.

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 design IAC valves are used. Ensure proper design valve is used during replacement.