Idle Air Control

The Idle Air Control (IAC) valve controls engine idle speed during engine load changes to prevent stalling. Mounted in the throttle body, the IAC controls air by-passed around throttle valve. Movement of IAC conical valve inward toward the seat decreases airflow while movement away from the seat increases airflow around throttle plate. The IAC valve moves in small steps called "counts", and can be measured with a "Scan" tester plugged into the Assembly Line Diagnostic Link (ALDL). Increase of counts indicates more air passing IAC valve.

Proper positioning of IAC valve at idle is determined by the ECM based on battery voltage, coolant temperature, engine load and engine RPM. When engine speed drops to less than desired RPM (with throttle plate closed), the ECM senses a near stall condition. The ECM then repositions IAC valve to prevent stalling.

Disconnecting and reconnecting of IAC valve with engine running will result in incorrect idle RPM. The IAC valve must be reset. IAC resets when ignition switch is turned from the "ON" to the "OFF" position. When servicing IAC, disconnect or connect only with ignition off to prevent required resetting of IAC.

The IAC valve affects only the idle system. If valve is stuck fully open, excessive airflow into the manifold creates a high idle speed. Valves 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.