The IAC (idle air control) valve in your Pontiac Sunfire controls the amount of air entering the throttle body to control the engine speed when the engine is at idle. The on-board computer opens the valve to raise idle speed or closes the valve to lower idle speed until the desired speed is achieved. The computer assumes the throttle plate in the throttle body is allowing a specific amount of air to bypass it and enter the engine. After adjusting the idle speed, you must reset the computer that controls the IAC valve.
Start the engine. Check for and repair any vacuum leaks in the Sunfire's vacuum hoses. Most vacuum leaks make a hissing sound when the engine is running. Allow the vehicle to run until operating temp is reached and the upper radiator hose becomes hot. Turn the engine off.
Unplug the IAC valve's electrical connector. The IAC on the Sunfire is located in the intake manifold below the throttle body. Lift the locking tab on the connector and pull it out of the IAC valve to unplug it.
Start the engine and adjust the throttle plate using a Torx driver until the engine idles at around 650 rpm. The throttle plate screw is located on the side of the throttle body near the throttle cables.
Turn off the engine. Disconnect the negative battery cable and reconnect the IAC electrical connector. Wait two minutes, reconnect the battery cable and start the engine. The computer should now control idle speed accurately.
