Idle Speed/Throttle Actuator - Electronic: Description and Operation

Fig. 65 Idle Air Adjusting Unit:

Engine idle speed is control by the intake collector mounted auxiliary air control (AAC) valve and the air cut valve. Both valves operate by metering the amount of air that is allowed to bypass the throttle plates. When the engine is cold, the air cut valve is fully open allowing the AAC to function normally. The ECU opens the AAC causing the engine to fast idle. Once the engine temperature begins to rise coolant flowing through the air cut valve causes the valve to close. When closed the AAC is limited to the amount of air that it can allow into the plenum. The idle then drops to curb speed where the limited function of the AAC is sufficient to properly maintain base idle speeds.