Idle Air Control (IAC) Valve

The purpose of the IAC valve is to control engine idle speed, while preventing stalls due to changes in engine load The throttle blade when closed allows a small amount of air into the intake manifold. However, most of the air for closed throttle engine operation passes through the IAC valve, bypassing the throttle blade. By moving a conical valve known as a pintle in toward the seat (to decrease air flow), or out away from the seat (to increase air flow), a controlled amount of air can be bypassed. If engine speed is too low, more air is bypassed in order to increase RPM. If engine speed is too high, less air is bypassed in order to decrease RPM. The PCM moves the IAC valve in small steps, called counts These can be measured and displayed with a scan tool, which plugs Into the Data Link Connector (DLC). The PCM calculates the proper position of the IAC valve during idle based upon the battery Voltage, the coolant temperature, the engine load, and the engine RPM.

If the RPM drops below specification and the throttle valve is closed, the PCM senses a near stall condition and calculates a new valve position in order to prevent stalling.
^ Engine idle speed is a function of total air flow into the engine. Idle speed is based on IAC valve pintle position, crankcase ventilation valve flow, throttle valve opening, bypass Office air flow, and calibrated vacuum loss through accessories.
^ Controlled idle speed is programmed into the PCM which determines the correct IAC valve pintle position to maintain the desired idle speed for all engine operating conditions and loads
^ The minimum idle air rate is set at the factory with a stop screw. This setting allows enough air flow by the throttle valve to cause the IAC valve pintle to be positioned a calibrated number of steps (counts), from the seat, during controlled idle operation.

If the IAC valve is disconnected or disabled with the engine running, the PCM may loose track of the IAC valve position causing erratic or incorrect Idle speed. if this occurs, reset the IAC valve by doing the following:
1. Start and idle the engine foe 15 seconds.
2. Turn the ignition switch to the LOCK/OFF position
3. Wait 15 seconds.
4. Restart the engine and inspect for proper idle operation.