Idle Air Control Valve

NOTE: Do not attempt to remove protective cap to readjust stop screw. Misadjustment may result in damage to Idle Air Control (IAC) valve or to throttle body.

The IAC valve is mounted on throttle body where it controls engine idle speed under command of powertrain control module (PCM)/engine control module (ECM). The PCM/ECM sends voltage pulses to IAC valve motor windings, causing IAC valve pintle to move in or out a given distance (a step or count) for each pulse. The pintle movement controls airflow around throttle valves which, in turn, control engine idle speed.

The desired idle speeds for all engine operating conditions are programmed into calibration of PCM/ECM. These programmed engine speeds are based on coolant temperature, park/neutral position switch status, vehicle speed, battery voltage, and A/C system pressure (if equipped).

The PCM/ECM "learns" proper IAC valve positions to achieve warm, stabilized idle speeds (rpm) desired for various conditions (park/neutral or drive, A/C on or off. if equipped). This information is stored in PCM/ECM "keep alive" memories. Information is retained after ignition is turned OFF. All other IAC valve positioning is calculated based on these memory values. As a result, engine variations due to wear and variations in minimum throttle valve position (within limits) do not affect engine idle speeds. This system provides correct idle control under all conditions. This also means that disconnecting power to PCM/ECM can result in incorrect idle control or necessity to partially press accelerator when starting until PCM/ECM relearns idle control.

Engine idle speed is a function of total airflow into engine based on IAC valve pintle position, throttle valve opening, and calibrated vacuum loss through accessories. The minimum throttle valve position is set at factory with a stop screw. This setting allows enough airflow by throttle valve to cause IAC valve pintle to be positioned a calibrated number of steps (counts) from seat during "controlled" idle operation. The minimum throttle valve position setting on this engine should not be considered "minimum idle speed", as on other fuel injected engines. The throttle stop screw is covered with a plug at factory following adjustment.

If IAC valve is suspected as cause of improper idle speed.