Idle Speed/Throttle Actuator - Electronic: Description and Operation

Engine idle speed is controlled by the Engine Control Module (ECM) through the IAC valve mounted on the throttle body. The ECM sends voltage pulses to the IAC motor windings causing the IAC pintle to move "IN" (retract) or "OUT" (extend) a given distance (number of steps) for each pulse, called counts. The pintle movement controls the airflow around the throttle valves, which in turn, controls engine idle speed.
- Engine idle speed is a function of total airflow into the engine based on IAC valve pintle position + Positive Crankcase Ventilation flow throttle valve opening + calibrated vacuum loss through accessories.
- "Controlled" idle speed is programmed into the ECM, 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 valves 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 and reconnected with the engine running, the idle speed may be wrong. In this case, the IAC valve can be reset by doing the following: Depress accelerator pedal slightly, start and run engine for five seconds, then turn ignition "OFF" for ten seconds.