Idle Air Control (IAC) System

IAC system consists of electrically controlled motor (actuator) which positions IAC valve in air by-pass channel around throttle valve. IAC valve is threaded into throttle body casting. ECM calculates desired position of IAC valve based upon battery voltage, coolant temperature, engine load and engine speed, to control idle speed and prevent stalls due to engine load changes.

If engine speed is lower than desired, ECM activates IAC motor to retract IAC valve. When IAC valve is retracted, more air is diverted around throttle valve to increase engine speed. If engine speed is higher than desired, ECM activates IAC motor to extend IAC valve. When IAC valve is extended, less air is diverted around throttle valve, decreasing engine speed. If engine speed falls below preset level and throttle valve is closed, ECM senses near stall condition. To prevent stalling, ECM will calculate IAC valve position based upon barometric pressure.

There are 3 different conical IAC valve designs used. First is 35° single taper. Second design, used on 4.3L engines with A/T, is dual taper. Third design is blunt. Ensure that correct design is used if replacement is made.