Idle Air Control (IAC) System

The IAC system consists of an electrically controlled motor which positions IAC valve in air by-pass channel around throttle plate of each TBI unit. The IAC valve is a part of the throttle body casting. The ECM calculates desired position of each IAC valve based upon battery voltage, coolant temperature, engine load and engine speed. It controls idle speed while preventing stalls due to engine load changes.

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