Idle Air Control (IAC) System

The IAC system consists of an electrically controlled motor which positions the IAC valve in the air by-pass channel around the throttle plate of each TBI unit. The ECM calculates the 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, the ECM activates the motor to retract the IAC valve. When the IAC valve is retracted, more air is diverted around the throttle plate, increasing engine speed. If engine speed is higher than desired, the ECM activates the motor to extend the IAC valve. When the IAC valve is extended, less air is diverted around the throttle plate, decreasing engine speed. If engine speed falls below a preset speed and the throttle plate is closed, the ECM senses a near-stall condition. To prevent stalling, the ECM will calculate an IAC valve position based upon barometric pressure.