The purpose of the Idle Air Control (iac) system is to control engine idle speeds while preventing stalls due to changes in engine load. The iac assembly, mounted on the throttle body, controls bypass air around the throttle plate. By extending or retracting a conical valve, a controlled amount of air can move around the throttle plate. If rpm is too low, more air is diverted around the throttle plate to increase rpm.
During idle, the proper position of the iac valve is calculated by the ecm based on battery voltage, coolant temperature, engine load, and engine rpm. If the rpm drops below a specified rate, the throttle plate is closed. The ecm will then calculate a new valve position.
Three different designs are used for the iac conical valve. The first design used is single taper while the second design used is a dual taper. The third design is a blunt valve. Care should be taken to insure use of the correct design when service replacement is required.
The iac motor has 255 different positions or steps. The zero, or reference position, is the fully extended position at which the pintle is seated in the air bypass seat and no air is allowed to bypass the throttle plate. When the motor is fully retracted, maximum air is allowed to bypass the throttle plate. When the motor is fully retracted, maximum air is allowed to bypass the throttle plate.
The ecm always monitors how many steps it has extended or retracted the pintle from the zero or reference position; thus, it always calculates the exact position of the motor. Once the engine has started and the vehicle has reached approximately 40 mph, the ecm will extend the motor 255 steps from whatever position it is in. This will bottom out the pintle against the seat. The ecm will call this position 0 and thus keep its zero reference updated.
The iac only affects the engine's idle characteristics. If it is stuck fully open, idle speed is too high (too much air enters the throttle bore) If it is stuck closed, idle speed is too low (not enough air entering). If it is stuck somewhere in the middle, idle may be rough, and the engine won't respond to load changes.