Idle Speed/Throttle Actuator - Electronic: Description and Operation

IAC Valve Installation:

DESCRIPTION:

^ The purpose of the Idle Air Control (IAC) Valve is to control engine idle speed, while preventing stalls due to changes in engine load.

^ The IAC valve, mounted in the throttle body, controls bypass air around the throttle valve. By moving the conical valve IN (to decrease air flow) or OUT (to increase air flow), a controlled amount of air can move around the throttle plate. If rpm is too low, more air is bypassed around the throttle valve to increase idle speed. If rpm is too high, less air is bypassed around the throttle valve to decrease engine rpm.

^ The IAC valve moves in small steps called "counts" which can be monitored by a "Scan" tool.

^ During idle, the proper position of the IAC valve is "learned" by the ECM based on battery voltage, coolant temperature, and engine rpm.

^ If the "learned" IAC valve position is incorrect, the ECM will command a reset. The reset will occur when the vehicle speed is above 30 mph.

^ If the rpm drops below a specified value, and the throttle plate is closed, the ECM senses a near stall condition. The ECM will then calculate a new value position to prevent stalls.

^ Different pintle designs are used for the IAC valve. Be sure to use the correct design, when replacement is required.

^ For diagnosis and testing, see COMPUTERIZED ENGINE CONTROLS.