Idle Speed/Throttle Actuator - Electronic: Description and Operation

Idle Air Control (IAC) Valve Assembly:

Idle Air Control (IAC) Valve Air Flow Diagram:

PURPOSE
The Idle Air Control (IAC) valve assembly is to control engine idle speed, while preventing stalls due to changes in engine load.

OPERATION
The IAC valve, mounted in the throttle body, controls bypass air around the throttle valve. By moving a conical valve known as a pintle, IN toward the seat (to decrease air flow), or OUT away from the seat (to increase air flow), a controlled amount of air moves around the throttle valve. If rpm is too low, more air is bypassed around the throttle valve to increase it. If rpm is too high, less air is bypassed around the throttle valve to decrease it.

The Engine Control Module (ECM) moves the IAC valve in small steps, called "counts." These can be measured by "Scan" tool test equipment, which plugs into the Assembly Line Diagnostic Link (ALDL) connector.

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 specification and the throttle valve is closed, the ECM senses a near stall condition and calculates a new valve position to prevent stalling.

If the IAC valve is disconnected and reconnected while the engine is running, the resulting idle rpm may be wrong and resetting of the IAC valve will be required.

After running the engine, the IAC valve will reset when the ignition is turned "OFF." When servicing the IAC valve, it should only be disconnected or connected with the ignition "OFF."

If the ECM is without battery power for any reason, the programmed position of the IAC valve pintle is lost, and replaced with a "default" value. This causes the engine idle to be unstable for a period of approximately seven minutes. To return the IAC valve pintle to the correct position, see "Service & Repair." Service and Repair

Different shaped pintles are used for different IAC valve applications. This throttle body (model B115) has an IAC valve with a 12 mm dual taper pintle. Be sure to use the correct part number, with the correct pintle shape and diameter, when replacement is required.

The IAC valve affects only the idle characteristics of the vehicle. If it is fully retracted, too much air will be allowed into the manifold and idle speed will be high. If it is stuck closed, too little air will be allowed in the manifold, and idle speed will be too low. If it is stuck part way open, the idle may be rough, and will not respond to engine load changes. A Code 35 should set if a controlled idle cannot be maintained.