Idle Speed/Throttle Actuator - Electronic: Description and Operation

IAC Valve Flow:

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

OPERATION
Bypass Air Control
The IAC valve, mounted in the throttle body, controls a portion of the bypass air. An orifice located between the throttle valves also supplies a constant amount of bypass air. By moving a conical valve known as a pintle, IN, towards the seat (to decrease air flow); or OUT, away from the seat (to increase air flow), a controlled amount of air can be bypassed. If engine speed is too low, more air is bypassed to increase RPM. If engine speed is too high, less air is bypassed to decrease RPM.

IAC Counts
The Powertrain Control Module (PCM) moves the IAC valve in small steps, called counts. These can be measured and displayed by a scan tool, which plugs into the Data Link Connector (DLC).

IAC Valve Positioning
During idle, the proper position of the IAC valve is calculated by the PCM, based on battery voltage, coolant temperature, engine load, and engine RPM. If the RPM drops below specification and the throttle valve is closed, the PCM senses a near stall condition and calculates a new valve position to prevent stalling.

ENGINE IDLE SPEED
Engine idle speed is a function of total air flow into the engine based on: IAC valve pintle position plus crankcase ventilation valve flow plus throttle valve opening plus bypass orifice air flow plus calibrated vacuum loss through accessories.

CONTROLLED IDLE SPEED
Controlled idle speed is programmed into the PCM, which determines the correct IAC valve pintle position to maintain the desired idle speed for all engine operating conditions and loads.

MINIMUM IDLE AIR
The minimum idle air rate is set at the factory with a stop screw. This setting allows enough air flow by the throttle valves to cause the IAC valve pintle to be positioned a calibrated number of steps (counts), from the seat, during controlled idle operation.

IAC VALVE RESET PROCEDURE
If the IAC valve is disconnected and reconnected with the engine running, the idle speed may be wrong. If this occurs, reset the IAC valve by depressing the accelerator pedal slightly, start and run engine for five seconds, then turn ignition "OFF" for ten seconds.