K 1500 Truck 4WD V8-5.3L VIN T (2000)
Idle Air Control Motor (IAC): Description and Operation
Idle Air Control (IAC) Assembly
The purpose of the 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 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 (3) in toward
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 in order to increase RPM. If engine speed is too high; less air is bypassed in order to decrease RPM. The PCM moves the IAC valve in
small steps, called counts. These can be measured and displayed with a scan tool, which plugs into the Data Link Connector (DLC). The PCM calculates
the proper position of the IAC valve during idle based upon the battery voltage, the coolant temperature, the engine load, and the 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 in or der to
prevent stalling.
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Engine idle speed is a function of total air flow into the engine. Idle speed is based on IAC valve pintle position, crankcase ventilation valve flow,
throttle valve opening, bypass orifice airflow, and calibrated vacuum loss through accessories.
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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.
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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.
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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, starting and running the engine for 5 seconds, and then turning the ignition OFF for 10 seconds.
