Firebird V8-5.7L VIN G (2001)
Idle Air Control Motor (IAC): Description and Operation
IDLE AIR CONTROL (IAC) VALVE
The purpose of the IAC valve is to control the engine idle speed, while preventing stalls due to changes in the engine load. The IAC valve (1), mounted
in the throttle body, controls the bypass air around the throttle valve (2). By moving a conical valve known as a pintle (3), in, towards the seat, in order to
decrease the air flow, or out, away from the seat, in order to increase the air flow, a controlled amount of air can be bypassed. If engine speed is too low,
more air is bypassed in order to increase the RPM. If engine speed is too high, less air is bypassed i order to decrease the RPM. The PCM moves the
IAC valve in small steps, called counts. These counts can be measured and displayed by a scan tool, which plugs into the Data Link Connector (DLC).
The PCM calculates the proper position of the IAC valve during idle based on 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 order to prevent stalling.
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The engine idle speed is a function of total air flow into the engine. The idle speed is based on the IAC valve pintle position, the crankcase
ventilation valve flow, the throttle valve opening, the bypass orifice air flow, and the calibrated vacuum loss through the accessories.
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Controlled idle speed is programmed into the PCM, which determines the correct IAC valve pintle position in order 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, or 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. Start and run the engine for 5 seconds, then turn the ignition OFF for 10 seconds.
