Riviera V6-3.8L SC VIN 1 (1995)
Idle Air Control (IAC) Valve: Description and Operation
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 plate. By moving a conical valve, known as a pintle, 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, the PCM will retract the IAC
pintle, resulting in more air being bypassed around the throttle plate to increase RPM. If RPM is too high, the PCM will extend the IAC pintle,
allowing less air to be bypassed around the throttle plate, decreasing RPM.
The IAC pintle valve moves in small steps called counts.
OPERATION
During idle, the proper position of the IAC pintle is calculated by the PCM based on battery voltage, coolant temperature, engine load, and engine
RPM. If the RPM drops below a specified value, and the throttle plate is closed (Thottle Position (TP) sensor voltage is between 0.20 to 0.74), the
PCM senses a near stall condition. The PCM will then calculate a new IAC pintle valve position to prevent stalls.
If the IAC valve is disconnected and reconnected with the engine running, the idle RPM will be wrong. In this case, the IAC has to be reset.
The IAC resets when the engine is started, runs over 800 RPM, and then turns "OFF."
When servicing the IAC, it should only be disconnected or connected with the ignition "OFF" to keep from having to reset the IAC.
The position of the IAC pintle valve affects engine start up and the idle characteristics of the vehicle. If it is open fully, too much air will be
allowed into the manifold. This results in high idle speed along with possible hard starting and a lean air/fuel ratio. If the valve is stuck closed, too
little air will be allowed in the manifold. This results in a low idle speed along with possible hard starting and a rich air/fuel ratio.
If it is stuck part way open, the idle may be high or low and will not respond to changes in engine load.
