DeVille V8-4.6L VIN Y (1998)
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 throttle plate when closed allows a small amount of air into the intake manifold. However, most of the air for closed throttle engine operation passes
through the IAC valve, bypassing the throttle plate. The IAC valve, mounted in the throttle body, controls air flow with a conical valve or pintle. By
moving the pintle in towards the seat, air flow is decreased. By moving the pintle out away from the seat, air flow is increased.
The PCM moves the IAC pintle out if engine speed is too low or in if engine speed is too high. The PCM controls IAC valve movement in small steps
called counts. The IAC valve position, measured in counts away from the valve seat, can be displayed on the scan tool.
The scan tool displays IAC position in counts. 0 counts indicates the PCM is commanding the IAC to be driven all the way into a fully seated position.
This is usually caused by a vacuum leak.
The higher the number of counts, the more air is being commanded to bypass the IAC pintle. 320 counts indicates a fully open position.
Prior to starting the engine and at idle, the PCM calculates the desired position of the IAC valve based on coolant temperature, engine load, engine speed
and battery voltage. While the throttle position is off idle, the IAC valve position follows or tracks throttle position to allow for smooth transitions from
open to closed throttle.
If the IAC valve is disconnected or disabled with the engine running, the PCM may loose track of the IAC valve position causing erratic or incorrect idle
speed. If this occurs, reset the IAC valve by doing the following:
1. The coolant temperature must be above 0°C (32°F).
2. The vehicle must be in Park (P) not Neutral (N).
3. Turn the ignition switch to Lock for 20 seconds.
