HOLDEN GEN III V8, Service Training

Page: 105 / 113

GEN III V8 ENGINE MANAGEMENT
IDLE AIR CONTROL VALVE
The purpose of the Idle Air Control (IAC) valve, is to control engine idle speed, and prevent engine stalling due
to changes in engine load at idle. The IAC valve, mounted in the throttle body, controls bypass air around the
throttle valve. By extending the pintle (to decrease air flow) or retracting the pintle (to increase air flow), a
controlled amount of air can move around the throttle valve.
If RPM is too low, more air is bypassed around the
throttle valve to increase RPM. If RPM is too high,
less air is bypassed around the throttle valve to
decrease RPM. The IAC valve moves in small
steps numbered from 0 (extended pintle, bypass
air passage fully shut) to 500 (retracted pintle,
maximum bypass airflow) as commanded by the
PCM.
If the IAC valve is disconnected or reconnected
with the engine running, the PCM can lose track of
the actual position of the IAC. This will also happen
if the PCM connectors, fuse F31, or battery
terminals are disconnected. If this happens, the
PCM will reset the IAC valve, after the engine has
been run for at least five seconds. The reset
procedure goes like this:
The PCM commands the IAC valve to shut the idle
air passageway in the throttle body. It does so by
issuing enough extend pulses to move the IAC
valve pintle fully shut in the bore, regardless of
where the actual position was. Then, the PCM
calculates the IAC valve is at a fully shut position,
and calls that position 0 steps. Next, the PCM
issues retract steps to properly position the pintle.
The IAC valve can also be reset with Tech 2.
The IAC valve affects only the idle RPM of the
engine. If it is open fully, too much air will be
allowed into the manifold and idle speed will be too
high. If it is stuck closed, too little air will be allowed
into the intake manifold, and idle speed will be too
low.
Page-100 GEN III V8 ENGINE MANAGEMENT