90-823225--1 10965E-26 - ELECTRONIC FUEL INJECTION (MULTI-PORT AND THROTTLE BODY)
IDLE AIR CONTROL (IAC) VALVE
The purpose of the IAC valve assembly 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 valves.
72800
Idle Air Control (IAC) Valve Assembly
By moving a conical valve known as a pintle, IN, to-
ward the seat (to decrease air flow), or OUT, away
from the seat (to increase air flow), a controlled
amount of air moves around the throttle valve. If RPM
is too low, more air is bypassed around the throttle
valve to increase it. If RPM is too high, less air is by-
passed around the throttle valve to decrease it.
The ECM moves the IAC valve in small steps, called
counts. These can be measured by scan tool test
equipment, which plugs into the DLC connector.
During idle, the proper position of the IAC valve is en-
gine load, and engine RPM. If the RPM drops below
specification and the throttle valve is closed, the ECM
senses a near stall condition and calculates a new
valve position to prevent stalling.
• Engine idle speed is a function of total air flow into
the engine based on IAC valve pintle position.
• “Controlled” idle speed is programmed into the
ECM, which determines the correct IAC valve
pintle position to maintain the desired idle speed
for all engine operating conditions and loads.
• The minimum idle air rate is set at the factory with
stop screws. 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 op-
eration.
• If the IAC valve is disconnected and reconnected
with the engine running, the idle speed may be
wrong. In this case, the IAC valve can be reset by
doing the following: Turn off engine, wait ten se-
conds, and restart engine.
72986
IAC Valve Air Flow Diagram
To Next Page
Loading...
