C 1500 Truck 2WD V6-262 4.3L VIN Z (1992)
Idle Air Control (IAC) System
Circuit Description:
The ECM controls idle rpm with the IAC valve. To increase idle rpm, the ECM moves the IAC valve away from it's seat, allowing more air to pass
by the throttle plate. To decrease rpm, it moves the IAC valve toward it's seat, reducing air flow by the throttle plate. A Tech 1 "Scan" tool will
read the ECM commands to the IAC valve in counts. The higher the counts, the more air allowed (higher idle). The lower the counts, the less air
allowed (lower idle).
Test Description:
Number(s) below refer to circled number(s) on the diagnostic chart.
1. The IAC tester is used to extend and retract the IAC valve. Valve movement is verified by an engine speed change. If no change in engine
speed occurs, the valve can be retested when removed from the throttle body.
2. This step checks the quality of the IAC movement in Step 1. Between 700 rpm and about 1500 rpm, the engine speed should change smoothly
with each flash of the tester light in both extend and retract. If the IAC valve is retracted beyond the control range (about 1500 rpm), it may
take many flashes in the extend position before engine speed will begin to drop. This is normal on certain engines, fully extending the IAC may
cause engine stall. This may be normal.
3. Steps 1 and 2 verified proper IAC valve operation while this step checks the IAC circuits. Each lamp on the node light should flash red and
green while the IAC valve is cycled. While the sequence of color is not important if either light is "OFF" or does not flash red and green, check
the circuits for faults, beginning with poor terminal contacts.
IAC VALVE RESET PROCEDURE:
^
Ignition "OFF" for 10 seconds.
^
Start and run engine for 5 seconds.
^
Ignition "OFF" for 10 seconds.
Diagnostic Aids:
A slow, unstable, or fast idle may be caused by a non-IAC system problem that cannot be overcome by the IAC valve. Out of control range IAC
Tech 1 "Scan" tool counts will be above 60 if idle is too low, and zero counts if idle is too high. The following checks should be made to repair a
non-IAC system problem.
^
Vacuum Leak (High Idle) - If idle is too high, stop the engine. Fully extend (low) IAC with tester. Start engine. If idle speed is above 800
rpm, locate and correct vacuum leak including PCV system. Also check for binding of throttle blade or linkage.
^
System too lean (High Air/Fuel Ratio) - The idle speed may be too high or too low. Engine speed may vary up and down and disconnecting
the IAC valve does not help. Code 44 may be set "Scan" 02 voltage will be less than 300 mV (.3 volt). Check for low regulated fuel
pressure water in the fuel or a restricted injector.
^
System too rich (Low Air/Fuel Ratio), - The idle speed will be too low. "Scan" tool IAC counts will usually be above 80. System is
obviously rich and may exhibit black exhaust smoke. Tech 1 "Scan" tool 02 voltage will be fixed above 800 mV (.8 volt). Check for high
fuel pressure, leaking or sticking injector. Silicone contaminated 02 sensors "Scan" voltage will be slow to respond.
^
Throttle body - Remove IAC and inspect bore for foreign material. See: Testing and Inspection/Symptom Related Diagnostic
Procedures/Rough, Unstable or Incorrect Idle/Stalling
^
A/C Compressor - Refer to "A/C Diagnosis" if circuit is shorted to ground. If the relay is faulty, idle problem may exist. See: Testing and
Inspection/Symptom Related Diagnostic Procedures/Rough, Unstable or Incorrect Idle/Stalling
^
If intermittent poor driveability symptoms are resolved by disconnecting the IAC, carefully recheck connections, valve terminal resistance,
or replace IAC.
