Hombre S Regular Cab 4WD V6-4.3L (1999)
Idle Air Control Valve: Testing and Inspection
Idle Air Control (IAC) System Diagnosis
Idle Air Control System Diagnosis
Circuit Description
The VCM controls idle RPM with the IAC valve. To increase idle RPM, the VCM retracts the IAC pintle from the seat, allowing more air to bypass the
throttle plate. To decrease RPM, it extends the IAC pintle valve in towards the seat, reducing air flow through the IAC valve port in the throttle Body. A
scan tool will read the VCM 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).
IAC Valve Reset Procedure
Without Scan Tool
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Start the engine for 20 seconds.
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Turn OFF the ignition for 10 seconds.
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Start the engine.
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Check for the proper idle speed.
With Scan Tool
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Connect the scan tool to the DLC.
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Turn ON the ignition leaving the engine OFF.
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Using the scan tool, select the IAC Reset in Special Functions.
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Activate the reset function and wait for the IAC counter to stop.
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Start the engine.
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Check for proper idle speed.
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. The following checks should be
made to repair a non-IAC system problem.
IMPORTANT: If for some reason the air intake to the engine is restricted, the VCM will attempt to compensate by backing out the IAC pintle to
maintain desired idle. If the engine has another source of air to the intake, the VCM will compensate by extending the IAC pintle, scan IAC counts may
reach zero as the VCM tries to maintain desired idle.
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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 crankcase ventilation system. Also, check for binding of
throttle blade or Linkage.
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Sticking throttle cable or throttle plate shaft.
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System lean (high air/fuel ratio) - Idle speed may be too high or too low. Engine speed may vary up and down, disconnecting IAC does not help.
May set DTC P0171. The scan tool will read Heated Oxygen Sensor (HO2S) output less than 300 mV (0.3 volt). Check for low regulated fuel
pressure or water in fuel.
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System rich (low air/fuel ratio) - Idle speed too low. Scan IAC counts are usually above 80. System obviously rich and may exhibit black smoke
exhaust. May set DTC P0172. The scan tool will read Heated Oxygen Sensor (HO2S) signal fixed above 800 mV (0.8 volt).
