Express 1/2 Ton Van V6-4.3L VIN W (1999)
1. Connect the scan tool to the DLC.
2. Turn ON the ignition leaving the engine OFF.
3. Using the scan tool, select the IAC Reset in Special Functions.
4. Activate the reset function and wait for the IAC counter to stop.
5. Start the engine.
6. 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.
^
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.
^
Sticking throttle cable or throttle plate shaft.
^
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.
^
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).
Check for high fuel pressure, leaking or sticking injector. Silicone contaminated HO2S scan voltage will be slow to respond.
^
Throttle body - Remove IAC valve and inspect bore for foreign material.
^
IAC Valve Electrical Connections - IAC valve connections should be carefully checked for proper contact.
^
Crankcase Ventilation Valve - An incorrect or faulty crankcase ventilation valve may result in an incorrect idle speed.
Refer to Rough, Unstable, or Incorrect Idle and Stalling.
^
A/C Compressor - Refer to A/C Compressor Control Circuit Diagnosis if circuit is shorted to ground. If the relay is faulty, an idle problem may
exist.
Refer to Rough, Unstable, or Incorrect Idle and Stalling.
^
If intermittent poor driveability or idle symptoms are resolved by disconnecting the IAC, carefully recheck connections, valve terminal
resistance or replace IAC.
An intermittent may be caused by any of the following conditions:
^
A poor connection
^
Rubbed through wire insulation
^
A broken wire inside the insulation
Thoroughly check any circuitry that is suspected of causing the intermittent complaint. Refer to Intermittents and Poor Connections Diagnosis. See:
Computers and Control Systems/Testing and Inspection/Symptom Related Diagnostic Procedures See: Computers and Control Systems/Testing and
Inspection/Component Tests and General Diagnostics
Test Description
Number(s) below refer to the step number(s) on the Diagnostic Table.
2. This test determines if the VCM is capable of closing the IAC valve.
3. This test determines if the VCM is capable of opening the IAC valve.
4. All engine mechanical related conditions should be checked and repaired before testing the IAC system.
