GM Lumina/Silhouette/Transport APVs 1990-1999 Repair Guide

Idle Air Control Valve

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OPERATION



The PCM controls engine idle speed by adjusting the position of the Idle Air Control (IAC) motor pintle. The IAC is a bi-directional motor driven by two coils. The pintle movement controls the air flow bypassing the throttle valves, which in turn, controls engine idle speed. The PCM pulses current to the IAC coils in steps (counts) to extend the IAC pintle into a passage in the throttle body to decrease air flow. The PCM reverses the current pulses to retract the pintle, increasing air flow. This method allows highly accurate control of idle speed and quick response to changes in engine load.

See Figure 1

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Fig. Fig. 1: Typical location of the IAC Valve-3.8L Engine

"Controlled" idle speed is programmed into the PCM, which determines the correct IAC valve pintle position to maintain the desired idle speed for all engine operating conditions and loads. "Minimum" idle air rate is set at the factory with a stop screw. 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 operation. It also helps prevent the throttle plate(s) from binding in the fully closed position in the throttle housing. The throttle stop screw is covered over at the factory.

See Figures 2 and 3



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Fig. Fig. 2: IAC valve wiring schematic-typical 3.8L engine



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Fig. Fig. 3: IAC air flow and wiring schematic-3.1L engine with TBI

TESTING



Preliminary tests for an IAC related problem include checking for the following conditions:



Poor connection at PCM or IAC motor. Inspect the harness connectors for backed out terminals, improper mating, broken locks, improperly formed or damaged terminals, and poor terminal to wire connection.
 
Inspect the wire harness for damage, including; proper routing, broken insulation, rub through, or broken wires.
 
A restricted air intake system. Check for a possible collapsed air intake duct, restricted air filter element or foreign objects blocking the air intake system.
 
Check the throttle body for objects blocking the IAC passage or throttle bore, excessive deposits in the IAC passage and on the IAC pintle, and excessive deposits in the throttle bore and on the throttle plate. Check for a sticking throttle plate. Also inspect the IAC passage for deposits or objects which will not allow the IAC pintle to fully extend.
 
Check for a condition that causes a vacuum leak, such as disconnected or damaged hoses, leaks at the EGR valve or EGR pipe to intake manifold, leaks at the throttle body, faulty or incorrectly installed PCV valve, leaks at the intake manifold, a brake booster hose that leaks or is disconnected, or a faulty component that uses engine vacuum to operate.
 

  1. If a scan tool is available, connect it to the system, and start the engine.
    1. Turn all accessories OFF (A/C, Heater, Rear Defroster, etc.).
    2.  
    3. Using the scan tool, command RPM up to 1500, down to 650, and then up to 1500 while monitoring Engine Speed on the scan tool. If the IAC valve follows these commands and holds the commanded engine speed, then it is good.
    4.  

  2.  
  3. If a scan tool is unavailable, then try the following test procedure:
    1. Unplug the IAC connector and check resistance between the IAC terminals. Resistance between terminals A & B and terminals C & D should be 40 to 80 ohms. If resistance is not within specification, the IAC valve should be replaced.
    2.  
    3. Check the resistance between the IAC terminals A to C, A to D, B to C and B to D. Resistance should be infinite. If not the IAC is faulty.
    4.  
    5. If the resistances are within specifications, check the IAC circuits for continuity back to the PCM. If the valve and circuits are functional, the PCM may be faulty.
    6.  

  4.  
  5. If intermittent poor driveability or idle symptoms are resolved by disconnecting the IAC, carefully re-check connections, and the IAC valve terminal resistance or replace the IAC.
  6.  

See Figures 4 through 8

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Fig. Fig. 4: Unplug the connector to the sensor



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Fig. Fig. 5: Read the resistance of terminals A and B in the IAC



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Fig. Fig. 6: When the IAC Terminals B and C are jumped, the reading should be infinity (Open Circuit)



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Fig. Fig. 7: The sensor plug is labeled/lettered for easy reference



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Fig. Fig. 8: Measure the resistance at the sensor prongs, check for corrosion and/or looseness.

REMOVAL AND INSTALLATION



Both original and replacement IAC valves have a special factory applied thread-locking compound applied to the screw threads. If the valve removed from the throttle body is being reinstalled, do not remove any of the thread locking compound that may remain on the threads.

3.1L Engine (VIN D)

  1. Disconnect the negative battery cable.
  2.  
  3. Remove the air cleaner assembly.
  4.  
  5. Detach the IAC valve electrical connector.
  6.  
  7. Remove the IAC valve assembly and gasket.
  8.  

See Figure 9

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Fig. Fig. 9: Idle Air Control Valve (IAC) and gasket


CAUTION
If the IAC valve has been in service, do not push or pull on the IAC valve pintle. The force required to move the pintle may damage the threads on the worm drive. Also, do not soak the IAC valve in any liquid cleaner or solvent, as damage may result. If installing a new IAC valve, be sure to replace with an identical part. IAC valve pintle shape and diameter are designed for the specific application. Measure the distance between the tip of the IAC pintle and the mounting surface, and if it measures greater than 11/8in. (28mm), use finger pressure to slowly retract the pintle. The force required to retract the pintle of a new valve will not cause damage to the valve.

To install:
  1. Clean the IAC valve gasket sealing surface, pintle valve seat and air passage. Use carburetor cleaner to remove carbon deposits. Shiny spots on the pintle or seat are normal, and do not indicate misalignment or a bent pintle shaft.
  2.  


WARNING
Do not use a cleaner that contains methyl ethyl ketone, an extremely strong solvent, and one that is not necessary for this type of deposit.

  1. Install the new gasket and valve assembly.
  2.  
  3. Using a box wrench or socket, tighten the valve to 13 ft. lbs. (18Nm)
  4.  
  5. Attach the electrical connector.
  6.  
  7. Install the air cleaner assembly.
  8.  
  9. Connect the negative battery cable.
  10.  
  11. To reset the IAC pintle position.
    1. With the key OFF ; depress the accelerator pedal slightly.
    2.  
    3. Start and run the engine for five seconds.
    4.  
    5. Turn the ignition OFF for ten seconds.
    6.  
    7. Restart the engine and check for proper idle operation.
    8.  

  12.  

3.4L Engines


WARNING
The IAC Valve may be damaged if installed with the cone (pintle) extended more than 11/8in. (28mm). Measure the distance that the valve is extended before installing a new valve. The distance from the idle air control valve motor housing to the end of the idle air control valve pintle should be less than 11/8in. (28mm). If it is longer, manually compress the pintle until the extension is retracted to less than 11/8in. (28mm).

  1. Detach the electrical connector from the Idle Air Control (IAC) valve and, if necessary for clearance, Throttle Position (TP) sensor.
  2.  
  3. Remove the two screws attaching the IAC valve to the intake manifold.
  4.  
  5. Remove the IAC.
  6.  
  7. Remove the IAC valve O-ring.
  8.  

To install:
  1. Install a new O-ring seal onto the IAC Valve.
  2.  
  3. Install the new IAC in the intake manifold.
  4.  
  5. Install the two screws into the IAC. Tighten them to 27 inch lbs. (3 Nm).
  6.  
  7. Reconnect the electrical connector to the IAC and to the TP sensor.
  8.  
  9. The IAC resets when the key is cycled ON then OFF .
  10.  

See Figure 10



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Fig. Fig. 10: Installation of an IAC Valve on a 3.4L Engine

3.8L Engines

The repair procedure for the 3.8L engine is the same as the 3.1L engine with the following exceptions:

  1. If the IAC valve is disconnected and reconnected with the engine running, the idle RPM will be wrong. In this case, the IAC has to be reset.
  2.  
  3. The IAC resets when the key is cycled ON then OFF .
  4.  

 
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