GM Lumina/Grand Prix/Cutlass Supreme/Regal 1988-1996

Idle Air Control (IAC) Valve

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OPERATION



Engine idle speeds are controlled by the ECM through the IAC valve mounted on the throttle body. The ECM sends voltage pulses to the IAC motor windings causing the IAC motor shaft and pintle to move in or out a given distance (number of steps) for each pulse (called counts). The movement of the pintle controls the airflow around the throttle plate, which in turn, controls engine idle speed. IAC valve pintle position counts can be observed using a scan tool. Zero (0) counts correspond to a fully closed passage, while 140 counts or more correspond to full flow.

Idle speed can be categorized in 2 ways: actual (controlled) idle speed and minimum idle speed. Controlled idle speed is obtained by the ECM positioning the IAC valve pintle. Resulting idle speed is determined by total air flow (IAC passage + PCV + throttle valve + calibrated vacuum leaks). Controlled idle speed is specified at normal operating conditions, which consists of engine coolant at normal operating temperature, air conditioning compressor OFF, manual transmission in Neutral or automatic transmission in D .

Minimum idle air speed 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 normal controlled idle operation.

The idle speed is controlled by the ECM through the IAC valve. No adjustment is required during routine maintenance. Tampering with the minimum idle speed adjustment is highly discouraged and may result inPremature failure of the IAC valve.

TESTING



See Figures 1 and 2

Click image to see an enlarged view

Fig. Fig. 1: Idle Air Control (IAC) valve wiring and terminal identification - 1988-93 vehicles



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Fig. Fig. 2: Idle Air Control (IAC) valve wiring and terminal identification-1994 and later vehicles

  1. Disengage the IAC electrical connector.
  2.  
  3. Using an ohmmeter, measure the resistance between IAC terminals A and B . Next measure the resistance between terminals C and D .
  4.  
  5. Verify that the resistance between both sets of IAC terminals is 40-80 ohms. If the resistance is not as specified, the IAC may be faulty.
  6.  
  7. Measure the resistance between IAC terminals B and C . Next measure the resistance between terminals A and D .
  8.  
  9. Verify that the resistance between both sets of IAC terminals is infinite. If the resistance is not infinite, the IAC may be faulty.
  10.  
  11. Also, with a small mirror, inspect IAC air inlet passage and pintle for debris. Clean as necessary, as this can cause IAC malfunction.
  12.  

REMOVAL & INSTALLATION



See Figures 3 and 4

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Fig. Fig. 3: The Idle Air Control (IAC) valve is mounted in the throttle body-1994 3.8L engine shown



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Fig. Fig. 4: Idle Air Control (IAC) valve components

  1. Disconnect the negative battery cable.
  2.  
  3. If necessary for access, remove the air cleaner.
  4.  
  5. Detach the electrical connector from the idle air control valve assembly.
  6.  
  7. Unfasten the retaining screws, then remove the IAC valve assembly from the throttle body. Clean the IAC valve O-ring sealing surface, pintle valve seat and air passage.
  8.  


WARNING
If the IAC pintle has been in service, DO NOT push or pull in the valve pintle. The force required to move the pintle may damage the the threads on the worm drive. Do not soak the valve in any liquid cleaner or solvent, as damage may result.

  1. Use a suitable carburetor cleaner (be sure it is safe to use on systems equipped with a oxygen sensor) and a parts cleaning brush to remove the carbon deposits. Do not use a cleaner that contains methyl ethyl ketone. It is an extremely strong solvent and not necessary for this type of deposits. Shiny spots on the pintle or on the seat are normal and do not indicate a misalignment or a bent pintle shaft. If the air passage has heavy deposits, remove the throttle body for a complete cleaning.
  2.  

To install:

If you're installing a new IAC valve, you must replace if with an identical part. IAC valve pintle shape and diameter are designed for the specific application.

  1. If you are installing a new IAC valve, measure the distance from the motor housing to end of the cone. If it is greater that 1 / 8 in. (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.
  2.  

The IAC valve pintle may also be retracted by using IAC/ISC MOtor Tester J-37027/BT-8256K. Do not soak the IAC valve in any liquid cleaner or solvent as damage may result.

  1. Install the new idle air control valve and tighten the retaining screws to 27 inch lbs. (3 Nm).
  2.  
  3. Attach the IAC valve electrical connections.
  4.  
  5. If removed, install the air cleaner assembly.
  6.  
  7. Connect the negative battery cable.
  8.  
  9. Reset the IAC valve as follows:
    1. Turn the ignition ON (engine OFF ), for 5 seconds.
    2.  
    3. Turn the ignition OFF for 10 seconds.
    4.  
    5. Start the engine, then check for proper idle operation.
    6.  

  10.  

 
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