GM Corsica/Beretta 1988-1996 Repair Guide

Idle Air Control (IAC) Valve

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OPERATION



See Figure 1

Engine idle speeds are controlled by the PCM through the IAC valve mounted on the throttle body. The PCM 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 counts correspond to a fully closed passage, while 140 counts or more corresponds to full flow.



Click image to see an enlarged view

Fig. Fig. 1: Cross-sectional view of the IAC valve air flow pattern

Idle speed can be categorized in 2 ways: actual (controlled) idle speed and minimum idle speed. Controlled idle speed is obtained by the PCM 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 a certain amount of air to bypass the throttle valves regardless of IAC valve pintle positioning. A combination of this air flow and IAC pintle positioning allows the PCM to control engine idle speed. During normal engine idle operation, the IAC valve pintle is positioned a calibrated number of steps (counts) from the seat. No adjustment is required during routine maintenance. Tampering with the minimum idle speed adjustment may result in premature failure of the IAC valve or improperly controlled engine idle operation.

TESTING



Except 1996 2.2L Engines and 1994-96 3.1L Engines

See Figure 2

  1. Visually check the connector, making sure it is connected properly and all of the terminals are straight, tight and free of corrosion.
  2.  
  3. Detach the IAC valve connector, then measure the resistance between the terminals A and B, and between terminals C and D:
    1. If the resistance between terminals A and B and terminals C and D measure 40-80 ohms, go to the next step.

      Click image to see an enlarged view

      Fig. Fig. 2: Idle Air Control (IAC) valve circuit - except 1996 2.2L Engines and 1994-96 3.1L Engines

    2.  
    3. If the resistance between the terminals A and B or between the terminals C and D does not meet specifications, replace the IAC valve.
    4.  

  4.  
  5. Check for continuity between the terminals A and D and terminals B and C:
    1. If there is no continuity between terminals A and D and terminals B and C, the valve is OK.
    2.  
    3. If the resistance between the terminals A and D or between terminals B and C does not meet specifications, replace the IAC valve.
    4.  

  6.  

1996 2.2L Engines and 1994-96 3.1L Engines

See Figure 3

  1. Visually check the connector, making sure it is connected properly and all of the terminals are straight, tight and free of corrosion.
  2.  
  3. Unplug the IAC connector and check resistance between the IAC terminals. Resistance between terminals A to B and terminals C to D should be 40-80 ohms. If resistance is not within specification, the IAC valve must be replaced.
  4.  



Click image to see an enlarged view

Fig. Fig. 3: Idle Air Control (IAC) valve circuit - 1996 2.2L Engines and 1994-96 3.1L Engines

  1. Check 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 valve is faulty.
  2.  
  3. If the resistance is within specification, check the IAC circuits back to the PCM for continuity.
  4.  
  5. If the valve and circuits are functional, the computer control module may be faulty.
  6.  

REMOVAL & INSTALLATION



See Figures 4, 5, 6, 7 and 8

On some models it may be necessary to remove the air inlet assembly.

  1. Disconnect the negative battery cable.
  2.  
  3. Detach the IAC valve electrical connector.
  4.  
  5. Remove the IAC valve by performing the following:
    1. On thread-mounted units, use a 1 1 / 4 in. (32mm) wrench.
    2.  
    3. On flange-mounted units, remove the mounting screw assemblies.
    4.  

  6.  



Click image to see an enlarged view

Fig. Fig. 4: Unplug the Idle Air Control (IAC) valve electrical connector



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Fig. Fig. 5: Unfasten the IAC valve-to-throttle body retaining screws



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Fig. Fig. 6: Carefully pull the IAC valve assembly away from the throttle body ...



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Fig. Fig. 7: ... then remove and discard the IAC valve O-ring and replace with a new one during installation

To install:
  1. Clean the mounting surfaces by performing the following:
    1. If servicing a thread-mounted valve, remove the old gasket material from the surface of the throttle body to ensure proper sealing of the new gasket.
    2.  
    3. If servicing a flange-mounted valve, clean the IAC valve surfaces on the throttle body to assure proper seal of the new O-ring and contact of the IAC valve flange.
    4.  

  2.  
  3. If installing a new IAC valve, measure the distance between the tip of the IAC valve pintle and the mounting flange. If the distance is greater than 1.102 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. If reinstalling the original IAC valve, do not attempt to adjust the pintle in this manner.
  4.  


WARNING
NEVER soak the IAC valve in any liquid cleaner or solvent!



Click image to see an enlarged view

Fig. Fig. 8: If installing a new IAC valve, measure the distance of the pintle extension (1)

  1. Install the IAC valve into the throttle body by performing the following:
    1. With thread-mounted valves, install with a new gasket. Using a 1 1 / 4 in. (32mm) wrench, tighten to 13 ft. lbs. (18 Nm).
    2.  
    3. With flange-mounted valves, lubricate a new O-ring with transmission fluid and install on the IAC valve. Install the IAC valve to the throttle body. Install the mounting screws using a suitable thread locking compound. Tighten to 28 inch lbs. (3.2 Nm).
    4.  

  2.  
  3. Attach the IAC valve electrical connector.
  4.  
  5. Connect the negative battery cable.
  6.  
  7. No physical adjustment of the IAC valve assembly is required after installation. Reset the IAC valve pintle position by performing the following:
    1. Depress the accelerator pedal slightly.
    2.  
    3. Start the engine and run for 5 seconds.
    4.  
    5. Turn the ignition switch to the OFF position for 10 seconds.
    6.  
    7. Restart the engine and check for proper idle operation.
    8.  

  8.  

 
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