GM Corsica/Beretta 1988-1996 Repair Guide

Idle Speed and Mixture Adjustments

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MINIMUM IDLE SPEED ADJUSTMENT



Various engine sensors send signals to the ECM. The ECM then sends signals to the (IAC) Idle Air Control Valve and the (TPS) Throttle Position Sensor to control the idle speed. The throttle stop screw, used for regulating minimum idle speed, is adjusted at the factory. The screw is covered with a plug to discourage unauthorized adjustments. The only time it may necessary to adjust the screw is when the throttle body is being replaced. The engine should be at normal operating temperature before making adjustments. This procedure is not necessary on 1992 vehicles equipped with the 2.2L engine.

2.0L Engine
1988 VEHICLES
  1. Remove the TBI bonnet and gasket.
  2.  
  3. Pierce the idle stop screw plug with an awl and apply leverage to remove it.
  4.  
  5. Connect a tachometer to the engine.
  6.  
  7. Set the parking brake and block the drive wheels.
  8.  
  9. Start the engine, put the transmission in park (neutral on manual transmission) and allow the engine rpm to stabilize.
  10.  
  11. Disconnect the idle air control connector.
  12.  
  13. Install tool J-36377, or equivalent in the idle air passage of the TBI unit. Verify that no air leaks exist around tool J-36377.
  14.  
  15. On vehicles with automatic transmission, place the selector lever in D (drive) before making a adjustment.
  16.  
  17. Adjust the idle stop screw to obtain 650 +- 25 rpm.
  18.  
  19. On vehicles with automatic transmission, place the selector lever in P (park). Turn the ignition OFF , remove the IAC passage plug tool and reconnect IAC valve electrical connector.
  20.  
  21. Use silicone sealant, or equivalent, to cover the minimum air adjustment screw.
  22.  
  23. Install the TBI bonnet. Replace the gasket if necessary.
  24.  

1989 VEHICLES
  1. Block the drive wheels and apply the parking brake. Run the engine to normal operating temperature.
  2.  
  3. Remove the air cleaner assembly or air duct.
  4.  
  5. Remove the hoses and plug any possible air sources on the tube manifold assembly.
  6.  
  7. Disconnect the throttle cable.
  8.  
  9. Ground the diagnostic test terminal in the ALDL connector.
  10.  
  11. Turn the ignition ON , leaving the engine OFF . Wait at least 30 seconds to allow the IAC pintle to seat in the throttle body.
  12.  
  13. With the ignition ON , the engine stopped, test terminal still grounded, disconnect the IAC valve electrical terminal.
  14.  
  15. Connect a tachometer to the engine or use a scan tool, if available, to monitor the engine speed.
  16.  
  17. Remove the ground from the diagnostic test terminal, place the transmission in PARK or NEUTRAL and start the engine. It may be necessary to depress the accelerator in order to start the engine. Allow the engine speed to stabilize.
  18.  

Before proceeding, make sure all the accessories are turned OFF and the engine cooling fan is not in operation.

  1. If the idle speed is not 450-550 rpm, adjust as necessary.
  2.  
  3. Install the throttle cable and make sure the idle speed is not affected.
  4.  
  5. Turn the ignition OFF and reconnect the IAC valve electrical connection.
  6.  
  7. Unplug and reconnect any vacuum ports which were plugged.
  8.  
  9. Install the air cleaner assembly or air duct.
  10.  

2.2L Engine
1990-91 VEHICLES

This procedure requires the use of a scan tool to plug into the ALDL terminal.

  1. Block the drive wheels and apply the parking brake. Run the engine to normal operating temperature.
  2.  
  3. Connect a scan tool to the ALDL connector.
  4.  
  5. Select the "Field Service Mode" on the scan tool. This will cause the IAC valve pintle to seat in the throttle body, closing the air passage. Wait at least 45 seconds, disconnect the IAC valve connector then exit the "Field Service Mode".
  6.  
  7. Place the transmission in PARK or NEUTRAL and start the engine. It may be necessary to depress the accelerator in order to start the engine. Allow the engine speed to stabilize.
  8.  

Before proceeding, make sure all the accessories are turned OFF and the engine cooling fan is not in operation.

  1. If the idle speed is not 450-550 rpm, adjust as necessary.
  2.  
  3. Turn the ignition OFF and reconnect the IAC valve electrical connection.
  4.  
  5. Reset the IAC valve pintle position as follows:
    1. Select "Engine rpm" on the scan tool.
    2.  
    3. Start the engine and hold the speed above 2000 rpm.
    4.  
    5. Select the "Field Service Mode" on the scan tool, for 10 seconds. This will reset the IAC pintle position.
    6.  
    7. Exit the "Field Service Mode", and allow the engine to return to idle.
    8.  
    9. Turn the ignition OFF , then restart and check for proper operation.
    10.  
    11. Unblock the drive wheels.
    12.  

  6.  

2.8L Engine
1988 VEHICLES

Various engine sensors send signals to the ECM. The ECM then sends signals to the (IAC) Idle Air Control Valve and the (TPS) Throttle Position Sensor to control the idle speed. The throttle stop screw, used for regulating minimum idle speed, is adjusted at the factory. The screw is covered with a plug to discourage unauthorized adjustments. Adjustment should not be necessary, however if it is necessary to gain access to the idle stop screw assembly use the following procedure:

Foreign material accumulating in the throttle body bore, on the throttle valve or on the throttle shaft, may result in a minimum idle speed that is not at specification. Clean the throttle body assembly as required and check the throttle assembly for smooth operation through the entire range of travel. Correct any problems found. Then, if necessary, adjust the minimum idle speed.

The engine should be at normal operating temperature before making adjustments.

  1. With the IAC valve connected, ground the diagnostic TEST terminal.
  2.  
  3. Turn ON the ignition, but do not start the engine. Wait at least 30 seconds.
  4.  
  5. With the ignition ON , disconnect the IAC electrical connector.
  6.  
  7. Remove the ground from the diagnostic lead and start the engine. Allow the system to go to closed loop.
  8.  
  9. Adjust the idle stop screw to:

    Automatic transmission - 550 rpm in drive
     
    Manual transmission - 650 rpm in neutral
     

  10.  
  11. Turn the ignition OFF and reconnect the connector at the IAC valve.
  12.  
  13. Start the engine and check the idle operation.
  14.  

1989 VEHICLES

This procedure requires the use of a scan tool to plug into the ALDL terminal.

  1. Back the throttle stop screw out until an air gap between the screw and the throttle lever can be seen by looking downward from above the throttle body.
  2.  
  3. Turn the stop screw until it just contacts the throttle lever.
  4.  
  5. Turn the stop screw in 1 1 / 2 turns further.
  6.  
  7. Place the transmission in PARK or NEUTRAL .
  8.  
  9. Block the drive wheel and apply the parking brake.
  10.  
  11. Connect the scan tool to the ALDL connector.
  12.  
  13. Start the engine and allow it to reach normal operating temperature and "Closed Loop". Run the engine until the IAC has relearned its controlled idle position.
  14.  
  15. Using the scan tool, observe the idle air control (IAC) valve counts. With the engine stabilized, the IAC valve should be 10-15 counts.
  16.  

The IAC valve counts should be check with all accessories and cooling fan OFF.

  1. If the IAC counts are out of the acceptable range, the adjustment may be tailored as follows:
    1. For more throttle valve opening, lower the IAC valve counts.
    2.  
    3. For less throttle valve opening, higher the IAC valve counts.
    4.  
    5. If it is necessary to change the adjustment more than one half turn either way, check for other causes of incorrect idle speed.
    6.  

  2.  

3.1L Engine
1990-92 VEHICLES

This procedure requires the use of a scan tool to plug into the ALDL terminal.

  1. Back the throttle stop screw out until an air gap between the screw and the throttle lever can be seen by looking downward from above the throttle body.
  2.  
  3. Turn the stop screw until it just contacts the throttle lever.
  4.  
  5. Turn the stop screw in 1 1 / 2 turns further.
  6.  

The ECM will need to relearn the IAC pintle position following battery reconnect. Refer to the Idle Learn Procedure for inputting this information immediately.

  1. Place the transmission in PARK or NEUTRAL .
  2.  
  3. Block the drive wheel and apply the parking brake.
  4.  
  5. Connect the scan tool to the ALDL connector.
  6.  
  7. Start the engine and allow it to reach normal operating temperature and "Closed Loop". Run the engine until the IAC has relearned its controlled idle position.
  8.  
  9. Using the scan tool, observe the idle air control (IAC) valve counts. With the engine stabilized, the IAC valve should be 10-20 counts.
  10.  

The IAC valve counts should be check with all accessories and cooling fan OFF.

  1. If the IAC counts are out of the acceptable range, repeat adjustment.
  2.  

IDLE LEARN PROCEDURE
3.1L ENGINE

Any time the battery is disconnected, the programmed position of the IAC valve pintle is lost, and replaced with a default value until the vehicle is driven and has time to relearn the correct IAC pintle position. To return the IAC valve pintle to the correct position immediately, perform the following:

  1. Reconnect the battery.
  2.  
  3. Install a Tech 1® scan tool.
  4.  
  5. Select IAC SYSTEM , then IDLE LEARN in the MISC TEST mode.
  6.  
  7. Proceed with the idle learn as directed by the scan tool.
  8.  

 
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