GM Celebrity/Century/Ciera/6000 1982-1996 Repair Guide

Gasoline Engines

Print

IDLE SPEED



Carbureted Models

On non-A/C models not equipped with Idle Speed Control (ISC), the idle speed is adjusted at the idle speed screw on the carburetor. Before adjusting, check the underhood sticker for any preparations required. On A/C equipped models which do not have an ISC motor, an idle speed solenoid is used. This solenoid is adjusted at the solenoid screw. Consult the underhood specifications sticker for special instructions. If the sticker is missing, consult your dealer for specifications.

  1. All idle settings are made with the engine operating in the closed loop. The air cleaner must be removed, along with the A/C and cooling fan OFF .
  2.  
  3. Apply the parking brake and block the drive wheels.
  4.  
  5. Disconnect and plug the vacuum hoses at the EGR valve and purge hose at the canister.
  6.  
  7. For the fast idle speed adjustment:
    1. With the automatic in PARK, manual in NEUTRAL, place the adjusting screw on the highest step of the fast idle cam.
    2.  
    3. Adjust if needed, to recommended rpm in PARK or NEUTRAL.
    4.  

  8.  
  9. For the curb idle speed adjustment:
    1. Adjust the curb idle speed screw to recommended rpm in DRIVE (AT) or NEUTRAL (MT).
    2.  

  10.  
  11. For the idle speed actuator plunger adjustment:
    1. Disconnect and plug the vacuum hose at the actuator.
    2.  
    3. Connect a vacuum source (a hand vacuum pump) and apply, 6 in. Hg. vacuum minimum to the actuator. The actuator plunger must be fully extended.
    4.  
    5. Adjust if needed, recommended rpm in DRIVE (AT) or NEUTRAL (MT).
    6.  

  12.  
  13. Unplug and reconnect the all the vacuum hoses: EGR valve, canister purge, and the idle speed actuator.
  14.  

Fuel Injected Models
THROTTLE BODY

No idle speed or mixture adjustments are possible on fuel injected engines unless the following:

This procedure should be performed only when the throttle body parts have been replaced. Newer models may not provide for this adjustment as the computer is responsible for idle control.

The following procedure requires the use of a special tool.

  1. Remove the air cleaner and gasket.
  2.  
  3. Plug the vacuum port on the TBI marked THERMAC.
  4.  
  5. If the car is equipped with a tamper resistant plug cover the throttle stop screw, the TBI unit must be removed as instructed above, to remove the plug.
  6.  
  7. Remove the throttle valve cable from the throttle control bracket to allow access to the throttle stop screw.
  8.  
  9. Connect a tachometer to the engine.
  10.  
  11. Start the engine and run it to normal operating temperature.
  12.  
  13. Install tool J-33047 into the idle air passage of the throttle body. Be sure that the tool is fully seated and that no air leaks exist.
  14.  
  15. Using a #20 Torx bit (0.16 in.-4mm), turn the minimum air screw until the engine speed is 675-725 rpm for automatic transaxle models in PARK, or 725-825 rpm for manual transaxle models in NEUTRAL.
  16.  
  17. Stop the engine and remove the special tool.
  18.  
  19. Install the cable on the throttle body.
  20.  
  21. Use RTV sealant to cover the throttle stop screw.
  22.  

MULTI-PORT/SEQUENTIAL INJECTION

The engine should be at normal operating temperature before making this adjustment. Newer engines do not provide for this adjustment as the idle is completely computer controlled. If the computer cannot control the idle, the most likely cause a defective idle air control valve. Test the IAC valve, check the engine for mechanical problems (for basic tune-up, vacuum leaks, proper compression) and check the rest of the fuel control system. The least likely cause is the computer itself, unless someone has recently worked on the computer or the electrical system. For models that are adjustable following the following procedures:

  1. Using an awl, pierce the idle stop screw plug (located on the side of the throttle body) and remove it by prying it from the housing.
  2.  
  3. Using a jumper wire, ground the diagnostic lead of the IAC motor.
  4.  
  5. Turn on the ignition, DO NOT start the engine and wait for 30 seconds, then disconnect the IAC electrical connector. Remove the diagnostic lead ground lead and start the engine. Allow the system to go to closed loop.
  6.  
  7. Adjust the idle set screw to 550 rpm for the automatic transaxle in Drive, and 650 rpm for the manual transaxle in Neutral.
  8.  
  9. Turn the ignition OFF and reconnect the IAC motor lead.
  10.  
  11. Using a voltmeter, adjust the TPS to 0.45-0.65 volt and secure the TPS.
  12.  
  13. Recheck the setting, then start the engine and check for proper idle operation.
  14.  
  15. Seal the idle stop screw with silicone sealer.
  16.  

MIXTURE ADJUSTMENT



Carbureted Models

See Figure 1

Mixture adjustments are a function of the Computer Command Control (CCC) system. The idle speed on models equipped with an Idle Speed Control (ISC) motor is also automatically adjusted by the Computer Command Control System, making manual adjustment unnecessary. The underhood specifications sticker will indicate ISC motor use. We strongly recommend that mixture adjustments be referred to a qualified, professional technician.



Click image to see an enlarged view

Fig. Fig. 1: Common label for a carbureted engine

The idle mixture screws are concealed under hardened plugs and mixture adjustments are not normally required. Since carburetor removal is necessary in order to gain access to the screws, the plug removal and adjustment procedures are covered in Fuel System .

  1. Preform the carburetor pre-set procedure.See Fuel System .
  2.  
  3. Run the engine on high step of the fast idle cam. until the engine cooling fan starts to cycle.
  4.  
  5. Run the engine at 3000 rpm and adjust the lean mixture screw slowly in small increments allowing time for dwell to stabilize after turning the screw to obtain an average dwell of 35°. If the dwell is too low, back the screw out, if too high, turn it in. If you are unable to adjust it to specifications, inspect the main metering circuit for leaks or restrictions.
  6.  
  7. Return to idle.
  8.  
  9. Adjust the mixture screw to obtain the average dwell of 25° with the cooling fan in the off cycle. If the reading is too low, back the screw out. If the reading is too high, turn it in. Allow time for reading to stabilize after each adjustment. Adjustment is very sensitive. Make a final check with the adjusting tool removed.
  10.  
  11. If you are unable to adjust to specifications, inspect the idle system for leaks, or restrictions.
  12.  
  13. Disconnect the mixture control solenoid when the cooling fan is in the off cycle, then check for an rpm change of at least 50 rpm. If the rpm does not change enough, inspect the idle air bleed circuit for restrictions or leaks.
  14.  
  15. Run the engine at 3000 rpm for a few moments, then note the dwell reading. Dwell should be varying with the average reading of 35°.
  16.  
  17. If the 35° is not the average dwell, reset the lean mixture screw pre step 3. Then reset the idle mixture screw to obtain a 25° dwell per steps 5 and 6.
  18.  
  19. If you have an average dwell, reconnect the systems disconnected (purge and vent hoses, EGR valve etc..), then reinstall the vent screen and set the idle speed.
  20.  

Fuel Injected Models

See Figure 2

No mixture adjustments are possible on fuel injected engines.



Click image to see an enlarged view

Fig. Fig. 2: Common label for a fuel injected engine

 
label.common.footer.alt.autozoneLogo