GM Buick/Oldsmobile/Pontiac/Full-Size 1975-1990 Repair Guide

Curb Idle

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ADJUSTMENTS



See Figures 1, 2, 3, 4, 5, 6 and 7

Rochester 2GC, GE, GV
  1. With the engine at normal operating temperature, remove the air cleaner and disconnect the air cleaner vacuum hose at the intake manifold. Plug the fitting with a clean rag.
  2.  
  3. Make sure the choke plate is open and the air conditioning is off.
  4.  
  5. Set the parking brake and block the drive wheels.
  6.  
  7. Disconnect the evaporative emission hose from the air cleaner.
  8.  
  9. Disconnect the hose from the E.G.R. valve. Plug the hose to the carburetor with a golf tee or pencil.
  10.  
  11. Connect a tachometer to the engine.
  12.  
  13. Check, and if necessary, adjust the timing.
  14.  
  15. On cars without A/C, open the throttle momentarily to extend the solenoid plunger. Turn the solenoid screw to adjust the speed to the curb idle rpm listed on the underhood sticker. Turn the idle speed screw to the specified rpm.
  16.  
  17. On cars with A/C, turn the idle speed screw to obtain the specified rpm. Disconnect the A/C compressor wire and turn the A/C on. Open the throttle momentarily to extend the solenoid plunger. Turn the solenoid screw to adjust the speed to the curb idle rpm listed on the underhood sticker.
  18.  
  19. Reconnect all hoses and wires. Disconnect the tachometer.
  20.  



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Fig. Fig. 1: 2GC curb idle adjustment, A/C equipped

1975-76 Rochester M4MC
  1. Run the engine to normal operating temperature. Remove the air cleaner and disconnect the air cleaner vacuum hose at the intake manifold. Plug the fitting with a clean rag.
  2.  
  3. Make sure the choke plate is open and the air conditioning turned off.
  4.  
  5. Set the parking brake and block the rear wheels. Connect a tachometer to the engine.
  6.  
  7. Disconnect and plug the carburetor hoses from the vapor canister and the E.G.R. valve. Plug the hoses with golf tees or pencils.
  8.  
  9. With the timing adjusted properly, set the slow idle screw to obtain 550 rpm (non-California 350 and 455 cu. in V8s) or 600 rpm (Calif.) in DRIVE . Adjust 400 V8s to 650 rpm in DRIVE . If the sticker specs vary from these, adjust to sticker specifications.
  10.  


CAUTION
When adjusting idle speeds with the vehicle in DRIVE, always have a helper in the vehicle with his or her foot on the brake and the parking brake fully applied.

  1. Adjust the idle speed-up solenoid on 350 and 455 V8s with air conditioning off to 650 rpm. The air conditioning compressor wires must be disconnected at the air conditioning compressor, and the transmission must be in DRIVE .
  2.  
  3. Reconnect the compressor wires and reconnect all hoses.
  4.  



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Fig. Fig. 2: Idle speed adjustment without solenoid

All Other Models To 1982
WITHOUT A/C
  1. On non-air conditioned vehicles equipped with the idle solenoid, open the throttle (with solenoid energized) to allow the plunger to extend.
  2.  
  3. Adjust the solenoid screw to obtain the specified solenoid energized idle rpm.
  4.  
  5. Disconnect the electrical lead attached to the solenoid. With the solenoid de-energized, adjust the idle speed screw to obtain the specified curb idle rpm.
  6.  



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Fig. Fig. 3: Idle speed adjustment with solenoid

WITH A/C
  1. Adjust the idle speed screw until the engine is turning the specified curb idle rpm.
  2.  
  3. Disconnect the air conditioning compressor clutch lead at the compressor.
  4.  
  5. Turn on the air conditioning to energize the idle solenoid. Place the transmission in DRIVE , making sure the parking brake is firmly applied and that a helper is in the vehicle with his or her foot on the brake. Open the throttle slightly to allow the solenoid plunger to fully extend.
  6.  


CAUTION
When adjusting idle speeds with the vehicle in DRIVE, always have a helper in the vehicle with his or her foot on the brake and the parking brake fully applied.

  1. Adjust the idle solenoid screw to obtain the specified solenoid energized rpm.
  2.  
  3. Reconnect the air conditioning compressor clutch lead and disconnect the tachometer.
  4.  

Idle Load Compensator Adjustment

To accomplish this adjustment, you'll need not only a tachometer, but a special adjusting tool J-29607, BT-8022 or equivalent, a3/32in. hex key, and a spare rubber cap, drilled to accept this key.

  1. Prepare the vehicle for adjustment by following instructions on the engine compartment sticker.
  2.  
  3. Connect a tachometer to the distributor side of the TACH filter or other connector. Remove the air cleaner. Disconnect and plug vacuum hoses going to: the Thermal Vacuum Valve, EGR valve, the canister purge port, and the ILC (Idle Load Compensator).
  4.  
  5. Back out the idle stop screw on the carburetor three turns. Make sure air conditioning is OFF. Set the parking brake, put the transmission in PARK , and block the drive wheels.
  6.  
  7. Start the engine. If it is not hot already, idle it until it is warm (water flowing through the radiator). Hold the jam nut with a wrench while you turn the plunger as necessary to obtain 725 rpm. Make sure to hold the nut securely as, if it turns, it could damage guide tabs.
  8.  
  9. Now, remove the plug and reconnect the ILC vacuum hose to the ILC port. Read the idle speed on the tach. It should be 500 rpm in DRIVE . Have someone apply the brake pedal and put the transmission in Drive before taking your reading. If the reading meets specification, proceed to Step 11. Otherwise, proceed with the steps below.
  10.  


CAUTION
When adjusting idle speeds with the vehicle in DRIVE, always have a helper in the vehicle with his or her foot on the brake and the parking brake fully applied.

  1. Stop the engine, remove the ILC, and then plug the vacuum hose leading to it with a rubber plug.
  2.  
  3. Remove the rubber cap from the center outlet tube of the ILC and, if there is one, remove the metal plug from the tube. Install the ILC back onto the carburetor and re-attach any items that were detached, such as the throttle return spring. Remove the plug from the vacuum hose and reconnect the hose to the ILC.
  4.  
  5. Install the rubber cap through which you have drilled a hole with the hex wrench passing through the hole in the cap. Engage the wrench with the adjusting screw inside the ILC.
  6.  
  7. Have a helper start the engine with the brake applied and put the transmission in DRIVE . Then, adjust the wrench until the rpm is 500. Turn the screw counterclockwise to increase speed and clockwise to decrease it. One turn equals about 75-100 rpm. When 500 rpm is attained, remove the wrench and drilled cap. Replace the complete rubber cap.
  8.  
  9. If the rpm is not correct, you'll have to repeat the adjustment, allowing for the amount and direction of the discrepancy. For example, if the rpm is 450, turn the screw one half turn counterclockwise. The adjustment is not correct until the engine runs at 500 rpm with the complete rubber cap installed.
  10.  
  11. With the engine running and transmission in DRIVE , as before, measure the distance from the jam nut to the tip of the plunger (Dimension A). It must not exceed 1 in. (25.4mm).
  12.  
  13. Disconnect and plug the vacuum hose again. Apply a vacuum source such as a vacuum pump to the port to fully retract the plunger. Now, adjust the idle stop screw on the carburetor float bowl to give 500 rpm in DRIVE . Then, put the transmission in PARK , and turn the engine off.
  14.  
  15. Remove vacuum hose plugs and reconnect all hoses. Reinstall the air cleaner.
  16.  



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Fig. Fig. 4: Adjusting Idle Load Compensator on 1983-1990 models

I.S.C. Adjustment

Used on the E4ME and E2ME carburetors, this device controls idle speed electronically via the Electronic Control Module. It is factory adjusted and does not require adjustment as a matter of routine maintenance. If diagnostic work on the vehicle indicates that idle speed is not to specification, it may be adjusted. A special tool J-29607, BT-8022 or equivalent is required to make the adjustment. The plunger has an unusual head to discourage tampering. You'll also need separate tach and dwell meters.

  1. Look at the unthreaded portion of the adjustable plunger, just below the head, for a letter code. If there is a letter, note what it is for later use, and then go on to the next step. Except on the 3.8L V6 in 1986-87, if there is no letter, use the special wrench to remove the plunger by unscrewing it. Then, measure the length (Dimension A) from the threaded end to the inner surface of the head. Note and record this dimension for later use. On the 3.8L V6 in 1986-87, use the second line of the chart, showing dimensions of 41 / 64 in. (16mm) and 5 / 16 in. (8mm).
  2.  



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Fig. Fig. 5: ISC plunger identification



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Fig. Fig. 6: Measuring ISC plunger length

  1. Prepare the engine for adjustments as detailed on the engine compartment sticker. Connect a tachometer, using the distributor side of the tach filter, if one is used.
  2.  
  3. Connect a dwell meter to the mixture control solenoid (M/C) dwell lead. Set the dwell meter (or read the meter) on the six cylinder scale regardless of the type of engine.
  4.  
  5. Turn the A/C off.
  6.  
  7. Start the engine and run it until the ECM system enters closed loop operation. At this point the dwell meter reading will begin to vary, indicating that the oxygen sensor is regulating fuel flow through a solenoid in the carburetor.
  8.  
  9. Now, turn the ignition switch off and unplug the connector going to the ISC motor.
  10.  
  11. Ground the D terminal of the motor connection (see illustration) with a jumper wire. Then, connect another jumper wire from the battery positive terminal to the C connection while simultaneously applying pressure with your finger to the plunger to help it retract. If the plunger is not assisted in this way, the internals of the ISC may be damaged. Also, as soon as the plunger is retracted, remove the 12 volt jumper to prevent damage to the ISC. Make sure you make the right connections. If you were to connect across terminals A and B , this also would cause ISC damage.
  12.  



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Fig. Fig. 7: Connections to retract and extend the plunger

  1. Block drive wheels and apply the parking brake. Start the engine and run it until the dwell meter reading varies. Then, put the transmission in DRIVE . Make sure the carburetor is not on the fast idle cam.
  2.  


CAUTION
When adjusting idle speeds with the vehicle in DRIVE, always have a helper in the vehicle with his or her foot on the brake and the parking brake fully applied.

  1. Now, note the rpm on the tach and adjust the carburetor slow idle stop screw (minimum authority) to the lower figure shown in the tune-up chart. Then put it back in PARK .
  2.  
  3. Now, ground terminal C again, and jumper the 12 volt source to terminal D to fully extend the plunger. Leave the power connected only long enough to extend the plunger, or the ISC may be damaged. Again, make sure you are making the right connection.
  4.  

With the transmission in PARK , use the special tool to preset the plunger for 1,500 rpm. Then, set the parking brake and block the drive wheels and put the transmission in DRIVE , have a helper in the car with the parking brake applied and his or her foot on the brake. Then, use the special tool to turn the plunger to get the higher figure (Maximum Authority) shown in the tune-up chart.

Now reapply power as described in Step 10 to make sure the plunger is fully extended. Recheck the Maximum Authority rpm to make sure it is still correct. Readjust if necessary.

  1. Now measure dimension B , as shown in the illustration. This is the distance from the back side of the plunger head to the front surface of the nose piece-the portion of the assembly the plunger fits into. This is the dimension you determined in Step 1. If the dimension should be too great, adjust the plunger with the special tool until it is within the specified limit.
  2.  
  3. Fully retract the plunger by jumping to connections as described in Step 7. Put the transmission in PARK . Turn the ignition switch off. Remove all instruments and jumper wires and reconnect the ISC motor connector. On engines using the E2ME 2-barrel carburetor, an INTERMITTENT trouble code will be sent. To clear this, you must remove battery voltage from the ECM (Electronic Control Module). With the ignition off, pull the fuse in the fuse box which is labeled ECM. The power must be interrupted for 10 seconds and MUST ONLY be interrupted with the ignition switch OFF .
  4.  

Throttle Body Fuel Injection

This is a simple procedure, but it is never performed in routine maintenance. It is required after throttle body replacement, only. See Fuel System for applicable procedures.

 
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