GM Caprice 1990-1993 Repair Guide

Carbureted Engines

Print

Idle speed and mixture settings are factory set and sealed; adjustment of the idle mixture requires special tools including an exhaust gas analyzer and should not be attempted by the do-it-yourselfer, but only by an authorized GM dealer.

A cover is in place over the idle air bleed valve, and the access holes to the idle mixture needles are sealed with hardened plugs to prevent the factory settings from being tampered with. These items are NOT to be removed unless required for cleaning, part replacement, improper dwell readings or if the System Performance Check indicates the carburetor is the cause of the trouble.

5.0L Engine
MIXTURE PLUG REMOVAL

See Figure 1



Click image to see an enlarged view

Fig. Fig. 1: Idle mixture needle plug removal

  1. Remove the carburetor from the engine, following normal service procedures to gain access to the plugs covering the idle mixture needles.
  2.  
  3. Invert carburetor and drain fuel into a suitable container.
  4.  


CAUTION
Take precautions to avoid the risk of fire.

  1. Place the carburetor on a suitable holding fixture, with the intake manifold side up. Use care to avoid damaging linkage, tubes and parts protruding from air horn.
  2.  
  3. Make 2 parallel cuts in the throttle body, 1 on each side of the locator points beneath the idle mixture needle plug (manifold side), with a hacksaw.
  4.  

The cuts should reach down to the steel plug, but should not extend more than1/8inch beyond the locator points. The distance between the saw cuts depends on the size of the punch to be used.

  1. Place a flat punch near the ends of the saw marks in the throttle body. Hold the punch at a 45 degree angle and drive it into the throttle body until the casting breaks away, exposing the steel plug. The hardened plug will break, rather than remaining intact. It is not necessary to remove the plug in a whole piece, but do remove the loose pieces.
  2.  
  3. If equipped with a 4 barrel carburetor, repeat the procedure for the remaining mixture needle plug.
  4.  
  5. Reinstall the carburetor using a new gasket.
  6.  

PRE-ADJUSTMENT PROCEDURE

See Figure 2



Click image to see an enlarged view

Fig. Fig. 2: Dwell meter connection

  1. Block the drive wheels, set the parking brake and place the transmission in P for vehicles equipped with an automatic transmission or neutral for manual transmissions.
  2.  
  3. Remove the idle mixture needle plugs and turn the idle mixture screw in until lightly seated, then back out approximately 3 3/8 turns using tool J-29030 or equivalent.
  4.  
  5. Reinstall the carburetor, if previously removed.
  6.  
  7. Do not install the air cleaner.
  8.  
  9. Disconnect and plug the hoses as directed on the Emission Control Information Label under the hood.
  10.  
  11. Check the ignition timing as shown on the Emission Control Information Label.
  12.  
  13. Connect the positive lead of a dwell meter to the mixture control solenoid test lead (green connector generally located on right fenderwell) and connect the other lead to ground. Set the dwell meter to the 6-cylinder position.
  14.  

MIXTURE ADJUSTMENT PROCEDURE

See Figures 3 and 4



Click image to see an enlarged view

Fig. Fig. 3: Idle air bleed valve identification letter location - 5.0L (VIN Y) Engine



Click image to see an enlarged view

Fig. Fig. 4: Adjusting idle air bleed - 5.0L (VIN Y) Engine

  1. Start the engine and allow it run until thoroughly warm and dwell begins to vary.
  2.  
  3. Check idle speed and compare to the specifications on the underhood Emission Control Information Label. If necessary adjust the curb idle speed.
  4.  
  5. With the engine idling in D for automatic transmissions or neutral for manual transmissions, observe the dwell reading on the 6 cylinder scale.
  6.  
  7. If the reading is varying within the 10-50 degree range, the adjustment is correct.
  8.  
  9. If the reading is not as specified remove the idle air bleed valve cover. Refer to Fuel System, Fuel System . With cover removed, look for presence (or absence) of a letter identification on top of idle air bleed valve.
  10.  

A missing cover indicates that the idle air bleed valve setting has been changed from its original factory setting.

  1. If no identifying letter appears on top of the valve, begin with Procedure A, below. If the valve is identified with a letter, begin Procedure B.
  2.  

Procedure A (No Letter On Idle Air Bleed Valve)

Presetting the idle air bleed valve to a gauge dimension is necessary only if the valve was serviced prior to on-vehicle adjustment.

  1. Install idle air bleed valve gauging Tool J-33815-2, BT-8253-B, or equivalent, in throttle side D-shaped vent hole in the air horn casting. The upper end of the tool should be positioned over the open cavity next to the idle air bleed valve.
  2.  
  3. While holding the gauging tool down lightly, so that the solenoid plunger is against the solenoid stop, adjust the idle air bleed valve so that the gauging tool will pivot over and just contact the top of the valve. The valve is now preset for on-vehicle adjustment. Remove the gauging tool and proceed as follows:
    1. Disconnect the vacuum hose from the canister purge valve and plug it.
    2.  
    3. Start engine and allow it to reach normal operating temperature.
    4.  
    5. While idling in D for automatic transmission or neutral for manual transmission, use a screwdriver to slowly turn the valve counterclockwise or clockwise, until the dwell reading varies within the 25-35° range, attempting to be as close to 30° as possible.
    6.  
    7. If the reading is not as specified, the idle mixture needles will have to be adjusted.
    8.  
    9. If the reading is within specifications, reconnect all hoses previously removed and install air cleaner.
    10.  

  4.  

Perform this step carefully. The air bleed valve is very sensitive and should be turned in1/8turn increments only.

  1. If unable to set dwell to 25-35°, and the dwell is below 25°, turn both mixture needles counterclockwise an additional turn. If dwell is above 35°, turn both mixture needles clockwise an additional turn. Readjust idle air bleed valve to obtain dwell limits.
  2.  
  3. After adjustments are complete, seal the idle mixture needle openings in the throttle body, using silicone sealant, RTV rubber, or equivalent. The sealer is required to discourage unnecessary adjustment of the setting, and to prevent fuel vapor loss in that area.
  4.  
  5. On vehicles without a carburetor-mounted Idle Speed Control or Idle Load Compensator, adjust curb idle speed if necessary.
  6.  
  7. Check, and only if necessary adjust, fast idle speed as described on Vehicle Emission Control Information label.
  8.  

Procedure B (Letter Appears On Idle Air Bleed Valve)
  1. Install idle air bleed valve gauging Tool J-33815-2, BT-8253-B, or equivalent, in throttle side D-shaped vent hole in the air horn casting. The upper end of the tool should be positioned over the open cavity next to the idle air bleed valve.
  2.  
  3. While holding the gauging tool down lightly, so that the solenoid plunger is against the solenoid stop, adjust the idle air bleed valve so that the gauging tool will pivot over and just contact the top of the valve.
  4.  
  5. The valve is now set properly. No further adjustment of the valve is necessary. Remove gauging tool.
  6.  
  7. Disconnect vacuum hose to canister purge valve and plug it.
  8.  
  9. Start engine and allow it to reach normal operating temperature.
  10.  
  11. While idling in D for automatic transmissions or neutral for manual transmission, adjust both mixture needles equally, 1 / 8 turn increments, until dwell reading varies within the 25-35 degree range, attempting to be as close to 30 degrees as possible.
  12.  
  13. If reading is too low, turn mixture needles counterclockwise. If reading is too high, turn mixture needles clockwise. Allow time for dwell reading to stabilize after each adjustment.
  14.  

After adjustments are complete, seal the idle mixture needle openings in the throttle body, using silicone sealant, RTV rubber, or equivalent. The sealer is required to discourage unnecessary readjustment of the setting, and to prevent fuel vapor loss in that area.

  1. On vehicles without a carburetor-mounted Idle Speed Control or Idle Load Compensator, adjust curb idle speed if necessary.
  2.  
  3. Check, and if necessary, adjust fast idle speed, as described on the Vehicle Emission Control Information label.
  4.  

IDLE LOAD COMPENSATOR (ILC) ADJUSTMENT

See Figure 5



Click image to see an enlarged view

Fig. Fig. 5: Idle Load Compensator (ILC) adjustment - 5.0L (VIN Y) Engine

  1. Prepare the vehicle for adjustments-see emission label.
  2.  
  3. Connect a tachometer (distributor side of TACH filter, if used).
  4.  
  5. Remove the air cleaner and plug vacuum hose to thermal vacuum valve (TVV).
  6.  
  7. Disconnect and plug the vacuum hose to EGR.
  8.  
  9. Disconnect and plug the vacuum hose to canister purge port.
  10.  
  11. Disconnect and plug the vacuum hose to ILC.
  12.  
  13. Back out the idle stop screw on the carburetor 3 turns.
  14.  
  15. Turn the air conditioning OFF .
  16.  


CAUTION
Before starting engine, place the transmission in the Park position, set parking brake and block drive wheels.

  1. With the engine running (engine warm, choke off), transmission in D position and ILC plunger fully extended (no vacuum applied), using tool J-29607, BT-8022, or equivalent, adjust plunger to obtain 650-750 rpm. Locknut on plunger must be held with wrench to prevent damage to guide tabs.
  2.  
  3. Remove plug from vacuum hose, reconnect hose to ILC and observe idle speed. Idle speed should be 425-475 rpm in D position.
  4.  
  5. If rpm in Step 10 is correct, proceed to Step 13. No further adjustment of the ILC is necessary.
  6.  
  7. If rpm in Step 10 is not correct:
    1. Stop engine and remove the ILC. Plug vacuum hose to ILC.
    2.  
    3. With the ILC removed, remove the rubber cap from the center outlet tube and remove the metal plug (if used) from this same tube.
    4.  
    5. Install ILC on carburetor and re-attach throttle return spring and any other related parts removed during disassembly. Remove plug from vacuum hose and reconnect hose to ILC.
    6.  
    7. Using a spare rubber cap with hole punched to accept a 0.090 in. (3/32 inch) hex key wrench, install cap on center outlet tube (to seal against vacuum loss) and insert wrench through cap to engage adjusting screw inside tube. Start engine and turn adjusting screw with wrench to obtain 550 rpm in D position. Turning the adjusting screw will change the idle speed approximately 75-100 rpm for each complete turn. Turning the screw counterclockwise will increase the engine speed.
    8.  
    9. Remove wrench and cap (with hole) from center outlet tube and install new rubber cap.
    10.  
    11. Engine running, transmission in D position, observe idle speed. If a final adjustment is required, it will be necessary to repeat Steps 12a through 12e.
    12.  

  8.  
  9. After adjustment of the ILC plunger, measure distance from the lock nut to tip of the plunger, dimension must not exceed 1 in. (25mm).
  10.  
  11. Disconnect and plug vacuum hose to ILC. Apply vacuum source such as hand vacuum pump J-23768, BT-7517 or equivalent to ILC vacuum inlet tube to fully retract the plunger.
  12.  
  13. Adjust the idle stop on the carburetor float bowl to obtain 500 rpm in D position.
  14.  
  15. Place transmission in P position and stop engine.
  16.  
  17. Remove plug from vacuum hose and install hose on ILC vacuum inlet tube.
  18.  
  19. Remove plugs and reconnect all vacuum hoses.
  20.  
  21. Install air cleaner and gasket.
  22.  
  23. Remove block from drive wheels.
  24.  

Differential Vacuum Delay Valve (DVDV) Adjustment

See Figure 6



Click image to see an enlarged view

Fig. Fig. 6: Differential Vacuum Delay Valve (DVDV) port identification-5.0L engine

The DVDV is located in the vacuum line between the Idle Load Compensator (ILC) and the vacuum source. It is used on all 5.0L engines (engine code Y).

The DVDV acts as cushioning device by slightly delaying the operation of the ILC until a constant vacuum change has occurred. Without the DVDV the ILC would react too quickly to changes in engine vacuum, causing a stalling or run-on condition.

To check the operation of the DVDV, install a vacuum gauge with a tee between the hose from the DVDV to the ILC. Install a vacuum pump to port 1 of the DVDV and apply 17.8 in. Hg while watching the other vacuum gauge, it should take 6-9 seconds for the vacuum to rise to 16.9 in. Hg Remove the vacuum gauge with tee, install the vacuum pump to port 2 and leave port 1 open. Air should flow through the valve after 0.5 in. Hg is applied.

 
label.common.footer.alt.autozoneLogo