Engine - Vibration At Low Speeds and At Idle
91chevy26
GMC NUMBER: 91-6A-162
GROUP: 6A Engine Mechanical
DATE: November, 1991
CORPORATE NUMBER: 166102
SUBJECT: ENGINE SHAKE AT LOW SPEEDS AND AT IDLE (MOUNT NEUTRALIZATION PROCEDURE)
MODELS: 1988 - 1991 S/T TRUCKS WITH 4.3L ENGINE
Some owners of 1988-91 S/T trucks equipped with 4.3L engines (VIN Code Z) may comment on excessive engine shake slightly above idle (800-1800 RPM). Due to the cylinder arrangement, design, and firing order, the 4.3L engine has inherent natural vibrations. Before attempting to repair any of these disturbances, it is necessary to compare it to another known good vehicle, same model, year, etc. Some engine shake is normal. However, if the engine shake is more than normal, it may be due to powertrain mount and/or exhaust system positioning and/or movement. The following procedure addresses the condition.
SERVICE PROCEDURE:
This diagnosis is designed to identify engine mount ground out. Pay special attention to engine mounts, transmission mounts, and exhaust system hangers. The vibration or ground out location must be isolated.
1. Apply parking brake.
2. Block the front wheels.
3. Place the vehicle in DRIVE (1st gear for standard transmission).
4. Bring engine rpm up to 1500-2000, listen for ground out (knocks or booming noise) and/or increased vibrations.
5. If no ground out is found, repeat step 3 with transmission in REVERSE.
NOTICE: DO NOT ACCELERATE AGAINST THE BRAKES FOR LONGER THAN 15 SECONDS OR DAMAGE TO THE VEHICLE MAY RESULT. CARE SHOULD BE TAKEN NOT TO OVERHEAT THE ENGINE OR TRANSMISSION. ALSO, CARE SHOULD BE TAKEN DURING DIAGNOSIS DUE TO THE FACT THAT SOME DISTURBANCES MAY BE CREATED DURING BRAKE TORQUE THAT NORMALLY DO NOT EXIST.
Note: Pay special attention to engine mounts making hard contact; check for proper clearance at tangs. If ground out or crushed mounts are confirmed, replace worn or torn components as required, and refer to the neutralization procedure below To neutralize the engine and align the exhaust system properly:
1. Allow vehicle to warm up fully. Raise vehicle and support with suitable safety stands.
Figure 1:
2. Loosen engine mount through-bolts (figure 1, item B) and transmission mount to cross-member bolts, and ensure the exhaust flexible hose is free to move.
Figure 3:
3. Loosen exhaust pipe/converter pipe clamp U-bolt (figure 3).
- Move U-bolt out of position.
- Heat the exhaust pipe/converter connection joint to loosen the converter.
Figure 2:
- Adjust converter to center the mounting pins in the insulators. This equalizes the hanger load at the insulators (figure 2).
- Move U-bolt back to original position and tighten.
4. Settle the powertrain by shifting from DRIVE to REVERSE with automatic transmission or 1ST to REVERSE with manual transmission, at least three times with engine at idle. Then turn engine off.
5. Tighten all fasteners with the powertrain in a relaxed position (vehicle in neutral). Torque to specifications given in the S/T service manual (X-9129).
Note: Be sure that the catalytic converter hanger and hanger pins are centered in the insulators. The pins should have 6.0 - 16.0 mm (1/4 in. - 5/8 in.) clearance as shown in figure 2.
Some residual vibrations may be normal. Compare with a vehicle of same model, year, engine/transmission combination, etc. to get a feel for what is normal.
WARRANTY INFORMATION:
For vehicles repaired under warranty, use:
Models Labor Operation Labor Time
S-trucks J1507 2.3 hrs.
T-trucks J1507 2.2 hrs.