Engine - Shake At Low RPM & At Idle, Engine Mount
Group Ref.: Engine MechanicalBulletin No.: 166102
Date: November, 1993
SUBJECT:
ENGINE SHAKE AT LOW SPEEDS AND AT IDLE (MOUNT NEUTRALIZATION PROCEDURE)
MODELS:
1988-92 S/T TRUCKS WITH 4.3L ENGINE (RPO LB4)
This bulletin is being revised to change the service parts availability information. Previous divisional publication numbers were:
Chevrolet 92-353-6
GMC Truck 92-6A-158
CONDITION
Owners of some 1988-92 S/T Trucks equipped with a 4.3L engine (VIN Code Z) may comment about excessive engine shake slightly above idle (800-1800 RPM).
CAUSE
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.
CORRECTION
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.
CAUTION:
DO NOT ACCELERATE AGAINST THE BRAKES FOR LONGER THAN 15 SECONDS. 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, 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.
2. Loosen engine mount through bolts (Figure 1, "B") and transmission mount to crossmember bolts and ensure the exhaust flexible hose is free to - move.
- 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.
- 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.
3. 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.
4. Tighten all fasteners with the powertrain in a relaxed position (vehicle in neutral). Torque to specifications in 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.
5. Replace the previous transmission mount with the revised mount (P/N 22145724 - for two wheel drive only) as shown in Figure 4. 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.
SERVICE PARTS INFORMATION
Parts are expected to be available from GMSPO on 8/2/93.
WARRANTY INFORMATION
For vehicles repaired under warranty, use: