ENGINE SHAKE AT LOW SPEEDS (MOUNT NEUTRALIZATION PROC.)

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 CANCELS AND REPLACES GMC TRUCK BULLETIN 91-6A-162, DATED NOVEMBER 1991. IT IS BEING REVISED TO ADD THE 1992 MODEL YEAR, AND TO ADD STEP #6 IN THE NEUTRALIZE ENGINE SECTION AS WELL AS TO ADD SERVICE PARTS INFORMATION AND WARRANTY INFORMATION.

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 cross-member bolts and ensure the exhaust flexible hose is free to move.

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.

- 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 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 (114 in. - 518 in.) clearance as shown in Figure 2.

6. 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

Part # Description Qty/Vehicle ------ ----------- ----------- 22145724 Transmission Mount 1

Parts may be obtained from Adistra Corp. by calling 1-800-627-5699.

WARRANTY INFORMATION

For vehicles repaired under warranty, use:

Models Labor Labor Time Operation ---------------------------------------------- S Vehicles J1507 See Applicable Add: R&R Trans Mt. K6723 Labor Time T Vehicles J1507 Guide

General Motors bulletins are intended for use by professional technicians, not a "do-it-yourselfer". They are written to inform those technicians of conditions that may occur on some vehicles, or to provide information that could assist in the proper service of a vehicle. Properly trained technicians have the equipment, tools, safety instructions and know-how to do a job properly and safely. If a condition is described, do not assume that the bulletin applies to your vehicle, or that your vehicle will have that condition. See a General Motors dealer servicing your brand of General Motors vehicle for information on whether your vehicle may benefit from the information.