Engine - Vibration at 1,800 to 2,00 RPM

91chevy14

Number: 91-308-6

Section: 6

Date: MAY 1991

Corp. Bulletin No.: 166002


Subject: ENGINE VIBRATION-PINCHED TRANSFER VENT HOSE


Model and Year: 1990-91 T TRUCKS WITH 4.3L ENGINES


CHEVROLET RECOMMENDS THAT A COPY OF THIS BULLETIN ALSO BE FILED UNDER SECTION 4A PROPELLER SHAFT.


Some 1990-91 "T" trucks equipped with 4.3L (LB4) engines may experience a vibration condition at approximately 1800-2000 rpm. This vibration can be induced by raising the engine rpm while the transmission is in park or neutral. The vibration may be felt throughout the entire vehicle; however, it may be most noticeable in the console area of the floorpan. It may be described as a "grounding out".

If this condition is encountered, it may be caused by the transfer case vent hose being pinched between the top of the transfer case and the bottom of the floor pan. When this condition exists, engine vibrations are transferred directly to the body.

Verify the routing location of the hose. If the hose is routed over the top of the transfer case, check to see if it can be moved freely. If it can be moved freely, it probably is not causing the vibration being felt when the transmission is in park or neutral. However, as a result of engine torque, it may transmit a vibration under hard acceleration. Remove the hose and inspect it for chaffing or damage and replace if necessary. Reposition the hose off to the left side of the transfer case so that it does not rub against the floor pan.

If the hose is pinched and cannot be removed, follow the service manual procedure for removal of the rear transmission mount to the crossmember. With the mount removed, lower the transmission and transfer case assembly until there is enough room to remove the pinched hose. Remove the hose and inspect it for chaffing or damage and replace if necessary. Reposition the hose off to the left side of the transfer case so that it does not rub against the floor pan. Follow the service manual procedure for reinstallation of the mount and torque fasteners to the appropriate specifications.

NOTE: Colder ambient temperatures can cause hoses to become more rigid (less flexible) and better capable of transmitting a vibration when pinched. This, combined with how severely the hose is pinched, may cause the vibration to be more noticeable at engine RPM's other than that listed.


WARRANTY INFORMATION:

For vehicles repaired under warranty use:

Labor Operation: T7246

Labor Time: 0.3 hr

If necessary to remove rear transmission mount:

Add: 0.4 hr. (Manual or Automatic Transmission)

NOTE: Labor Operation is coded to base vehicle coverage in the warranty system.

Trouble code: 92