PROM - Sustained Detonation/Knock

Group Ref.: Engine Fuel & Emissions

Bulletin No.: 376508

Date: October, 1993

SUBJECT:
SUSTAINED DETONATION/KNOCK (APPLY STANDARD DIAGNOSTIC PROCEDURE/INSTALL NEW PROM)

MODELS:
1992 C/K AND S/T TRUCKS, G AND M/L VANS

THIS BULLETIN IS BEING REVISED TO CORRECT THE SERVICE PROM KIT NUMBERS FOR THE FEDERAL C (BHSM) AND THE CALIFORNIA C (BHUT) CALIBRATIONS. PLEASE DISCARD PREVIOUS DIVISIONAL PUBLICATIONS:

CHEVROLET 92-285B-6E

GMC TRUCK 92-6E-162

GM OF CANADA 93-6E-128

CONDITION:

Some owners of 1992 S/T, C/K, M/L or G trucks equipped with 4.3L (LB4) V6 (VIN Z) engines may comment that the engine exhibits sustained detonation (knock) during acceleration or climbing a grade. The sustained detonation (knock) occurs after the engine reaches normal operating temperature. The detonation may occur at part throttle or wide open throttle operation.

NOTE:
Some burst detonation (knock) during "tip-in" acceleration and/or at shift points is considered normal.

CAUSE:

Some engines may have higher than intended compression ratios after accumulating carbon deposits.

CORRECTION:





Follow standard detonation diagnostic procedures in the appropriate service manual. If the cause of the detonation cannot be determined and eliminated after standard diagnostic procedures, the condition may be corrected by the installation of the correct field fix calibration PROM from the table.

NOTE:
The manual transmission calibrations contain the neutral gear rattle update (increased idle speed). If a vehicle is experiencing neutral gear rattle, hardware changes and the new calibration are required. For a description of neutral gear rattle and the required hardware changes, see Corporate Bulletin 267201.

NOTE:
The S/T series automatic transmission calibrations contain the torque converter clutch (TCC) lock-up update. For a description of the TCC update, see Corporate Bulletin 137107.

WARNING:
Use of a detonation field fix calibration PROM in a nondetonating engine may result in degraded driveability. General Motors strongly recommends the concern be properly identified as sustained detonation occurring after the engine is fully warmed to normal operating temperature.