PROM - Sustained Detonation/Knock While Accelerating
Number: 92-285B-6ESection: 6E
Date: AUGUST 1993
Corporate Bulletin No.: 376508
ASE No.: A1, A8
Subject:
SUSTAINED DETONATION/KNOCK
Model and Year:
1992 C/K, M/L, G, S/T TRUCKS WITH 4.3L (LB4) ENGINE
THIS BULLETIN CANCELS AND SUPERSEDES DEALER SERVICE BULLETIN NO. 92-285A-6E, DATED SEPT. 1992. ART WORK HAS BEEN ADDED. ALL COPIES OF 92-285A-6E SHOULD BE DISCARDED.
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 Dealer Service Bulletin No. 92-187A-7B, dated Jan. 1993.
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 Dealer Service Bulletin No. 93- 75-7A, dated Jan. 1992.
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.
PARTS
WARRANTY INFORMATION:
For vehicles repaired under warranty, see illustration.