SUSTAINED DETONATION/KNOCK (APPLY STD DIAG. PROC./INST.. Chevrolet Blazer

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.

PROMS 1992 LB4 VIN Z

TRANS AXLE EMISSION SERVICE B/C SCAN OLD B/C* PLATFORM RPO RPO CLASS PROM KIT CODE I.D. CODE ---------------------------------------------------------------------------- S/T MY2 GU2 NA5/NA6/NB2 16196188 BHWN 6235 AULR/BHSF S/T MY2 GU4 NA5/NA6/NB2 16196192 BHWP 6245 AULS/BHSH S/T MY2 GU6 NA5/NA6/NB2 16196199 BHWR 6255 AULT/BHSJ S/T MD8 ALL NA5/NA6 16192257 BFNF 2295 BARL/AWHH S/T MD8 ALL NB2 16192260 BFNH 2305 BARM/AWHF

PROMS 1992 LB4 VIN Z

TRANS AXLE EMISSION SERVICE B/C SCAN OLD B/C* PLATFORM RPO RPO CLASS PROM KIT CODE I.D. CODE --------------------------------------------------------------------------- C1,C2/K1,K2 MT8 ALL NA5/NA6+NA1 16196134 BHSN 6205 BCZP/AULJ K1,K2 MG5 ALL NA5/NA6+NA1 16196134 BHSN 6205 BCZP/AZUS C1,C2/K1,K2 MT8 ALL NB2+NA1 16196153 BHUU 6225 BCZR/AULM K1,K2 MG5 ALL NB2+NA1 16196153 BHUU 6225 BCZR/AZUT C1,C2 MG5 ALL NA5/NA6 16196152 BHSM 6195 BDCW/AULH C1,C2 MG5 ALL NB2+NA1 16196131 BHUT 6215 BDCX/AULL C1,C2/K1,K2 MD8 ALL NA5/NA6 16192249 BFNC 2275 AULF C1,C2/K1,K2 MD8 ALL NB2 16192253 BFND 2285 AULK

G1,G2,G3/M1 MD8 ALL NA5/NA6 16196101 BHSA 6105 AULN/BCKU G1,G2,G3/M1 MD8 ALL NB2 16196108 BHSC 6125 AULP/BCKT

L1 MD8 ALL NA5/NA6 16196104 BHSB 6115 AYAW/BCNU L1 MD8 ALL NB2 16196111 BHSD 6135 AYAX/BCN

* Old broadcast code is included to help the technician determine if a previous field fix has been installed. If a broadcast code other than one appearing on this list is present, contact Technical Assistance Network (TAN) for instruction.

Parts are currently available from GMSPO.

WARRANTY INFORMATION:

For vehicles repaired under warranty:

LABOR TIME (in hours)

LABOR OP M L G C K S T -------- -------------------------------- T0500 - PROM Replace 0.6 0.6 0.6 0.6 0.6 0.6 0.6

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.