Engine - Cold Knock
NUMBER: 92-T-178CORP.REF.NO: 376105
DATE: September 1993
SECTION: 6A, 6E3, V6
SUBJECT:
COLD ENGINE KNOCK - INSTALL CHECK VALVE OIL FILTER/REPLACE PROM
MODELS/YEARS:
1991-92 BRAVADA
CONDITION:
Some 1991 and early 1992 Bravadas with the 4.3L (VIN Z, LB4) engine have been reported to exhibit "cold knock" on start up. "Cold knock" usually occurs after the vehicle has been completely warmed, then parked for 8 or more hours in ambient temperatures of 2~C/35~ F or less. "Cold knock" can be separated into three distinct categories.
A. Short Duration - Harsh deep metallic knock that usually lasts from 1 to 10 seconds. Generally classified as a bearing or rod knock.
B. Valve Train - Light clatter, tick or click that may last up to 1 minute.
C. Piston Slap - Metallic knock that occurs only under load. Piston slap may last as long as 5 minutes.
CORRECTION - CATEGORY A: Short Duration Knock
^ For 1992 LB4 4.3L VIN Z ONLY:
Install PF52 oil filter. Install a revised PROM that reduces the engine spark advance after the engine is started. The reduction in spark lowers the cylinder pressure and eliminates the knock. The revised PROM will NOT eliminate a piston slap (Category C) or valve train noise (Category B) concern. The base cold knock PROMs contain the previously released calibration update for torque converter clutch (TCC) lock-up (see Dealer Technical Bulletin 92-T-34). If a vehicle has had a detonation fix PROM installed previously, select the combined detonation and cold knock fix PROM for the application.
Note:
Use of a detonation fix PROM in a non-detonating vehicle may result in degraded driveability.
GMSPO will be temporarily stocking four (4) PROMs for each light duty 1992 model year LB4 application.
^ Base
^ Detonation Fix
^ Cold Knock Fix
^ Combination Cold Knock and Detonation Fix
The detonation fix (detonation fix only) PROMs will not be restocked by SPO when the current supply is exhausted. If a detonation fix PROM is required after the existing stock is exhausted, the combination cold knock and detonation PROM is to be used. SPAC (Special Parts Assistance Center) will have information available on each PROM. Select the PROM from the table. Old broadcast code (OLD B/C code) information has been supplied to help determine if a detonation fix PROM has been installed previously. If the Old B/C code can be found in the first table, a detonation fix has not been installed.
PROMs are currently available from GMSPO.
1992 LB4 VIN Z
For vehicles with "cold knock" only
For vehicles with "cold knock" having previously installed detonation field fix PROM
- OR -
For vehicles with "cold knock" and detonation
For 1991-92 model year LB4 engines, two actions have been found to show a high probability of success in curing short duration cold knock (Category A). General Motors suggests performing the following steps.
Note:
These steps are only applicable to short duration cold knock, and will not eliminate a knock occurring under load. These actions will not eliminate a knock lasting more than 10 seconds, nor a knock occurring in a hot or warm engine.
Step 2: Change the oil; use a synthetic oil meeting the API Service SG or SG/CE standard.
Note:
This action is only required once; conventional (non-synthetic) API Service SG or SG/CE oil may be reinstalled at the next oil change.
CORRECTION - CATEGORY B: Valve Train Chatter, Tick, or Click. For 1992 Bravadas (adjustable lash) LB4 (VIN Z 4.3L V6) engine, adjust valve lash. Service Manual Update 92-U-44 provides information on this procedure.
Investigation of "cold knock" is continuing. Updates will continue to be provided when available.
Parts are currently available from GMSPO.
WARRANTY INFORMATION:
Labor Operation:
J0950 - Filter and Oil Replace - Use published labor time.
For 1992 LB4 vehicles
Labor Operation: T0500 - PROM replace
Labor Time: 0.5 hours