Engine - Cold Knock
Number: 93-155A-6ASection: 6A
Date: AUGUST 1993
Corporate Bulletin No.: 376105R
ASE No.: A1, A8
Subject:
COLD ENGINE KNOCK
Model and Year:
1991-93 C/K, R/V, M/L, S/T, G TRUCKS WITH 4.3L, 5.7L AND 7.4L ENGINES
THIS BULLETIN CANCELS AND SUPERSEDES DEALER SERVICE BULLETIN NO. 93-155-6A DATED MARCH 1993. THE CORRECTION SECTION HAS BEEN CHANGED COMPLETELY. COPIES OF 93-155-6A SHOULD BE DISCARDED.
CONDITION:
Some late model LB4 (VIN Z 4.3L V6), L05 (VIN K 5.7L V8), and L19 (VIN N 7.4L V8) truck engines 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 35 degrees Fahrenheit 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 Service Bulletin No. 92-75-7A) if the vehicle has an automatic transmission or neutral gear rattle if the vehicle has a manual transmission (see Dealer Service Bulletin No. 92-187A-7B). If a vehicle has had a detonation fix PROM installed previously, select the combined detonation and cold knock fix PROM for the application. See Dealer Service Bulletin No. 92-285B-6E for more information on detonation.
NOTE:
Use of a detonation fix PROM in a non-detonating vehicle may result in degraded driveability.
SPO 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. Special Parts Assistance Center will have information available on each PROM. Select the PROM from the table. Old broadcast code (OLDB/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. If the broadcast code cannot be found in the tables, call the appropriate marketing division technical assistance group.
PROMs are expected to be available from GMSPO August 30, 1993.
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 1991-93 model year L05 and L19, 1991 and 1993 LB4 engines, two actions have been found to show a high probability of success incuring 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. Neither will these actions eliminate a knock lasting more than 10 seconds, nor a knock occurring in a hot or warm engine.
Step 1:
Install a check valve oil filter
Original Filter Install
PF35 AC PF1218
PF51 AC PF52
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.
If an LB4 (VIN Z 4.3L V6) or an L05 (VIN K 5.7L V8) continues to exhibit short duration cold knock after performing steps one and two and accumulating at least 200 miles, call the appropriate marketing division technical assistance group.
CORRECTION - CATEGORY B:
Valve Train Chatter, Tick, or Click.
For 1992 or 1993 vehicles equipped with a Romulus built (adjustable lash) LB4 (VIN Z 4.3L V6) engine, adjust valve lash. Bulletin 376107 provides information on this procedure and on how to identify Romulus-produced engines.
Investigation of "cold knock" is continuing. Updates will continue to be provided when available.
Parts are expected to be available on August 30, 1993.
WARRANTY INFORMATION:
Labor Operation: J0950 - Filter and Oil Replace
Labor Time: Use published labor time.
For 1992 LB4 vehicles:
Labor Operation: T0500 - PROM replace
Labor Time: 0.5 hours