Ignition - Detonation or Spark Knock, Diagnosis

Group Ref.: Engine Fuel & Emission
Bulletin No.: 376520
Date: February, 1994

SUBJECT:
DETONATION OR SPARK KNOCK (SERVICE PROCEDURE)

MODELS:
1990-94 CADILLAC DEVILLE, FLEETWOOD (FWD), SIXTY SPECIAL, ELDORADO, SEVILLE EQUIPPED WITH 4.5L OR 4.9L ENGINES (VIN B-RPO L26, VIN 3-RPO LW2, VIN 8-RPO L06)

CONDITION:

Some owners may experience detonation or spark knock under light to medium throttle loads with the A/C on only.

CAUSE:

The condition may be caused by poor engine grounds, incorrect ignition timing, improper EGR operation, restriction in the engine cooling system, poor fuel quality, incorrect spark plug heat range, or carbon build up on the faces of the pistons or the valves in the cylinder heads.

CORRECTION:

To repair this condition use the following procedure:

1. Verify minimum of 91 octane fuel with less than 10 percent ethanol. Poor fuel quality has been the most common cause of detonation on these engines.

2. Verify correct spark plug heat range.

3. Check for restrictions in the cooling system or engine overheat.

4. Verify base ignition timing is set to 10 degrees BTDC. If premium fuel is not used, ignition timing can be retarded to a maximum of 6 degees BTDC. (Some engine performance loss may be noted.) Refer to DSIB 91-1-23 (Corporate Bulletin 916304R).

5. Verify proper EGR operation per Mml chart C-7 in the appropriate Service Information Manual. If vacuum to EGR is inadequate, inspect vacuum hose for restrictions, blockage, EGR, and solenoid operations. Refer to proper Labor Operation in appropriate Labor Time Guide.

6. Enlarge the EGR inlet orifice located in the EGR solenoid to 1/32 (.03125 or .79375 mm) using a 1/32 drill bit. This orifice should not be enlarged greater than 1 mm or surge and/or idle roughness conditions could occur as a result of too much EGR flow.

7. Run Top Engine Cleaner (P/Ns 10550002, 1052626 or equivalent) through the engine.

NOTE:
Although TEC does not affect catalyst performance when used as directed, misuse of the product may result in appreciable losses in oxidation catalytic converter efficiency due to high temperatures caused by the extra rich mixture. For this reason TEC must be used according to its label with particular attention to the following:

a. Allow the engine to run at idle for 10 to 15 minutes before adding TEC.

b. Remove the air cleaner.

With engine equipped with Air System (AIR):

c. Divert the AIR from the converter in the following manner: Disconnect both the AIR Switch Solenoid and the AIR Divert Solenoid. CKT 639 Pink/Black.

d. Pour half a can of TEC into the throttle body at a slow, steady rate with the engine idling.

e. Pour remaining half can of TEC into throttle body at a fast enough rate to stall engine. Let TEC soak in engine a minimum of 8 hours.

f. Start engine and idle 10 to 15 minutes. Increase engine speed to 2000 rpm. Pour another can of TEC into throttle body at a slow, steady rate so the engine does not stall.

g. With engine speed at 2000 rpm, at a slow, steady rate so the engine does not stall, pour two quarts of water through the throttle body to de-carbon the piston faces and the valves in the cylinder heads.

h. Restore AIR management systems. Reconnect AIR Switch Solenoid and AIR Divert Solenoid. CKT 639 Pink/Black.

WARRANTY INFORMATION:
For vehicles repaired under warranty, use:

Labor Operation: T3423

Labor Time: 0.5 hour.