INFO. ON SERVICE/DIAGNOSTIC PROCEDURES FOR LS0/LR0 ENGINES

SUBJECT: INFORMATION ON SERVICE/DIAGNOSTIC PROCEDURES FOR 6.OL/7.OL TBI ENGINES

VEHICLES AFFECTED: 1989-93 C6/C7/B7/P6 TRUCKS (RPOS LSO AND LRO)

This bulletin is to inform dealer technicians of the importance of following correct service/diagnostic procedures. The following is a list of do's and don'ts for the 6.OL and 7.OL engines.

1. The procedure for setting ignition timing must be followed; it is explained on the label located on the engine air cleaner housing. It should be emphasized that the EST timing connector must be unplugged to put the ignition in by-pass mode before making any timing measurements. The timing should be one of the first checks made when an engine is brought in for poor driveability or a piston failure. Incorrectly set base timing could be the reason for the problem, and should be verified.

2. To prevent corrosion, the distributor cap terminal material has been changed from zinc to aluminum beginning with late 1991 model year production. A campaign bulletin (GMCL IN05, GM TRUCK 91-E-13) was issued to replace parts on earlier models. Delco-Remy caps with aluminum terminals can be identified by the absence of rectangular vent notches on the top end of the terminals where they meet the top surface of the cap plastic. The plastic may be white or black. This notch identification does not apply to aftermarket caps. It is important to avoid after-market caps as their terminals are of unknown materials. When aftermarket caps are replaced for driveability problems, the customer should be informed that continued use of these caps could lead to engine failure.

3. The correct spark plugs for 1989 through 1993 model year medium duty TBI engines are AC R44TS for the 6.OL (LSO) engine and AC R43TS for the 7.OL (LRO) engine. The correct gap for these spark plugs is 0.035 in. 0.9MM Under no circumstances should the heat ranges or gap be changed, as plug fouling or pre-ignition could result.

4. Proper spark plug wire routing and the use of correct spark plug wires, boots, and shields is extremely important.

OEM wire clips must be kept intact to avoid inductive crossfire between the wires. Specifically, cylinders 5 and 7 are particularly susceptible to crossfire if they are not kept separated correctly, as they are with the OEM clips.

To protect the spark plug boots, both the manifold mounted and the boot sleeved heat shields must be used. If the boot shields are removed, the boots will fail due to overheating and a misfire will result. Also, the liner between the boot and this shield must be kept intact to prevent high voltage arcing through the boot to the shield.

Only the longer ribbed style boot should be used. The boot shield is designed to engage this boot for easier removal. The shield can be used to turn the boot to free it from the spark plug and then to pull the boot off of the spark plug. Do not remove the boot by pulling on the spark plug wire.

To properly reinstall the plug boots, first insert the shield fully into the cylinder head around the spark plug. Next, push the plug boot over the spark plug insulator until the terminal is fully seated. Failure to engage the terminal can lead to core erosion due to heat build up from arcing between the terminal and spark plug post. In time, the core will erode until this becomes a completely open circuit. This will lead to pre-ignition and piston damage.

6. The use of an ignition analyzer oscilloscope is recommended to identify the presence of crossfire or misfire. A routine ignition check should be made a part of periodic preventative maintenance inspections to identify problems that could lead to future engine damage.

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.