For 1990-2009 cars

# INFO. ON COATED PISTON FOR CYLINDERS 5 & 6

#### SUBJECT: INFORMATION ON COATED PISTON FOR CYLINDERS 5 AND 6

#### MODELS: 1996 BUICK SKYLARK, CENTURY 1996 CHEVROLET BERETTA, CORSICA, LUMINA, MONTE CARLO 1996 OLDSMOBILE ACHIEVA, CUTLASS CIERA, CUTLASS SUPREME 1996 PONTIAC GRAND AM, GRAND PRIX WITH 3100 V6 ENGINE (VIN M - RPO L82)

STARTING JULY 1, 1996 ALL 3100 V6 ENGINES FOR BOTH REGULAR PRODUCTION AND SERVICE WERE BUILT WITH A COATED PISTON IN CYLINDERS 5 AND 6 TO ELIMINATE THE POTENTIAL FOR PISTON SCUFFING. 1997 PRODUCED ENGINES FOR BOTH REGULAR PRODUCTION AND SERVICE WILL BE BUILT WITH THESE PISTONS IN CYLINDERS 5 AND 6. THE NEW PISTON IS BLACK IN COLOR ON THE SKIRTED AREA AS COMPARED TO A SHINNY ALUMINUM COLOR FOR A NON-COATED PISTON. CYLINDERS 1-2-3 AND 4 WILL CONTINUE TO BE PRODUCED WITH A NON-COATED ALUMINUM PISTON IN PRODUCTION. THE COATED PISTON WILL REPLACE THE STANDARD SIZE SERVICE ALUMINUM PISTON AND WILL SERVICE ALL SIX (6) CYLINDERS. THE COATED PISTON IS ONLY AVAILABLE IN THE STANDARD SIZE. ANY OVERSIZE PISTON WILL BE NON-COATED.

ONE OF THE MOST DIFFICULT OF ALL TROUBLESHOOTING TASKS IS TO DETERMINE THE SOURCE OF THE CUSTOMER NOISE CONCERN. CONSIDER THE FOLLOWING DIAGNOSTIC HELPFUL HINTS:

#### PROCEDURE:

1. THE USE OF A ENGINE STETHOSCOPE, ELECTRONIC ENGINE EAR AND A DIAGNOSTIC SCANTOOL MAY HELP ISOLATE THE NOISE TO A COMPONENT. 2. NOISES THAT ORIGINATE IN THE VALVETRAIN TYPICALLY ARE NOT AFFECTED BY SHORTING OUT CYLINDERS. 3. A MOMENTARY LIGHT MECHANICAL NOISE WHEN THE ENGINE IS FIRST STARTED IS OK. THIS HAPPENS BECAUSE THE VALVE LIFTERS WILL DRAIN DOWN WHEN THE ENGINE IS NOT RUNNING. THE VALVE LIFTER NEEDS A FEW SECONDS TO FILL WITH OIL AFTER THE ENGINE IS STARTED. THE USE OF THE PROPER OIL FILTER (PF 47) WILL HELP MINIMIZE THIS DUE TO THE OIL DRAIN BACK VALVE INCORPORATED INTO THIS FILTER. THIS FEATURE MAY NOT BE PRESENT IN OTHER BRANDS OR FILTER TYPES. 4. A SHARP METALLIC KNOCK AT CRANKSHAFT SPEED COULD BE CAUSED BY LOOSE TRANSMISSION TORQUE CONVERTER BOLTS OR CRACKED FLEXPLATE. INSPECT THE FLEXPLATE FOR CRACKS AND MAKE SURE THE TORQUE CONVERTER BOLTS ARE PROPERLY TIGHTENED. 5. A THUMP OR KNOCK NOISE AT IDLE COULD BE CAUSED BY AN ACCESSORY DRIVE BELT THAT HAS PILLED. PILLING IS THE TRANSFER OF BELT MATERIAL INTO TINY BALLS THAT ARE DEPOSITED INTO THE GROOVED AREA OF THE DRIVE BELT. REMOVE THE ACCESSORY DRIVE BELT AND START THE ENGINE. IF THE NOISE IS GONE, INSPECT THE BELT FOR PILLING. ALSO INSPECT THE DRIVEN PULLEYS OF THE ACCESSORY DRIVE FOR DIRT OR PAINT BUILD UP IN THE GROOVED AREAS. 6. A SHARP HEAVY CONTINUOUS METALLIC KNOCK NOISE COULD BE A ROD KNOCK. USE A SCANTOOL SUCH AS A TECH II TO DISABLE THE FUEL INJECTOR TO ISOLATE THE AFFECTED CYLINDER. 7. A LIGHT TICKING NOISE AT A HOT IDLE COULD BE A VALVETRAIN NOISE. INSPECT THE ROCKER ARMS AND PUSHRODS. ALSO CHECK THE VALVE GUIDE TO VALVE STEM CLEARANCE AS OUTLINED IN THE SERVICE MANUAL. 8. A LIGHT START UP RATTLE ON A COLD ENGINE COULD BE PISTON PIN NOISE. IF MORE THAN 1 CYLINDER IS AFFECTED, THE NOISE MAY SOUND MARBLED OR LIKE A DIESEL. THIS NOISE USUALLY GOES AWAY AFTER THE ENGINE HAS RUN FOR 5 MINUTES. USE A SCANTOOL TO ISOLATE THE AFFECTED CYLINDER BY DISABLING THE FUEL INJECTOR. 9. A HEAVY KNOCK OR TICKING NOISE MOST NOTICEABLE AT IDLE COULD BE CAUSED BY A SCUFFED PISTON. YOU CAN ISOLATE THIS NOISE TO A PARTICULAR CYLINDER BY USING A SCANTOOL TO DISABLE THE FUEL INJECTOR.

#### IMPORTANT: A TRUE CAUSE FOR THE NOISE MAY NOT BE DETERMINED UNTIL THE ENGINE IS DISASSEMBLED AND INSPECTED.

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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.

COPYRIGHT 1996. GENERAL MOTORS CORPORATION. ALL RIGHTS RESERVED.