ENGINE PISTON NOISE-1981'B'AND 'C' VEHICLES WITH 4.1L V-6

SOME EARLY 1981 4.1 LITER V-6 EQUIPPED VEHICLES MAY EXHIBIT TWO SEPARATE AND DISTINCT ENGINE NOISES DURING COLD START-UP. THE NOISE MAY BE EITHER A HOLLOW TYPE SOUND THAT DISAPPEARS IN LESS THAN TWO MILES OF DRIVING, OR SECONDLY, A SOUND LIKE A STUCK OR COLLAPSED LIFTER WHICH MAY DISAPPEAR AFTER SEVERAL MILES OF DRIVING. THE CAUSE IS EITHER A LOOSELY FITTED PISTON OR AN IMPROPERLY MACHINED PISTON. PISTON SLAP CAUSED BY A LOOSELY FITTED PISTON, CLEARANCE OVER .003 INCH, WILL RESULT IN A HOLLOW-TYPE SOUND THAT WILL DIMINISH IN A FEW MILES OF DRIVING. TO DETERMINE WHICH CYLINDER(S) ARE CAUSING THE CONDITION, SHORT OUT ONE SPARK PLUG AND LISTEN FOR A DECREASE IN THE NOISE. THIS WILL INDICATE A PROBLEM WITH THAT CYLINDER. REPEAT THE PROCEDURE FOR EACH CYLINDER SINCE MORE THAN ONE MAY BE THE CAUSE. REPLACE ANY PISTON WITH CLEARANCE OVER .003 INCH. THE NOISE WHICH SOUNDS LIKE A STUCK OR COLLAPSED LIFTER AND LASTS LONGER THAN PISTON SLAP IS PROBABLY CAUSED BY AN IMPROPERLY MACHINED PISTON. INVESTIGATION HAS REVEALED THAT SOME PISTONS USED IN EARLY 1981 ENGINES HAVE AN OFF ANGLE CAM GRIND WHICH CAUSES THE LIFTER-TYPE NOISE WHEN THESE PISTONS ARE INSTALLED IN CYLINDERS 1, 3 OR 5 (LEFT BANK OF THE ENGINE). SHORTING OUT CYLINDERS WILL INDICATE THE CONDITION IS RESTRICTED TO THE LEFT BANK. IT IS RECOMMENDED THAT ALL THREE PISTONS IN THE LEFT BANK BE REPLACED. MOST V-6 ENGINE NOISE CAN BE ATTRIBUTED TO THIS CONDITION. A PISTON THAT IS INCORRECTLY CAM GROUND WILL NOT AFFECT ENGINE DURABILITY SO PISTON REPLACEMENT SHOULD OCCUR ONLY FOR COMMENTS OF ENGINE NOISE. UNLIKE THE 6.0 LITER ENGINE, THERE IS ONLY ONE SIZE STANDARD SERVICE PISTON (P/N 25505374) AVAILABLE. THIS IS A HIGH LIMIT (HIGHEST DIAMETER PISTON IN THE TOLERANCE RANGE) PISTON AND WILL PROPERLY FIT MOST CYLINDERS. TWO OVERSIZE PISTONS, A .010" AND .030" OVERSIZE, ARE ALSO AVAILABLE. TO CORRECTLY FIT PISTONS TO CYLINDERS, REFER TO PAGE 6A-26 OF THE 1980 V-6 SERVICE INFORMATION SUPPLEMENT. IN THOSE CASES WHERE SERVICE PISTON FIT IS TOO TIGHT, IT WILL BE NECESSARY TO HONE THE CYLINDER BORE TO PROVIDE PROPER CLEARANCE. PISTON RING COMPRESSOR, J-22965, SHOWN IN FIGURE 6A-55, IS TOO SMALL FOR THE 4.1 LITER PISTON. AN ADJUSTABLE RING COMPRESSOR SUCH AS J-8037 OR EQUIVALENT CAN BE USED.

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.