ENGINE KNOCK DIAGNOSIS OF NOISE

SOME 1980 VEHICLES EQUIPPED WITH A 2.5L L4 ENGINE MAY DEVELOP AN ENGINE KNOCK. ENGINE KNOCK DIAGNOSIS IS EXTREMELY DIFFICULT IN TODAY'S LIGHTWEIGHT SYSTEMS. THE FOLLOWING ARE OBSERVATIONS THAT SHOULD MAKE DIAGNOSIS EASIER. CARBON KNOCK - CARBON KNOCK SOUNDS MUCH LIKE PISTON SLAP, WITH THE FOLLOWING EXCEPTIONS CARBON KNOCK IS USUALLY LOUDEST ON A FLOAT OR DECELERATION ON A FREE ENGINE WHEN COLD BUT MAY NOT DISAPPEAR WHEN COMPLETELY WARM. PISTON SLAP WILL APPEAR ON A NEW ENGINE WHILE CARBON KNOCK COMES AFTER MILEAGE ACCUMULATION. TIMING GEAR NOISE - LOOSE OR IMPROPERLY SEATED CAMSHAFT TIMING GEARS ARE USUALLY LOUDEST WHEN WARM. THEY ARE SPEED SENSITIVE ONLY, NOT LOAD SENSITIVE. CAM GEAR NOISE WILL BE EVIDENT AT A WARM IDLE AND SOUNDS MUCH LIKE A LOOSE TIMING CHAIN NOISE EXPERIENCE IN THE PAST. REMOVING DRIVE BELTS AND PLACING A SOUNDING DEVICE ON THE TIMING POINTER WILL GIVE DEFINITE INDICATION OF TIMING GEAR NOISE. TORQUE CONVERTER BOLTS - LOOSE TORQUE CONVERTER BOLTS SOUND MUCH LIKE A BEARING KNOCK AND WILL GIVE SEVERAL RAPS ON A QUICK ACCELERATION (ENGINE RUNNING FREE). WITH THE TRANSMISSION IN GEAR, THE NOISE MAY OR MAY NOT APPEAR AT IDLE DEPENDING ON THE IDLE SMOOTHNESS. CHECK TORQUE CONVERTER TO FLYWHEEL BOLTS BEFORE ATTEMPTING TO REPAIR ANY BEARING- TYPE KNOCK. REPAIR PROCEDURE FOR CARBON KNOCK THE ENGINE SHOULD BE DECARBONED BEFORE ANY REPAIRS ARE ATTEMPTED. IF THE KNOCK HAS NOT DISAPPEARED, NORMAL SERVICE OPERATIONS SHOULD BE USED TO DETERMINE PISTON TO CYLINDER WALL CLEARANCE AND CORRECT AS NECESSARY. IF THE KNOCK GOES AWAY BUT COMES BACK AFTER SOME MILEAGE ACCUMULATION, THE FOLLOWING PROCEDURE SHOULD BE USED. REMOVE THE CYLINDER HEAD, THOROUGHLY CLEAN ALL CARBON BUILD-UP FROM TOP OF PISTON AND CYLINDER HEAD SURFACE. ANY CARBON PARTICLES ACCUMULATED BETWEEN THE PISTON AND CYLINDER WALL SHOULD BE BLOWN OUT WITH COMPRESSED AIR. REASSEMBLE WITH A NEW CYLINDER HEAD GASKET AND SHIM KIT, PART NUMBER 10022914. THE SHIMS ARE REQUIRED TO COMPENSATE FOR AN INCREASED THICKNESS OF THE CYLINDER HEAD GASKET AND ARE TO BE INSTALLED BETWEEN THE ROCKER ARM BALL AND RETAINING NUTS (SEE FIGURE 1). FIGURE 2 AND 3 SHOWS A MEANS OF IDENTIFICATION OF THE SUBJECT CYLINDER HEAD GASKET. ANY MECHANICAL REPAIR INVOLVING REMOVAL OF THE CYLINDER HEAD SHOULD BE VISUALLY INSPECTED AS TO THE TYPE CYLINDER HEAD GASKET ON THE VEHICLE. IF THE MECHANICAL REPAIR IS NOT RELATED TO A CARBON KNOCK, THE ORIGINAL TYPE GASKET, PART NUMBER 10006328, SHOULD BE USED. IF THE GASKET REMOVED HAS THE IDENTIFICATION NOTCHES, THE CARBON KNOCK GASKET, PART NUMBER 10022914, MUST 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.