ENGINE NOISE TICKING EXCESSIVE ROD SIDE CLEARANCE

Subject: DIAGNOSING ENGINE NOISES (#5 MYSTERY NOISE) - 3.8L V-6 AND 4.1 L V-6 ENGINES

The following is provided for your information and may prove helpful in resolving a particular engine noise condition on the above engines. The noise we refer to, sometimes called #5 mystery noise, is a result of excessive fore and aft end play of the rod on the crank pin journal.

The noise is normally heard when the engine is just above idle and under a light load (car in gear). The noise can best be described as a light ticking sound much like overhead noise but appears to come from the bottom part of the engine and is most pronounced at the rear of the front wheel opening. The level of the noise will generally decrease if the EST harness is unhooked at the distributor. Also it can generally be shorted out on #5 cylinder.

Should you encounter a vehicle exhibiting this type noise, rod side clearance (fore and aft movement) at the crank pin journal should be checked. If the clearance exceeds .015 on #1, #3, or #5 cylinders, replace the rod with a new one (part number 25516444). The new rod is .010 thicker and will reduce clearance to the low side of published limits.

The above clearance check may be performed with a dial indicator or feeler gauge. If the check is made with a feeler gauge, it should always be made between the rod and the thrust surface on the crankshaft at the crankshaft counter balance weight area and not in the area of the thin wall between the adjacent rods (see illustration). The clearance check should be made in more than one place on the crankshaft and care should be taken to be sure feeler gauge is down all the way onto the crank and not sitting on top of narrow raised thrust surface adjacent to rod.

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.