TIMING GEAR NOISE/KNOCK 2.5L INFORMATION

1985 S, T, AND M MODELS WITH 2.5L L4 ENGINES

Some 1985 S, T, and M model vehicles equipped with the 2.5L L4 engine may experience a knocking condition from the engine. The following information is being provided to outline characteristics of timing gear noise (knock) and to assist in diagnosis of this condition.

Loose or improperly seated camshaft gears can produce a knocking type noise from the engine, usually after the engine is warmed up. This condition is speed sensitive and is not load sensitive.

A good indication of a cam gear noise is to remove the drive belts, start the engine and, using a sounding device (stethoscope) placed on the timing cover, listen for a sharp rapping noise while slowly accelerating the engine between 700 and 1,000 rpm. Timing gear noise is most audible at about 800 rpm. It can be heard the loudest at the timing gear cover and at the oil pan between #3 and #4 cylinders.

If an engine noise has been diagnosed as timing gear related, the following procedure for checking timing gear backlash should be followed.

1. Remove rocker arm cover and rocker arms.

2. Remove timing gear cover.

3. Check camshaft end clearance to determine if gear is fully seated. End clearance should be .0015" to .0050" (.038 mm to .127 mm).

4. Install the crank pulley and retaining bolt. Torque bolt to 220 N.m. (160 ft. lbs.).

5. With a magnetic dial indicator positioned on the front face of the engine block, place the dial indicator on one tooth of the camshaft gear.

6. Rotate the camshaft gear back and forth. Camshaft gear backlash should be .0005" to .0095" (.012 mm to .241 mm). Check the backlash in four (4) areas, 90 DEG apart.

7. If the camshaft end clearance exceeds .0050"(.127 mm) or gear backlash exceeds .0095" (.241 mm) in the above checks, it will be necessary to replace the timing gears and/or cam gear thrust plate, or both.

8. If gear lash exceeds the .0095" (.241 mm) specified, replace both the camshaft and crank shaft gears.

Refer to Section 6A3 of the S 15 Light Duty Service Manual for proper camshaft removal and reinstallation procedures.

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.