COWL 'POP'/TAPPING NOISE, TRIM HOOD FLANGE/RELIEVE STRESS
Subject: COWL 'POP'/TAPPING NOISE (TRIM HOOD FLANGE/RELIEVE FENDER STRESS/ LOOSEN-SHIM FRONT BODY MOUNTS)
Model and Year: 1983-91 S/T TRUCKS
THIS BULLETIN CANCELS AND REPLACES TRUCK SERVICE BULLETIN NO. 89-2-139, AND 86-2-128, (GROUP 2B-LIGHT DUTY CAB) TO UPDATE THE SERVICE PROCEDURE AND SERVICE INFORMATION. ALL COPIES OF 89-2-139 AND 86-2-128 SHOULD BE DISCARDED.
Some 1983-1991 S/T pickup and utility vehicles may experience a "popping" noise from the cowl area (at the base of the "A" pillar in front of the speaker grills). The sound may emanate from either side of the vehicle. The exact cause of this condition is currently unknown.
Cowl "pop" is defined as a tapping noise at the lower corner of the windshield or base of the "A" pillar that usually, but not always, occurs when the vehicle is cold (below 45 degrees F). The sound is similar to the noise made when tapping a pencil point on the windshield.
Some success has been achieved in correcting this condition by trimming areas of hood flange that may contact the cowl vent panel, by relieving any stress that may be created by the front fender mounting sequence, and by loosening the #1 body mount and/or shimming one or both front body mounts.
SERVICE PROCEDURE:
NOTE: Typically this condition occurs only when the vehicle sheet metal is at a temperature below 45 degrees F. The condition may temporarily disappear if vehicle evaluation is performed after a vehicle has been in a warm environment.
REMOVAL OF HOOD FLANGE MATERIAL:
Some ST vehicles experiencing a cowl "pop" tapping noise may be corrected by trimming the back edge of the hood flange. On a small number of vehicles, excess material on the rear edge of the hood flange contacts the front of the cowl vent panel causing a tapping noise when the vehicle goes over a bump.
1. Visually inspect for witness marks in the painted area of the cowl vent panel to determine if the hood flange contacts any cowl panel surfaces.
2. If witness marks are not present, it is not necessary to perform Steps 3 and 4. Continue to "Relieving Fender Mounting Stress" Section.
If witness marks are present, trim off approximately 1/8" of the hood flange only in the areas contacting the cowl vent panel using Jilson Tool 1451 SS or equivalent.
3. Refinish the bare metal on the trimmed areas and on matching cowl vent panel areas to prevent corrosion.
RELIEVING FENDER MOUNTING STRESS:
Some success has been achieved in correcting the cowl "pop" tapping condition by relieving the stress caused by the front fender mounting sequence.
1. Remove windshield wiper arms using Kent-Moore Tool J-8966 Remover and installer, or equivalent.
2. Remove cowl vent panel assembly.
3. Loosen the four rear fender mounting bolts to zero torque. (One of the upper fender mounting bolts is located underneath the hood hinge seal.)
4. Retorque the two lower fender bolts to 20-34 N.m (15-25 Lbs Ft).
5. The upper hinge mounting bolt should be left at zero torque. Apply Loctite 242 thread locking compound (GM P/N 12345382), or equivalent, to hold in place. Retorque the remaining upper fender bolt to 20-34 N.m (15-25 Lbs Ft).
6. Reinstall cowl vent panel assembly and windshield wiper arms.
Loosen and Shim #1 Body Mount:
1. Loosen the #1 body mount to zero torque.
2. Raise the body off the mount to obtain clearance between the floor and the top of the body mount.
3. Cut a 4" x 4" square of 1/32 inch thick teflon or nylon (obtain locally) and drill a 5/8" hole through the center. Insert this between the floor pan and the body mount and lower the body.
An alternative material is three washer pump shields, P/N 22063244. If three washer pump shields are used, cut three shields (sheets) to size and locate a 5/8" hole in the center of each shield. Place the three shields between the floor pan and mount, then lower the body on the mount.
4. Apply Loctite 242 thread locking compound (GM P/N 12345382), or equivalent, to the body mount bolt and retorque to 60-80 N.m (44-59 Lbs Ft).
NOTE: Some insulating materials may compress over time resulting in a loss of torque to the body mount bolt. For this reason, nylon or teflon 1/32" thick and the thread locking compound on the bolt are being specified.
Use the most applicable Labour Operation and Time from the Canadian Labour Time Guide.
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.
