S10/T10 P/U 2WD V6-262 4.3L (1988)
Cowl: Customer Interest
Cowl - Popping/Tapping Noise
Number:
91-351-10
Section:
10
Date:
JUNE 1991
Corporate Bulletin No.: 162001R
Subject:
COWL "POP"/TAPPING NOISE
Model and Year:
1983-91 S/T TRUCKS
THIS BULLETIN CANCELS AND SUPERSEDES DEALER SERVICE BULLETIN NO. 86-58, DATED APRIL 1986. THE SERVICE
PROCEDURE HAS BEEN UPDATED AND THE 1987-91 MODEL YEARS ADDED. ALL COPIES OF 86-58 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 simiiar 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.