Car:
2002 Pontiac Grand Prix 3.8L V-6, automatic transmission
202,000 miles
Problem:
Owner states the vehicle vibrates at around 50 mph when accelerating. Check engine light on, and tire pressure warning light on.
Test Drive:
Vehicle begins vibration at about 50mph and continues above this speed. Vibration is a slight side way shimmy, instead of up or down, or front to rear of car when driving. Hard braking from 50mph produces massive shudders throughout car body. When driving at high speed and shifting into neutral to disengage drivetrain, the vibration is not as present as with drivetrain engaged.
Check engine lights produces mass air flow sensor code and egr system malfunction. However, engine is running smoothly, and not stumbling at any speed, or while parked, or in neutral. Fast to low engine idle speed is smooth and no indications of stumbling or hesitation.
Tires were at 25psi. Filled to 35psi and pressed the Tire Pressure Monitor button to calibrate the warning system at 35psi. Test drive again, and vibration is still present.
Visual inspection beneath car and found driver side CV axle inner boot broken, clamp loose. Passenger side CV axle boots in good shape. Check bearing free play and steering knuckle free play, none found in either system.
Checked all motor mounts, and they are in good shape, no tears or worn rubber.
Brake calipers have some free play in them from worn down brake pads, however this doesn't seem out of the ordinary from normal wear and caliper free play, as brake calipers sort of float when brakes are not being applied. Brake pads have 20% life left in them.
Diagnosis:
Suspect the driver side CV axle, the low brake pad levels to be main culprits which are causing massive vibration above 50mph, and when braking. Also tires are worn, and have 30% life left.
Fix:
1. Replaced driver side CV axle. The original axle inboard boot was torn, and did have some lube left over, but not much. While it was not dried out, rust was present on the roller bearings within the inboard CV joint.
2. Both front rotors turned. The shop technician who turned the rotors indicated the rotors were warped, but were they were able to turn them ant still remain in spec. While most of the rotor was turned true to flat, there was a semi-circular arc that was not touched during the turning process, about 1/4 inch wide at it's widest point. Tech stated that the rotor was indeed flat all the way around despite the half arc present towards the outer edge.
3. New brake pads installed.
4. Front wheels balanced at shop. Each tire required 2lbs of weight to the inside of the wheel rim after removing the original wheel weights from the outside of the wheel rims.
Post work assessment:
Vehicle vibration problem is resolved. No vibration at any speed up to 80 mph. No vibration at any speed with hard braking.
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