Nasty Vibration Above 65 Mph And When Braking

[Quack11]

91 LS 400 with 141K and in excellent running condition until last week. Car vibrates real bad above 65 MPH and sometimes at slower speed. The car does the same when the brake is applied, the problem is so bad you can see the gear shifter moving back and forth. This does not happen all the time and I've noticed that if I take off rapidly, the car drives awesome until its time to stop. I've changed both front and rear brake pads and rotors. Inspected the entire front and rear suspension, checked engine, transmission, differential and drive shaft mounts. I am not too satisfied with the condition of the drive shaft mount and the differential mounting bushing. Any assistance will be greatly appreciated. I searched the forum for previous post on this issue but did not find any. Thanks.

[Quack11]

I also did a wheel balance and alignment with inspection of all suspension bushings, everything looks good.

[curiousB]

bent rim which should show up on tire balance. Maybe your tire guys aren't too thorough. Rotate front to rear tires and see if problem changes. If same problem in front then its bent spindle. Hit a curb or pothole lately.

[Quack11]

I balanced the wheel and rotated them three times in the last week since problem started. The car was driven maybe a total of 50 miles in the last month and the road driven is in great condition. No maintenance was done on it prior to this issue. Transmission shifts as advertised, no issues so that is why I narrowed it down to drive shaft mount, plus I inspected the center support and it does not look too healthy, the donut portion has many small cracks and it is not centered but I have nothing to reference it to.

[andre]

hello,

had similar symptoms = the problem was a stucked valve in a brake caliper

try, after driving for few minutes, to spit on a brake rotors ( to check if they are hot )

and if your spit bounces of , you know they are VERY HOT...

and then you will know it will be the stucked valve

good luck

[VBdenny]

You don't have to spit on the rotor to see if it is hot, just feel the wheel after driving awhile and if it is much hotter than the other side, you know it might be sticking. A vibration that developed so quickly indicates that something changed. I would be looking very closely at the tires if the rotor proves not to be overheated. A belt can break in a tire at any time and that would certainly cause such a vibration.

[Quack11]

Thanks Andre, I was questioning the brake being overheating due to a bluish color on the rotors which is an indication of excessive heat. I will check this on my next run. The tires are in excellent condition with no deformation of any kind so i've ruled out any bad tires idea. Will keep the forum updated on any progress.

[RDM]

Thanks Andre, I was questioning the brake being overheating due to a bluish color on the rotors which is an indication of excessive heat. I will check this on my next run. The tires are in excellent condition with no deformation of any kind so i've ruled out any bad tires idea. Will keep the forum updated on any progress.

There's your answer, a blued rotor means the caliper is sticking, and the rotor is likely warped. That would lead to the vibrations you're getting, especially when braking.

[curiousB]

There's your answer, a blued rotor means the caliper is sticking, and the rotor is likely warped. That would lead to the vibrations you're getting, especially when braking.

I agree blue neans a lot of heat was there. Probably have to replace rotor at the same time with a new or rebuilt caliper. Other symptoms (other than heat) are slight pulling to one side and lousy gas mileage.

[Quack11]

There are two small phillips head screws that secure each rotor to the hub. Can the vibration I described be created if none of these screws (four total for both front rotors)are installed?

[curiousB]

There are two small phillips head screws that secure each rotor to the hub. Can the vibration I described be created if none of these screws (four total for both front rotors)are installed?

Probably not. Those screws are just to hold it in place before the wheel goes on. The true holding mechanism is the wheel and the five bolts sandwiching the rotor to the spindle hub.

If the rotors or hub are dirty or some debris on them that could cause rotor to not sit flush on hub face which would cause a wobble. That probably wouldn't explain the blue color to the rotor. I think sticky caliper is best bet.

[Quack11]

Problem solved......hopefully. I replaced the four rotors with brand new one but the rear rotors are defective. I returned them for replacements and the problem is gone. Just got off our local high way and at 90+ she is smooth sailing and stops like a parachute was attach to the back. At 141+K miles, I can't believe how smooth and quiet this car is. The suspension still absorb every defect in the road and the cabin is still sound proof. How much longer can these cars go? I'm the second owner and the car is still 97% original. Thanks all.

[andre]

:)

good news...

my LS is '92 and done 350klm and is still humming like a bee...

after 4 years i still enjoy the refinemets of this engine = the best engine

and comment about how silent it is inside...

ps.

how much longer can theese cars go?

for as long as there is a petrol in the tank :)

regs

[VBdenny]

Hey hey, nothing like a problem solved. I liked the part about 90+ since I run mine that fast on the Jersey Turnpike sometimes. When these puppies are running right, there isn't much close.

[curiousB]

Problem solved......hopefully. I replaced the four rotors with brand new one but the rear rotors are defective. I returned them for replacements and the problem is gone. Just got off our local high way and at 90+ she is smooth sailing and stops like a parachute was attach to the back. At 141+K miles, I can't believe how smooth and quiet this car is. The suspension still absorb every defect in the road and the cabin is still sound proof. How much longer can these cars go? I'm the second owner and the car is still 97% original. Thanks all.

Make sure to check they aren't overheating. A dragging brake shoe or sticky caliper piston could leave pressure on pad causing it to heat up and warp. Your parts supplier won't give you another set if you return with warped ones again. After a lengthy drive are the rotors and surrounding metal hot (don't touch directly you'll get a bad burn).

[Quack11]

Good call, the left rear caliper (original part) was difficult to push back with the C-Clamp so I will be replacing it this weekend.

[curiousB]

Good call, the left rear caliper (original part) was difficult to push back with the C-Clamp so I will be replacing it this weekend.

Before replacing it I'd just check temperature of that rotor/wheel vs the others after a bit of a drive. If its about the same then no problems. If the pad is dragging it will be a lot hotter than the others.