Strange vibration

[J.DeFeo]

I have a vibration problem! I had several, and now I'm down to just one.

It's not wheel/tire roundess or balance since I just had both of those checked on a GSP9700 and they're perfect.

It's not thrust arm bushings since there's no change in the vibration during braking.

It happens once I get up to highway speeds - it's a vibration that almost seems in sync with the engine speed, but it's not - I can push the clutch to the floor on the highway and the vibration remains. Which means it's either driveshaft or some other suspension component. It is related ONLY to road (or driveshaft) speed, not engine speed. It is not noticeable until above 60 MPH.

Here's the strange part though - it basically disappears after 10-15 minutes of driving. It can be felt through the armrest BARELY, but that could just be uneveness in the road. It is very noticeable when I first get going in the morning but by the time I've been driving for a bit it's more or less gone. I pray to god that it isn't the center bearing (I just redid the clutch and replaced it, I pre-loaded it according to the Bentley instructions, new guibo and transmission mounts as well.

I noticed two rubber mounts on the rear stabilizer are cracked and look pretty deteriorated. The bushings up front all look fine. Could those rear mounts be causing this? I'm pretty sure my engine mounts need to be replaced as well, but because the vibration has no correlation to engine speed I'm skeptical that they could be causing this.

Would it be worth dropping the exhaust and checking the center bearing just to make double sure it's on right? I'm 99.9% sure it is, I just did it maybe a month ago. The bolts were flush with the back end of the mounting holes; i.e. the bearing was pushed as far towards the front of the car as it would go (about 1/4", as specified in the bentley manual) then bolted down tight.

[Paul in NZ]

sounds like your tires...can get a flat spot overnight,once they are warmed up and been spinning for a bit they even out.

[J.DeFeo]

sounds like your tires...can get a flat spot overnight,once they are warmed up and been spinning for a bit they even out.

Does that happen even if they're brand new? They have like 200 miles on them. Is there a way to fix that?

[Morgenster]

Does that happen even if they're brand new? They have like 200 miles on them. Is there a way to fix that?

I remember a post that says especially when new and it gets better.
I had the exact same problem. Then I had the wheels changed and new tires.
I'm pretty sure the old wheels and tires were badly balanced because they were mounted in a hurry at the shop.
It was much better after the change-over but still noticeable and now a month and a half later (3000kms on new tires) it's virtually gone.

[J.DeFeo]

Ok, the tires now have about 5,000 miles on them and still the same problem.

I'd just like to make absolutely sure that this isn't a driveshaft issue. Let's say I put the two pieces of the driveshaft back together at the U-joint 180 degrees off (I did mark it but I guess it's possible); this would vibrate at low speeds too, right? By virtue of only vibrating at high speeds it pretty much rules out that screwup?

The vibration is temperature-dependant. It's worse on cold days and almost unnoticeable on warm days. It also completely diappears at around 78mph.

I noticed that the vibration gets slightly worse at 70ish mph if I let off the gas with the clutch still engaged. I read that this indicated a rear-end problem. Sway bar links, maybe?

[Morgenster]

Ok, the tires now have about 5,000 miles on them and still the same problem.

I'd just like to make absolutely sure that this isn't a driveshaft issue. Let's say I put the two pieces of the driveshaft back together at the U-joint 180 degrees off (I did mark it but I guess it's possible); this would vibrate at low speeds too, right? By virtue of only vibrating at high speeds it pretty much rules out that screwup?

The vibration is temperature-dependant. It's worse on cold days and almost unnoticeable on warm days. It also completely diappears at around 78mph.

I noticed that the vibration gets slightly worse at 70ish mph if I let off the gas with the clutch still engaged. I read that this indicated a rear-end problem. Sway bar links, maybe?

OK now that's scary and exactly the same as my problem. Now that winter is setting in the vibration's came back and disappear when the car is warmed up (especially after idling some time). I'm thinking more along the lines of guibo, center bearing or diff mount.
What I think both of us should try now is see if the vibration persists when we declutch at that speed.
If the problem persists when you declutch it's likely a tire/suspension problem. If not It's driveshaft-related.

On another note: does your car have a tendency to buck when cold?

[J.DeFeo]

OK now that's scary and exactly the same as my problem. Now that winter is setting in the vibration's came back and disappear when the car is warmed up (especially after idling some time). I'm thinking more along the lines of guibo, center bearing or diff mount.
What I think both of us should try now is see if the vibration persists when we declutch at that speed.
If the problem persists when you declutch it's likely a tire/suspension problem. If not It's driveshaft-related.

On another note: does your car have a tendency to buck when cold?

Letting off the clutch doesn't change anything, but even with the trans in neutral the driveshaft is still spinning, which wouldn't necessarily rule out a driveshaft related problem. Both my guibo and center bearing are basically brand new, less than 10k miles on them.

Yes, my car bucks when I let off the throttle completely when it is cold occationally. I've had it happen in 1st-3rd gear. When the engine is warmed up it doesn't happen.

I'll have to check my diff mounts. I replaced the transmission mounts at the same time as the guibo/center bearing and the old ones were pretty worn (smooshed about 2/3 the height of the new ones).

[ber55ber55]

Hey guys, I have slightly the same problem.

I dont have any sounds/vibrations at low speed.

At around 65-70 and above there is an extreme vibrate.

I recently redid the guibo, centering bearing, new fluid in tranny, and new tires. I was also thinking it is the diff. Either fluid is low, needs to be replaces, or the bushing are toast.

When i hit the gas it persists, when i roll at high speeds in gear, it persists, but when i put the car in neutral or step on clutch......gone....

Its not affecting my driving so im not toooooo worried YET. Let me know if you guys find something. Thanks

[Brandon J]

2 things come to mind. Flat spot tires and hard cold suspension. The tires are flat spotting more in the cold. The tires are cold so it is less compliant for an exisiting problem. The suspension is cold so the shocks do not work as well until warmed-up.

So, it kind of sounds like there is an existing problem that gets masked when the tires and shocks (and or suspension) get warm and more compliant. When you did the center bearing, was it aligned correctly. I have seen several center bearings that were put in with the preload, but not installed perfectly perpendicular to the driveshaft.

Sometimes the thrust arms don't give a vibration when braking. Sometimes the bushing sits better when braking and there is no vibration. So don't rule the thrust arms just yet. Since you do feel it in the arm rest, great description, then yes we do look at the rear. If not tires, sometimes wheels can give this, rear bearings, rear halfshafts, shock mounts, rear contol arm bushings, front bearings, etc.

Great description of the problem btw. Good Luck.

[Sam-Son]

have something similar to this and I think it may just be the tires they have about 15k miles on them and my avons are known for getting abit...rough when they get to this mileage...well I Hope it the tires