Hello all,
I'm getting a pretty bad vibration: Some days there's nothing at all, some days it's terrible and I've almost pulled off the road. It starts around 40-45 mph but once it's started it can keep vibrating even down to around 25-30 mph. No difference if I change gear down to 4th or 3rd.

I thought it was the rear engine mount, which I've changed for a new one but no difference, though initially I thought it helped. Today I'm planning to check the bolts holding it on haven't come loose. Anything else obvious I should be checking? It feels identical to my older 96 LS which was completely fixed with a new rear mount.

Thanks for any help!

 

Where do you feel the vibration specifically? Can you feel it through the steering column? Do you feel it in the floor? Does it feel more intense in the front or the rear?

I had a bad vibration once I could feel through the column due to losing wheel weights.

 

The whole car shakes, I think it's speed related but its hard to tell. Pure guess it's around 10 Hz. I know it's not the wheels since I put my ice tires on recently and same thing.

 

When it starts, put the car into neutral, does the vibration remain? Should help narrow down where it's coming from, and, whether parts of it are rotational (wheels) or driveline.

When you say rear motor mount, do you mean the transmission mount? The LS400 has 2 motor mounts along each side of the engine, and then one single transmission support\mount thingy way at the "rear".

 

Good idea on the neutral thing, I will try that my next drive tomorrow. Thanks for the great idea!

And yes, the transmission mount, just above the center crossmember. I always thought that was known as the rear engine mount.

Hey if it's a problem with the drive shaft, is it ever obvious from just pulling and heaving around if anything is loose? Something is definitely really badly worn, but the vibration only happens on some days, may be temperature related by I haven't noticed and strong patterns yet. If it happens again I will post a video here of how it sounds.

 

we'll have to know if it's speed dependent or rpm dependent... are certain road surfaces worse than others?

these engines are super smooth to begin with so even a bad mount wouldn't cause such a severe vibration, my guess is a combination of tire balance and worn suspension bushings

edit: ok didn't see the part about the winter tires making no difference, now i'd look into drive shaft/prop shaft

 

There is an argument for the center support bearing being worn, and the play from that causing driveline vibrations. And the neutral test won't rule that out, as the driveshaft is always spinning.

It's hard to get a look at it, as it's covered by shielding panels so you would need to take those off to get a close inspection. But you should be able to at least visually take a look at it from the rear of the car.

Stroock is also right - even something as simple as worn control arm bushings (as I've learned) can have a big impact on ride quality and micro-vibrations introduced into the car. But even with those, the car should ride fine on glass pavement. When they're worn, they amplify any road imperfections into general NVH.

These are just a couple of blind guesses, these things are hard to track down.

 

inconsistent vibrations that are so bad you need to pull over would almost always point to the driveline/driveshaft. i suspect the center carrier bearing as well; when it gets to a certain harmonic it deflects and causes bad vibrations! low speeds it's smooth. check there.

unbalanced wheel/tire and torn bushings can also contribute but not cause such a racket that you need to pull over.

 

+1 to Timmy's suggestion. I'd check the drivetrain especially the guibo.

 

The driveshaft is my next project. My guibo coupler has some hairline cracks developing, nothing seriously concerning yet, but will be the next major thing which needs to be addressed.

I suspect most, if not every car, will be in a similar situation. Maybe not necessarily requiring immediate service, but those driveshaft components are going to need some love before long.

How exactly can the guibo contribute to vibrations or harshness? I know the center support bearing deteriorating can allow the driveshaft to flex in ways it shouldn't, causing issues. How does the guibo coupling do that?

 

It’s basically a rubber bushing that spins with the driveshaft absorbing a certain degree of resonance.
We all know what happens when rubber parts age…

 

Thanks all! I did change the front right control arm a month ago, but that was a different issue - knocking on road bumps, especially around corners. That part is solid now - although I will add that the bolts came loose a little and I thought it was just a really bad arm! It's alright now.

What's the level of difficulty on these drive shaft components, like changing the guibo? I try to do most stuff myself if it's not going to take me days!

 

This guy does it in his driveway

 

Just a quick update - vibration unchanged on going into neutral.

Haven't figured out the pattern yet, but some days its smooth as from the factory. Trying to find time for this - getting new home heatpump installed tomorrow lol

 

gotta get under there and check out the driveshaft, after the heat returns to your home !