Help me to troubleshoot. Rear shaking at speed
 

Hey guys, I did a lot of searching before posting this and came across far too many "maybe"'s and partial information.

I'm trying to take an informed route to figure out what is plaguing my car because it's easy to take everyone's unique advice and spend several hours and a TON of money on diagnosis labor and parts.

So here's the setup:

1997 UCF21 Celsior. Same as your 98-2000 LS400.

Recent maintenance: replaced engine mounts and inspected transmission mounts. All good now. Wheels and tires balanced. Alignment done.

There is a pretty annoying shake coming from the rear (shakes the seat, not steering wheel) of the car that starts around 110 mph and is pretty annoying until about 135kph where it tapers off and is very minor (yet present nonetheless) It feels EXACTLY as it would if your rear wheels were way out of balance.

I have already double checked and re-balanced the wheels/tires. One wheel was out 2 ounces but even after correcting this the shake was just as present. The vehicle's alignment is spot on.

I'm extremely nervous about what to try next or how to diagnose further simply because parts are NOT aplenty and they are far from cheap. Trailing arm bushings are $100 + shipping even on eBay and that may not be the issue. I could also be looking at a driveline issue but it's a huge bill and a lot of downtime for me to remove the driveshaft and bring it to a shop here to be checked out.

This is such a good car but I am cleaning up after the previous owners lack of care and am slowly getting murdered with bills. Since importing it I have changed every fluid in need of service .. replaced brakes, tires, bulbs, filter, a collapsed motor mount etc etc etc.

What say you guys? Help me out.

How many miles/Km on car? Rear carrier bushings and any number of bushings in the rear multi-link can be due for replacement.

56000 miles

could be driveshaft bushings and/or center carrier bearing.

does the vibration come in and out smoothly even when rpm/speed are constant?

Big difference
 

[/QUOTE] starts around 110 mph and is pretty annoying until about 135kph [/QUOTE]
So, which is it, mph or kph?

fixed. stay tuned for my next regretful thread.

the tire had de-lined so even though it balanced fine ... twas not round.

u mean the tread separated? at 110 mph? ur lucky ur alive

He's from SK, which I'd guess to be Saskatchewan canada so it's likely km/h but he may have an imported US car...

And I was going to say if the tires are balanced and the car is aligned properly, the most likely cause is a separated or bulged tire.... And definitely lucky to be alive if you were actually going 110mph.