I'm the proud new owner of a used, 1991 LS 400. The car has been maintained very well and is beautiful both inside and out - it's magnificent! I've been unable to diagnose and rectify a minor problem with a high frequency vibration/rumble that begins at about 55 kmh and increases with speed. I don’t believe this ‘tremor’ is an inherent design characteristic of the car but that something 'wrong' is causing it.

The vibration resonates a bit through the steering wheel and the transmission tunnel. The vibration continues (i.e., no change) if the car is put in neutral (i.e., drive shaft is disengaged) at high speed suggesting it’s not a drive shaft, u-joint or drive line fault. The car tracks 'true' and does not veer one way or the other. I've had new tires installed, balanced and re-balanced again, and the front and rear wheels aligned but the problem still persists.

I've accessed a couple of other Lexus owner sites and queried about vibrations – answers ranged from possibly bad tires (not likely - I have new Yokohamas), poorly balanced tires (not likely, they've been balanced and re-balanced), lug nuts on the brake pads not being tightened in the proper sequence or torqued improperly, idler arms, bent axle(s) or wheel bearings. I have been assured by the mechanic that everything mechanical (i.e., the entire front end, brakes, the drive shaft, the 'U' joint, etc., etc.) is in excellent shape and is very solid, 'tight' and well within specifications.

Before I embark on what could be a very expensive 'search' by a process of elimination and repair and replace through the local Lexus dealership, has anyone with a LS400 had any experience with a similar problem or educated guesses as to what might be the most likely cause(s) for this problem?

Any help would be much appreciated.

 

I have the same symptoms in my 92 at low speeds. I have dismissed the suspension as everything seems tight up on the rack, I am looking at either a bearing (a stretch) a driveshaft bearing (or coupler), a top strut mount (not isolating road feel from the chassis) or maybe a metal on metal trans mount. Does the car do that at idle, up at drive RPM's? Get back with me on this, well solve this together. GReat car but the way.

 

I've seen this before. Have a mechanic or yourself look at the transmission mount. The transmission mount is probably sitting on the "stop". This is a small raised flat area directly in the middle of the mount. If the top metal part of the mount is touching this raised area, the mount is collapsed. All of thevibrations from the transmission will be transferred through this metal "stop",through, the crossmember, and into your car. Also place a prybar between the engine mount and the metal mounting surface. If there is more than minimal movement you also have collapsed mounts. They do not need to be broken to cause a problem. It is amazing what a worn or collapsed mount will change.

 

My 1990 LS400 has the same problem - Vibration at 55 to 60 and continuing as speed increases.
Car has 165,000 miles the rear Trans mount was replaced at 127,000 miles. ( local shop before I owned car).
Ok now at Toyota dealer - trans mount is bad plus the rear dif. bushing are bad - Tech believes that the rear dif. bushing are causing the trans mount to go bad.

So new trans mount plus rear dif. bushing about $500 in repairs.

Hope this fixes the problem.