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! Unfortunately, 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 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.

My first recommendation would be to take it to a Lexus dealer. They know their cars inside and out. Being a technician, I see 400's all day. Congrats on the purchase, it truely is a fine auto. Have them check the ball joints, and radius arms. Being as old as it is, the rubber bushings are more than likely worn. Where the lower control and caster arms mount to the frame, there are large pieces of rubber that dampen the loads. I've seen plenty that the caster bushings crack and don't absorb as much as they should. Ball joints on those are known to go out after 100k+. Next start looking at motor mounts and tranny mount. Being a higher speed vibration, that's all I can think of off the top of my head. I highly recommend taking it to a dealer so you have a trained set of eyes look at the car. You'll have to pay the diagnostic fee which shouldn't be more than $100, but you'll know exactly what it is and you have it on paper. You don't have to have it fixed there either... Though I do recommend it. Personal preference doesn't always match the wallet though.