Does any one know what make an LS400 vibrate during deceleration? I have already replaced the engine and trans mounts, also completely rebuilt the front suspension. The car seem to only vibrate when you take your foot off the gas. It does it at any speed and it always does it when decelerating.
It is felt in the steering wheel but a little in the seat as well.

 

could be your giubo bushing on your driveshaft, but check the simple stuff first like wheels. are they balanced properly? lug nuts are tightened? what's the condition of the rear suspension? if you rebuilt the front end, the rear end will be in the same condition too.

 

Thanks for the ideas timyOtool. What throws me is that it only shakes when you take foot off the gas, and it is in the steering wheel. Do you think a dealer will be able to diagnose? I can't afford to just guess and start putting parts on in hopes of a cure.

 

check your rotors also

 

Rotors may need resurfacing or new pads. Simple stuff first

 

sounds as if the front rotors are warped

 

It does not do it on braking, just when you let off the gas.

 

does it do it while coasting down in neutral as well?? or only in gear??

if it doesn't do it in neutral then I would suspect a driveline/driveshaft differential issue

 

Yes it does it coasting in coasting in neutral.

 

Okay coast in neutral and apply the brake, does that stop it? When torque is removed in drive then it shakes, it then has to be something that torque holds pressure against. That would be rear wheel bearings, the cv shafts, and the driveline other wise known as drive shaft. So the problem is with one of those, you check by getting under the car and jacking the rear wheels off the ground and moving things by hand and turning things by hand looking and feeling. And what timmyOtool says as well.

 

It does not shake while accelerating but does shake as soon as gas is let off. Putting it in neutral does not change the feel at al. And the last piece of the puzzle is I just changed both rear axles because old boot were leaking. That is when I noticed the vibration.

 

If it happened after changing the CV axles then that is the cause of the vibration, pretty simple stuff. Looks like one is bad or both, its pretty hard not to install correctly.

 

They are brand new axles. How do i test them to see if they are bad?

 

If you're into this type of thing, here's an interesting youtube series on driveline joints and how you can get vibration even with brand new parts.

 

Great video. Thanks. It never quite got to the diagnostic part though. I watched all 3 videos in the series. Do you know where I can find 4,5 and 6?