fasted1953

OK, I've searched for hours and haven't found anything on this. 99 GS400, 106K mi. Going around 45 to 55 mph. If I coast then hit the gas, ( soft hard doesn't matter) when I hit the gas, there is a mild thump then accelerates. Like there's slack or sag somewhere on coasting and accelerating puts it back in position. Doesn't happen at any other speed or situation. Its slight and can't tell where it's coming from. Engine mount, u joint bushings? No clue.

Any suggestions?

neojungle1

iTrader: (8)

could be your transmission mount going or motor mount do you feel the car trying to go in one direction evertime you accelerate

fasted1953

No, it doesn't behave any differently except for that bump. Can't tell where in the drivetrain it is from the feel.

Oh, and it doesn't do it in neutral revving the engine so I suspect it's not a motor mount. Just guessing though.

mex lex

iTrader: (4)

Check the driveshaft rubber couplings. Sounds like its your culprit.

neojungle1

iTrader: (8)

^^^^ i second that too good call on it

fasted1953

Took it in to my mechanic this morning. Was able to replicate it for him. He suspects its the pumpkin bushings, but said it could be the rubber couplings. I'll post again when I get some more info. Could also be diff fluid. He said that something is "leaning one way" when coasting and acceleration takes that slack out or pushes it back. He described it as slight but I know that it will only get worse, and drive me nuts while doing so.

fasted1953

Got the call. Its the pumpkin bushings. The rubber bushings that hold the differential housing in position. They said there are six of them that should be changed. A couple of them on top are big as your fist, and they're sagging. There's also two in the back. I think the other two are on his boat. Parts are on national backorder, so they estimate a week to get the parts. I must not be the only one with the problem. Mechanic says those bushings do wear our on GS4's.

Also told me that my diff fluid is tired, and my front brakes are down to 2mm or so. I see an expensive, busy weekend ahead. I'm leaving the bushing work to them.

50 bucks for them to change the diff fluid. I should probably do that myself, dont you think?

AirRideFan

iTrader: (1)

FASTED1953- How much did they charge to do those 6 pumpkin bushings on your GS4 bro? I need to replace mine, ASAP. It's DRIVING ME NUTS!!! The car doesn't feel like it did when I bought it.. (although I remember even last year, about 10k miles ago when I first drove the car home to Pittsburgh (from Dallas) it DID have a real slight lil thump...now it's MUCH MUCH worse, and very noticeable.

Do you know if ALL of your bushings on the diff were the culprit or just the BIG, top two?

Let me know brother... Thx

Navaz

Let us know if it worked for you. Ive been trying to figure that problem out for ten years. I don't think diff mounts are the problem. When I reset te ecu the problem goes away for a while then comes back. I think it's electrical.

Navaz

https://www.clublexus.com/forums/per...eleration.html
my old post for the same problem.
Since then i have replaced.
Transmission & mounts, installed rmm 3.76 / supra diff brand new with mounts. All drive shafts. I even tried swapping out the main ecu with no fix. My lexus delership told me that they will not charge me for whatever part fixes the problem, however i have to pay for parts that dont fix the problem so far im in over $10,000. I have sort of given up. If you solve the issue please chime in with the solution.