Roundlake

iTrader: (1)

My cars a 99 NB1

Built motor, ms3 turbo

Brand new poly engine and diff mounts. All bolts have been triple checked including PPF height.

this spring I upgraded to a 6 speed and 3.6 torsen from UK. Both seem to be working well but the amount of drivetrain slop I have is bad.

When letting off the throttle the whole car seems to move or shift back and forth slightly. Also when getting onto the throttle there can be some movement. If I'm aggressive with coming on and off the throttle 2 or 3 times quickly, I can see the shifter move a bit forwards and backwards. I really hate it and would like to fix it before I AUTO X the car. Also letting out the clutch slow with low rpms makes a similar noise.

The car wasnt doing this with the old 5 speed and 4.1 torsen which leads me to believe the axles and driveshaft are fine.

Is it possible the Diff needs adjustment?
not sure what else to check or try.

sixshooter

iTrader: (12)

The shifter shouldn't be moving around much if the motor mounts and diff mounts are new poly.

And neither being messed up should change the direction of the car on or off throttle.

Check for worn rear control arm bushings and possibly rotating alignment bolts back there (not uncommon).

And please double check all of the mounts on engine and diff for properly seated poly and that all the bolts remain tight. Poly makes bolts loosen due to extra vibes if not Loctite(d).

Soy

I have this same exact issue ever since i upgraded to a 4.1 torsen from an open diff. dont have a solution for you sadly. I think my issue has something to do with backlash on the ring and pinion, my backlash comes with serious gear noise on decel. im about at the point of just buying another diff or getting it sent somewhere for a refurb

Roundlake

iTrader: (1)

I double checked all the mounts today. They are tight and good. Rear bushings are good.

Maybe the tune needs some more work with fueling on and off throttle?
or backlash in diff?

WigglingWaffles

In for answers, have been living with the same issue.
Always suspected a mix of slop within the driveshaft, a junky 6 speed or backlash on a eudm 3.6 fuji but my old 4.1 torsen and 5 speed was the same way. Sorta chaulked it up to my tuning skills being subpar for on/off throttle stuff.

Roundlake

iTrader: (1)

Well I let you know what I find out. Probably going to take the diff out soon and get it checked. And in the spring ill ask my tuner about the on/off throttle.

Icedawg

If there is too much backlash in the diff you can test it without removing it. Just lift the back end and rotate the wheels back and forth to feel the free play. Check against spec.
Check the drive shaft for free play back and forth (testing pinion and bearing free play, should be none) and for bad u joints (rotational free play before engaging diff).
Inspect sub frames front and back for signs of cracking.

Roundlake

iTrader: (1)

Awesome thanks. Do you know what the spec is for the backlash?

Icedawg

not exactly, but it should be less than a quarter of a turn. You need to be careful to make sure you are only exploring the backlash. The wheel applies enough torque that is harder to judge. If you take the wheel off and just rotate the brake rotor you do not have as much torque and it is easier to tell the difference between turning the gears versus just the backlash.

A quarter turn means it is well worn. Less than an eighth means it is in pretty good shape. If it is the cause of your problem it should be well on the high side.
I think it is more likely to be suspension part problems, as others have suggested.
You have checked the engine and diff bushing bolts, what about the long PPF to trans and diff bolts? Especially the diff, with the odd design used to secure the threaded portions on the PPF, if those fail they will spin and never tighten properly.

Roundlake

iTrader: (1)

Thanks for the great info. I'll take a look at the diff and see what I find. All the ppf bolts have been re torqued yesterday. None moved and were all tight. I did install coilovers at the same time as 6 speed and torsen diff. I just don't see it being suspension bushings because it wasn't doing this before the 6 speed and diff swap....
my dad (mechanic) checked the rear bushings and says they are fine. Theres slight movement with pry by from side to side on rear control arm bushings. This is only when you stick the pry bar into the end of bushings and pry moving the whole control arm over a slight bit. It returns to center after.

. I believe this is normal and was confirmed by him as well.