Harsh clutch shudder in 2001 Bugeye WRX

[JB9]

Hi all. I know this has probably been discussed a lot, but I hope you guys are willing to help me anyway. After reading a few threads on this, I feel like my car is worse than others with shudder issues.

This is my first Subaru and this issue really is taking away from the experience.

I bought the car with a very bad clutch. It was so bad that it started slipping as soon as you got on any boost at all. Also did feel some shudder on take-off and downshifts without proper rev matching.

So I bought a China Clutch + Flywheel kit. I've done this for at least two other cars in the past and they've been absolutely fine, so I figured: Why not?

Installed it all, did my best to torque the pressure plate down bit by bit in a star pattern, etc. As usual.

Upon first test drive, I quickly realized that the car now has extreme clutch shudder. Drivetrain (and thus, the whole interior etc) vibrates a LOT when slipping the clutch to take off or downshift etc. Its unbearable. Since I felt some of this with the old clutch as well (looked like a stock one), I figured the problem must be somewhere else...

So I've changed: Both engine mounts, all gearbox mounts and the torque mount.

Pretty much no difference at all. Diff seems to be firm in its bushings.

It is like this both when warm and cold. Possibly a bit better when cold but still unbearable.

So now my question is: What else is likely to be causing this? Here are some thoughts:

* Low ride height and therefore some kind of binding issue in driveshafts/axles? Car is on BC coilovers and currently has about 10mm between wheel arch and tyre up front, about 25mm in the rear. Seems unlikely since its absolutely fine when clutch is fully engaged and fully disengaged.
* Driveshaft/propshaft and/or its support bearing and (U?) joints?
* Rear diff mounts/bushings?
* Rear diff?
* The gearbox?
* Or could it be as simple as the china flywheel or pressure plate being warped from factory?

I should also add that the China clutch seems to be a very stiff one.

 

[Ecmbuster]

The "china" assembly can be warped or out of balance.
With a dial indicator - >
Check for crankshaft runout
Check for flywheel runout
Check the transmission input shaft bearing / gear noise Check the output shaft bearing / gear noise.
Is there a pilot bushing / bearing inside the crankshaft?
A hoist test, running or engine off can also check the rotation of the drive line for abnormalities.
A "safe" down hill coast in neutral will eliminate a defective driveline vibrations.
ie: differential, wheel bearings, axle shafts, drive shaft etc.
Torque everything to specifications with a torque wrench.

 

[JB9]

Thank you. Did some coasting today and it feels super smooth. Acceleration and deceleration is fine too. Only two things to note is that the gearbox sometimes needs double clutching to not grind when going in to 1st (synchros probably), and that a noise is heard from the rear diff on engine braking/decel. Hard to describe, but it sounds both a bit like scraping and whining at the same time, so I guess the rear diff is going bad, even though there is no noise on coasting or acceleration. But would a slightly bad rear diff cause clutch shudder?

Haven't done any other testing yet.

Fresh pilot bearing was installed when installing the new flywheel and clutch.

 

[Ecmbuster]

Drain the oil at the rear differential and look for metal debris. Check for too much "back lash" at the pinion, rumbling bearings on the pinion and side gears. Check all the wheel bearings for correct torque, including the wheels.
As for the transmission, grinding should not be felt, drain its oil and look for metal debris.

 

[JB9]

Small update on this:

Installed FWD fuse and removed the driveshaft/propshaft. Drove the car a little bit. Car felt much better, but thats probably because it had no load on the rear output so the center diff softened the harshness. I could still feel the clutch shudder/vibrations, just not as bad.

If they had been completely gone when doing this test, I guess the fault would have been somewhere in the rear, but now I feel like it definitely has to be gearbox, flywheel, or clutch. So lets skip the diff, rear axles, driveshaft etc for now.

Bent crankshaft surely wouldn't feel fine when clutch is fully engaged or disengaged, right? And I feel like it also would make the engine leak oil out of the rear main seal. Engine runs smooth and has no oil leaks (changed the rear main seal and the oil separator plate or whatever its called when doing the clutch change).

Oh btw: The front inner CV joints felt a bit sloppy on the stubs coming off the gearbox/front diff. Not sure if thats considered normal, and/or if that could cause this annoying behaviour. I feel like it shouldn't.