88 Clutch not releasing, help pls

[jclporsche]

Did a rebuild to 3.4L, added a Wavetrac ltd slip, KEP alum pressure plate, had my clutch disc recovered with kevlar at a local industrial brake supply company. Drove about 800 mi break in in the hills nearby. Yes the kevlar is grabby. it wants to buck, especially in reverse. I got used to it, have to almost pop the clutch to avoid the bucking.
One day the clutch pedal started loosing the ability to release the clutch, I had to double clutch it to avoid grinding gears. I figured the hydraulic cylinders were worn. Originals & 24 years old as far as I know. So I put in a new master & slave clutch cyl. Used a pressure bleeder. Seamed a little better, but as I drove it got bad again. Particularly after a fairly hot take off. Tried adjusting the rod at the master cyl to get more push. Only helped very slightly and now it is at the last 6mm of it's engagement in the clevis.
So I'm thinking, what could be wrong? is the clutch disc coming apart or buckling? Maybe that kevlar recover was not such a good idea. Chit. Out she comes.
The disk is flat, not coming apart. Some burn marks, but probably from me letting it slip a bit once or twice to try and break it in to the plates to see if that would help the bucking & grabbing.
Here are pics of the p plate, disc front & back and flywheel, which was resurfaced at rebuild. They are all flat, not warped. The disc measures 9.2-9.3mm thick on the Kevlar.
Could it be expanding when it gets hot and warping?
Any ideas why it wont release?

[Grady Clay]

My first observation is that there is possibly something wrong with the disc.

First, a standard disc (even reconditioned) should not ‘judder’ as you describe.
There is a spring system between the two friction surfaces on the disc that should prevent this.
Perhaps this system was damaged in the reconditioning.

Another ‘clue’ that something is amiss is the local over-heating of the disc, PP and flywheel (red arrows).
It seems there is disc ‘clamping’ around the outer and inner perimeters and not in the middle of the disc.
I suspect this is related to the ‘judder’.









I suspect that if you simply install a new Sachs OEM disc, everything will work properly.

Best,
Grady

[jclporsche]

Thanks Grady,
That was kinda my plan before I got it apart (get a new disc I mean). I read somewhere Porsche recommends only new replacement discs, never recover one. I figured they were just trying to sell parts, but now I know better. So much for saving a few bucks.

[Porscheman912]

Wow, that is very strange, never seen that one before.

[dshepp806]

Very nice observation notes, Grady,...thanks!

Doyle

[jclporsche]

Ok, new Sachs disk is in and it is still not releasing! The only thing left that it could be (besides possible air in the line, and I ran about a quart of fluid thru using a pressure bleeder gitting no bubbles) is the the release fork. It has slight wear on the finger tips that pull the throw out brg. I didn't take a pic of it. It didnt look that bad, no cracks or bending. I dont know how much is worn off, could 1 or 2mm loss there cause this?
I will order one and try to change it without droping the engine & trans again, though I'm getting damned efficient at doing that.

[Flat6pac]

I will order one and try to change it without droping the engine & trans again, though I'm getting damned efficient at doing that.
__________________
MMMM good luck....
Bruce

[fastfredracing]

Have you looked very carefully at the fingers of the pressure plate. We had similar issues with a buddies Carrera, and upon taking it apart , found slight cracks in 3 or 4 of the fingers of the diagphram of the pressure plate. The markings on the clutch, and flywheel were similarly localized. Does the pedal feel firm, and start to disengage the clutch near the top of it's travel? They can be a bugger to bleed properly sometimes.

[aerodyn930]

++1 on the pressure plate, We recently had a similar issue in a customer's car, Same pressure plate , We found the press. plate diaphragm spring cracked near the fulcrum point,
test the press plate by placing it in a hyd press and using a suitable arbor duplicate the pull of the throw out fork by pushing on the throw out bearing, you may find a discrepancy by watching the surface of the press. plate as you do this.

After you find your problem verify that you have the correct travel in the clutch master cylinder as you can damage a press plate by moving it beyond it's design limits.

Last if bleeding the clutch becomes a problem, Hook up a suitable hose between the right rear caliper and the clutch slave cyl. , Open both bleeder valves and pump your brakes to purge the clutch system, Close both bleeder valves and you should have a nice firm clutch.

Hope this helps, Lou

[jclporsche]

Fastf, Thanks for the thought, it's a brand new KEP light weight pressure plate. So the fingers are new. The pedal feels normal, yes firm all the way through it's stroke. Just not releasing until the pedal is ON the floor board, even then the clutch is slightly engaged still. Can't get into gear with the engine running.
I'm thinking I might insert a couple of shims down through the inspection opening in the bell housing and put them between the fork and the throw out bearing tabs. Just to see if replacing possible wear off those fingers will make it function. Then I will know its a worn release fork.

[jclporsche]

Here is a gif of my clutch fork during full pedal travel. it is turned 90 deg, I couldn't rotate the gif. The movement is about 20mm at the top (where the slave cyl pushes). Anyone know if that is normal full stroke? I can move it about 2mm with a screwdriver before it pulls the thrust brg tight.

[jclporsche]

At rest, no one in the car, clutch fully engaged it has about 2mm of movement at the top of the fork before it pulls up to tight on the thrust bearing assembly. So I wondering if I put a new fork in with unworn fingers, that should give me the additional dimension to fully uncouple the clutch? I don't think I can get the shims in there like I was thinking, just to try it out.
What I' struggling with is how it went from totally normal clutch function to not disengaging in a matter of just a few miles. Could the new throw out bearing with its un-broke in pull tabs have accelerated the wear of the old release fork fingers? Are the release fork fingers surface hardened? Maybe it wore through the hardened surface over the years, then just happened to finish it off during my break in?
I will be taking it out and replacing the fork, just want to cover every possibility. A fourth engine pull will.... well I don,t want to think about it (make me an expert, yah, that's it).

[jclporsche]

Thanks for all your comments so far, you guys are always very helpfull.
Anyone know about the stroke dimensions at the top of the clutch fork as viewed through the access port on the trans bell?

[Tippy]

People have reported issues with KEP PP's. I've had no issue with 2 different ones.

I have a Kevlar disk and it's buttery smooth. I dare say that is not Kevlar from the looks. From what I've read too, Kevlar virtually does not wear out. I have launched my car several times in 2nd WOT and never made much of a witness mark on the PP nor flywheel - the beauty of Kevlar.

Did you change out the TO bearing guide tube???

[jclporsche]

Tip,
The t.o. guide tube was updated about 40k mi ago at the last clutch job. It is the updated diameter & was in good condition. Reason I replaced disc & pp this time is it got a little oil or grease on disk & I wanted more grip for the new motor.
The locally recovered Kevlar disc is out now, replaced with a brand new Sachs from our host. The first time I pulled out of the garage, no more jerking & grabbing like th Kevlar one did, but again, the clutch was engaging about 1" off the floor. The next thing I know, I can disengage it at all again.

[jclporsche]

I have about 1200 mi on the KEP pp, didnt start having trouble (other than the gerking in 1st & reverse) until the last 100 mi or so.

[Tippy]

Looking at it again, I see that the fact it wasn't releasing is why it is blackened initially making me think it wasn't Kevlar since mine stated a nice yellow after many miles of use.

Oil on disks can produce the "on/off" sensation since oil tries to suck the two together tight until the bond is broken making a jerky release over and over.

[jclporsche]

Tippy,
When this all went together it was a freshly rebuilt engine, no more oil on the clutch or flywheel. It's clean & dry in there now. The burn marks are from me loosing it when it started to grab & buck one or two times. I took it up to about 3500 rpm & slipped the clutch a second or two thinking I could smooth out the bucking. It only produced a bad burning kevlar smell, didnt change the grabbiness of it. In fact soon after that is when the non-release issue began. First I figured the hyd cyl's were going. Changed them both out and no fix. That's when I thought the disk was warping or coming apart. Then I got a new Sachs clutch disc to put in. Thought that would solve everything.
Only thing left that I can see is the fork and the short rubber hyd line. everything else is new now.

[jclporsche]

Well I finally got time to get back into this problem. The new KEP pressure plate has 2 cracks! Monday I will try to find out if it will be replaced under warranty. I may just put the old Sachs pp back in (20 lbs vs: 12 lbs, and not as much pressure?).
The release fork has a little wear, but not bad looking to me, what do you guys think? Another $250 for a new one worth it?
I also noticed the Sachs pp has travel stops where the KEP doesn't.

[jclporsche]

I just want to be driving her again!
Thanks again for any & all advice!