help diagnosing clutch failure mode please

[jasper]

2002 996 C2

190,000 km

Driving home two days ago in traffic. Go for shift from first to second, clutch pedal drops to floor sharply, gear engages but balks hard, clutch pedal comes back to top position weakly as if actuated only by the helper spring.

I manage to get home - making total of two dozen or so more shifts, each shift is the same, light clutch pedal pressure on way down, balks hard going into gear, clutch pedal only comes up about half way, then spring up the rest of the way - as if the clutch helper spring is doing the work.

No slippage detected, although I used only first and second gear so no lugging. Very difficult, but not impossible to select first while stopped at traffic lights.

History wise - a factory reman engine was installed by a dealer under warranty at 70,000 km (long before I owned it). There is no record of new clutch being installed at that time but I have to assume that one was.

The clutch has been behaving well up to that point. No slippage or chattering detected prior to failure.

No leakage detected at either the master or slave, the reservoir level is much higher than the clutch hydraulics line offtake and it has not moved.

I am trying to rule out hydraulics because I need a reason to drop the transmission so I can replace the IMSB. A friend lost a motor last week and I'm feeling nervous. I started a thread over at Rennlist to document this job but I can't get cleanly past the first step - ruling out the hydraulics.

Thread is here: http://forums.rennli...retty-sure.html

The car is in the air, belly pans are off, and I've removed the clutch slave from the transmission, hydraulic lines are still connected. There is no fluid behind the boot on the slave. I am able to push the slave piston in by hand - it gives light spring pressure only. I have not tried to push it all the way in.

Before I removed the slave I had a helper press the clutch pedal in and out. With my ear to the bulkhead I could a rushing/wooshing fluid noise as he did so. I also verified that only the clutch helper spring is brining the pedal up.

So is your money on a broken pressure plate or simply a failed master cylinder seal???

thanks

jasper

Edited May 5, 2012 by jasper

[RFM]

When the slave cylinder is removed from the transmission, with the hydraulic lines still connected, there should be a high resistivity while pushing the rod back into the slave cylinder. I would go for a check of the hydraulic system in first place.

[jasper]

Thanks - will try this again and report back. I pushed the piston in last night, but not all the way. I could feel only light spring tension. I didn't get in far enough to push against the hydraulics. I note that the hydraulics are good enough to allow me to select gears, and that this pressure remained good enough that I could select gears even after some time had passed.

God forbid I have to bolt the slave back up and try bleeding the system.

[jasper]

OK after a real check I can report that the piston can be pushed all the way in, with considerable pressure required from the heel of my hand. I can also report that when I reach up and in to the bell housing I can move the throwout lever quite a few milimetres with no resistance whatsoever (it flops around) but that I can not depress the clutch diaphragm springs. Not a fair test though..I have no leverage whatsoever of the clutch diaphragm from where I was lying on the floor.

Please keep comments coming!

[jasper]

Hows this for a test:

Removed the actuator rod from the slave.

Pushed the piston all the way in and inserted a 3/8" punch into the piston where the rod normally rides.

clamped the system so that the piston can't move out when the clutch pedal is depressed.

Go inside the car and tried to press down the clutch pedal and it moved about an inch and stopped. I pushed pretty hard - but not enough to blow the seals obviously.

Seems to me the hydraulics are good.

What say you?

Anyone?