Help diagnosing clutch slipping (E36 endurance racecar content)

I have an E36 that we campaign in ChampCar.  We've done a handful of events so far, but only 2 full weekends (8+7), one in late 2019 and one last month.

At both of these events we had wet/damp Saturdays and dry Sundays.  At both events, we lost the clutch on Sunday; in 2019 we lost it after about 5 or 6 hours of racing and had to get towed in, last month it hung on until about 6.5 hours and our driver was able to limp it to the finish line.  In both cases, after the car cooled down it seemed to drive fine.

After 2019 I replaced the clutch with an OEM Sachs M3 replacement.  I don't know if the clutch was original at that point but I do know that we took a slightly abused 328is with 120k miles endurance racing with whatever clutch it happened to have in it, so I wasn't surprised that it was used up. I WOULD BE surprised if the replacement clutch has already been used up, but maybe that's just wishful thinking.  It has approximately 25 hours of racing on it, about half of that in wet conditions.

There IS fluid leaking out from the bellhousing.  It appears to be transmission fluid but I can't be certain.  So... working theories are that the input shaft seal is leaking when the transmission gets hot and oil is getting flung onto the clutch, or the OEM replacement clutch we put in, despite being an M3 clutch, is just not up to the task of endurance racing and needs to be replaced already.

Is there *any* way I can diagnose this without pulling the transmission again, and is there a 3rd possibility that doesn't involved a weekend's worth of work?  Please?  Maybe?

When I was going through clutch hell on my E36 track rat, I found a 1" hole, and a boroscope to be a very useful tool in clutch diagnostics.  Even a cheap $30 one from Amazon is a pretty handy bit of kit for the toolbox.  On a positive note, after you pull it 3 or 4 times, you'll get really good at it (assuming you aren't doing it on the back).

A weekends worth of work in the garage is infinitely better than one in the paddock.  Did you use the M3 flywheel and pressure plate?  I think the clutch disc is the same between the 328 and M3, and the pressure plate and flywheel are different.

I don't think it should be worn out with two weekends, unless one of the drivers is doing something very wrong.  Probably contaminated. :(

Thinking out loud here...is it possible that the throwout bearing is sticking on the input shaft when it gets hot? Hydraulic issue keeping the clutch from engaging?

Tyler H (Forum Supporter) said:

Did you use the M3 flywheel and pressure plate?  I think the clutch disc is the same between the 328 and M3, and the pressure plate and flywheel are different.

It was a single mass flywheel kit from Bimmerworld (the one where you have to intentionally put the clutch disk in backwards), so it was all "designed" to work together.

I hadn't thought about the throwout bearing.  I was sorta hoping it was hydraulic related but those typically result in the clutch not releasing, as opposed to being stuck partially released.

Well... I suppose it will be easier to pull the engine this time around, since I used the opportunity last time to remove all the junk that didn't need to be there...

Fluid contamination will definitely drastically reduce clutch life. Maybe talk to the drivers too, if they're having any issues rev matching and are slipping the clutch on downshifts, excessive heat can also lead to premature wear. Could also be air in the clutch line/slave cylinder leading to partial clutch engagement on shifts?

Bad TOB is an interesting idea. Was that replaced with the clutch in 2019? 

On our 318Ti it had two issues we suspected were a bad clutch but turned out to be 1:  a blown slave cylinder leaking inside the bellhousing.  It failed kinda gradually (unlike a Miata) and was an easy fix.

The second was weirder.   We broke a front sway bar mount which allowed so much front roll that the open diff allowed rolling burnouts at 80mph in 4th gear in an easy corner.  First time it happened I was sure the clutch was slipping (who expects a burnout at 80 with 100 rampaging horsepowers?).  Nope, bad suspension and E36 M3ty open diff.

Just thought I'd update this in case anyone finds this thread in the future.  I bought a cheap boroscope on Amazon and... nothing.  I could see that the clutch was indeed inside the bellhousing and not much else.  I ended up pulling everything and the rear main seal had been replaced at some point, very poorly.  There was a big gash in the seal surface from a seal puller and a bunch of oil had made its way into the flywheel and was being flung all around.

The transmission input shaft seal was also leaking but that leak was pretty well contained inside the TOB guide.

I replaced all the seals and she survived 4 hours of racing this past weekend, so I think I'm good now.