zf transmission leak at the track

[Rev Engineer]

I can't figure it out! I replaced the transmission seals before I completed my swap. I don't appear to leak any oil when driving on the street, but every time I get off the track I have oil dripping onto the cats and whiffs of smoke coming out from under my car. Everything in front of the tranny and bellhousing is dry. The fluid is very clean and has to be trans fluid. The underside of the car is wet with oil from the rear of the tranny and back. After 3-4 track days, I found the tranny .5L down - total capacity is 1.2L. I topped it off and went through the PITA to re-replace both rear seals. No oil leaking after a few days of street driving. Everything looked good until I ran another track day and more smoke.

Maybe it's leaking out the top vent?? Or, maybe it's plugged and pressure is building up inside the tranny? The car gets run hard at the local track - lots of time above 100 mph. I'd suspect that being a problem, but plenty of e36's never have this problem. I'm using Royal Purple transmission fluid. Same stuff that I've got in my e36 m3 and I've run that car at the track too. I'm about ready to give up and resign myself to toping the fluid off after every track event. Any ideas? Maybe I should try fitting a vent catch-can?

[1990m3]

are the crush washers on your fill and drain plugs fresh? Is it leaking from the shifter seal or the output shaft seal? Tough to tell I'm sure while it's in the car...

[kroberts]

I know this sounds like another PITA but you could clean the exterior of the trans and add some UV dye to your oil. Then check it after your next track session.

[PiercedE30]

How does the detent cover look? It's possible it could be leaking around there.

[Rev Engineer]

Thanks for the suggestions. There are no crush washers used for the drain or fill. It's hard to tell if it's the shifter seal or the output, both appear wet as well as everything around the backside of the tranny. UV dye sounds like a neat idea. I might give that a try. The trans is already very clean, just wet with oil.

Not sure about the detent cover. Is that near the top near the shifter?

The strangest thing is that it only leaks when I'm on the track. Weird!

[lateapex31]

Is the trans overfilled? Is the car level in both longitudinal and lateral axes when filling? If you just jack the front up or just one side it is easy to overfill and then it'll piss out of the breather. Royal Purple is also rubbish oil, perhaps is is getting too thin when up to racing temps and is more prone to creep up the inside of the case and out the breather during long corners.

[matt]

Definitely sounds overfull and coming out the vent.

[Rev Engineer]

Unfortunately, it's not overfilled. Tonight I spent some more time underneath the car studying the leak. To start, I topped it back off. After about 1.5 hours on the track, I've lost 250ml of fluid. I topped it off before this last event and that time I was down 500ml over 3-4 track days.

The drivers side of the car is more wet that the passenger side. The oil starts in front of the output/guibo on the drivers and isn't wet until after the guibo on the passenger side. The left rear trailing arm shows sign of oil and the right trailing arm shows just a little. Overall, the underside of the drivers side is much more wet.

At the last track event, I was having some fun with the go-pro mini camera and had it mounted on the back bumper. Through some right-hand turns, it looks like there might be puffs of smoke coming out the back. Somehow, I think the fluid is leaking out the side or top of the trans on right-hand turns. The gubio it completely oil-free and I'm guessing that the seals are not leaking.

Not knowing what else to do, I cut up a rag and zip-tied it loosely over the top of the vent. I'll check it after my first run group. Should be interesting.

As far as the Royal-Purple being rubish... I really like the way the synchos feel with it in the ZF. Redline MTL has always felt too notchy to me and I prefer the shift feel of stock ATF over Redline. On the other hand, perhaps it's not cut out for track usage and is getting to thin. I might try to round up some Redline before the next event and change it over. Certainly worth a try.

[matt]

The vent is on the top right isn't it? Maybe try replacing the vent?