transmission blowing fluid out the vent...why?

Posted this on bimmerforums and didn't get much response. Hoping some of you can weigh in with opinions/suggestions. I've never encountered anything like this in all my years of messing with cars.

My 93 740i w/ 203,000 miles just recently decided to blow a lot of tranny fluid out the vent hole on a two hour interstate run. Saw smoke behind me (fluid on the exhaust pipe), pulled over and the back of the car had a good misting of fluid. I first thought it was oil, but later realized it was trans fluid. A couple of days after that trip the 'trans program' popped up on the display a 'failsafe' that locks the tranny in 4th gear, I guess to minimize potential damage from whatever the ailment is. I did a pan drop/filter/castrol multi vehicle import fluid fill about 30K miles ago. Now after dropping the pan (it was clean) and putting in 6-7 more quarts of the Castrol, all is well. But what would cause this and will it happen again?

Trans is a ZF 5HP30 five speed and there is some debate over what fluids are and are not approved to use. ZF calls for LA2634 fluid which is really expensive. The Castrol MV Import AFT says on the back of the bottle 'replaces LA2634'.

I found this thread: http://forums.bimmerforums.com/forum/showthread.php?1430455-93-740iL-5HP30-gushing-trans-fluid-from-vent and one of the posters added Lucas Trans Fix to stop the foaming. I don't normally trust snake oils, but am thinking about doing this myself. Anyone have any similar experiences?

I wouldn't put that Lucas trans fix crap in anything other than an old ChryCo slushbox on its last legs. It seems to have a 50/50 success rate, with it either fixing everything or killing everything in one fell swoop.

Hate to say, but your mistake was probably skipping the LA2634 fluid. ZF 'boxes are PICKY about their fluid. Multi-vehicle fluids are normally "adequate" for multiple vehicles but especially with high mileage (approx 170k when you first changed it if I'm reading right) the trans is not going to take kindly to any kind of additive that was never there in the first place.

The foaming issue is a good start, I guess. Maybe. Any pinched lines running to the cooler? What kind of shape is the cooler in? I take it when you filled it up, you used the proper procedure of 1) add fluid, 2) with vehicle running and on jack stands, run it through all the gears and back to park, 3) add more fluid, 4) repeat until trans takes no more fluid?

Autos are a PITA to troubleshoot no matter what but ZF boxes are even worse.

Trans fix is pretty much really thick ATF with some seal swellers, it will help a trans with badly worn pistons/rings/seals apply hard enough to keep things from slipping and fix external leaks with the sweller and the viscosity. Also works great in a really loose manual box. Besides that, leave it on the shelf.

Was the fluid you drained burnt at all? Whats the cooler setup like? Any thermostats involved? Though I would think a BMW would know if its trans were overheating.

Yeah, used the right filling procedure...which is a PITA. Why couldn't they put a dipstick in? Fluid that came out was not burned and pan & filter were clean. I am pretty afraid of using the Trans Fix and probably won't. Not sure I want to invest $250 or so into the 'proper' fluid either...

For that generation ZF you are probably ok with the multipurpose oil... The oil the factory used back in 93 has likely been superseded now anyways...

Transmissions blowing fluid out the vents happen usually for 1 of 2 reasons... Over heating, or over filling... If you are positive you haven't over filled it, then chances are you have a cooler issue, or a slight slipping issue thats causing the transmission to over heat...

We've used synthetic Dexron after rebuilding ZF transmissions for years, and also use if for flushes. Some of them have gone 220k with no issue, none have ever suffered failures on the Dexron. Even used it in VW DSG transmissions and the world has so far failed to end.

In short, I doubt that it is the fluid's fault.

Restricted cooler flow is another kind of cooler issue that can cause the trans to push fluid out the vent.

Is the oil cooler completely separate from the radiator, or part of the radiator?

Oil cooler is separate from radiator, right in front of the trans. Thanks for all the advice. Esp. good to know I am OK w/ the Castrol ATF.