Folks -

A thread I started over on the 944 list mutated into a discussion of ATF "leaks" on my 928 ( see post #44 @ https://rennlist.com/forums/924-931-...l#post14507652 ).

I'd like to move that conversation over here since it isn't relevant to the 944 and this is really the only way I know of to do that.

The subject is what Joel aptly refers to as the "ATF Drainback Problem". He's the second person (and no, I can't remember the first) who's discussed a problem with ATF draining out the vent hole of a correctly filled transmission that sits unused for a few weeks.

I'd like to hear from more folks who've experienced this problem and about any solutions people have come up with? Since checking the fill on a 928 transmission is a bit of a black art to begin with, anything that preserves the fill and keeps an operator from having to repeat the process every spring ( in my case ) would be welcome.

Regards,

 

Scott,

There is not much of a black art to filling the auto unit correctly but the procedure has to be followed rigidly. Overfill it and it comes out of the vent as a friend of mine in his Cayenne turbo S once found out when following me! Once the surplus had blown out the problem ceased.

Whether anything else can cause this remains to be seen. The procedure relies on having the ATF correctly distributed in the system in the hot condition. Once the motor is static some of the ATF drains from the torque converter into the sump and thus why the procedure. As I am aware there is nothing to hold all the ATF in the torque converter when the system is shut down so some of it drains back. Doubtless someone will have a more complete explanation if are other potential issues.

 

Thanks for that Fred, it not only confirms the diagnosis I think, but also points to a solution.

I believe what you're suggesting here is that the WSM procedure results in an overfill that eventually drains through the vent as Joel suggests? If that's the case, there's no corrective action beyond cleaning up the mess? Following the WSM procedure a 2nd (3rd, 4th, etc.) time will only result in another overfill and subsequent ATF "dump"?

Regards,

 

I am only interpreting what Fred has written, not trying to put words in his mouth, but it seems to me that what he said was "follow the WSM, it works", which has been the case for me with my 928 for >10 years, >80K miles. YMMV.

 

Not at all- what I suspect is happening is that some folks do not strictly follow the procedure even though they tell you they did and end up overfilling the system such that it spits the stuff out [as happened to me].

This can happen for a number of reasons such as not having the system up to temperature, not cycling through the gears as specified or simply doing something dumb like not have the car level. Reading the level accurately is not the easiest of tasks, especially if working off axle stands- get the thing up in the air on a hoist and life is suddenly a lot easier.

Whether or not there is an inherent fault condition that can cause this to happen remains to be seen

 

I would think so too, but this hasn't been my first rodeo either. I took the most important factor as being level, and went to great pains to level the platform before filling it. I used a 4 post lift and leveled the car in all dimensions at the wheels. If the chassis wasn't level I wouldn't have detected it. This is exactly why I refer to the process as described by the WSM as a "black art". It's a lot like measuring Mean Sea Level.

The bottom line appears to be that if ATF gets blown off by the transmission at rest, it's found it's own natural equilibrium and doesn't indicate a leak, a failure, or any reason at all to replace fluid?

 

Just for clarification Fred, and the edification of the audience, in your mind how does "not strictly follow[ing] the procedure" and "overfilling the transmission" differ?

It seems we're describing the same thing?

 

Scott,

This happened on my 928 and at the time had only been handled by the main dealer. After the incident, I took the car back to the main agents, asked them to check the level and see if there were any signs of a leakage [from wherever]. They checked it out, could not find any fault but could see it had blown out of the vent. The incident happened on a hot day and I was not hanging around.

They concluded that it had most likely been overfilled inadvertently by one of their mechanics and it had simply found its own level. Never had a problem since.

So to answer your query, one of the parameters is to have the transmission fully warmed up before carrying out the requisite- if this is not the case presumably the level has not fully expanded to its hot volume, one sets the level based on this condition and when it does eventually reach full hot it expands more and pukes out of the vent. Thus the procedure has not been followed accurately and an overfill condition inadvertently occurs even though the owner believes it has been done correctly.

In my case the system seemingly found its own level as they did not add anything or find the level to be low. Whether or not such has happened to you remains to be seen.

 

Thanks Fred. That's what I was looking for.

Regards,

 

When I have experienced this issue the trans ends up way underfilled after the fluid drains back. It happens if I let the car sit for a few weeks. Starting and idling the car for a while seems to forestall it.

-Joel.

 

https://rennlist.com/forums/928-foru...orrection.html

 

Scott,

I can only relate to you what I experienced. I had ATF spat from the vent and no where else- it was a one off event and has never re-occurred since.

If the check valve from the torque converter is leaking, as the torque converter high point is greater than the oil level in the tranny body, it will want to leak back under gravity. If this happens then the ATF level will increase and then as per the other thread, has the possibility of finding other weak links that could leak. If they are tight then no oil will leak and the ATF will return to the torque converter as soon as the engine starts to rotate as that drives the ATF pump in the tranny [or so I understand]. If this happens normal service is resumed and the ATF should not emanate from the vent line.

Check valves are good at stopping back flow but they are not great when it comes to tight shut off. Presumably Porsche knew this, and thus why there are two check levels- one for a cold motor to give the mechanic something to shoot for during initial fill, and the hot mark for a precise final check. As the torque converter is at a higher level this will not fill initially and thus the thing has to be run to get the level check correct. Thus with the unit running and cold the ATF should be at the lower level and when hot at the higher level.

If on the other hand, the unit is overfilled as per my experience, it will find its way out and maybe under more extreme conditions, it has to reach certain temperatures and/or operating speed to cause this to happen.

There is always an explanation for everything that happens.