overfill atf, a problem?

[Jerry_NJ]

Okay, this has been discussed in the distance past, I didn't find anything posted in recent years.

One post says, just like engine oil (especially in the vertical orientation for pistons) that too much oil can result in frothing - air bubbles in the fluid.

That said, I drained my 2004 Forester's (with 55K miles) ATF and measured the amount taken out. That was just over 1 gallon of warm not hot fluid. My measure was the AFT filled an empty 1 gallon plastic milk bottle a little past the normal full bottle level when purchased with milk in it. I then installed one gallon of fresh ATF. I tried dipping cold and hot and find too much wetting of the dip stick to get a reading. Keep in mind this car has thousands of miles on it with the fill level I must have duplicated based on making in = out. There has never (yet) been a problem with the transmission. I also recall always having trouble reading the dip stick, same can be said for the engine oil dip stick. There must be oil/atf in the associated filler tubes.... that's really overfilled.

I have a hand vacuum pump and suspect I can pump some ATF out of the filler tube and see what happens to the reading on the dip stick. For now the transmission is shifting great, albeit I haven't driven more than about 10 miles at a run. Again, the car has been driven all day and part of the night (over 12 hours) at highway speeds with what I believe is the same fill level as it now has.

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[hardball75006]

I don't think you over filled. When I did a drain and fill on my Legacy it took 4 quarts.

Are you checking the the fluid level with the engine running? When I check mine with the engine running, the level is right on the money. When the engine is off it looks over filled.

[Jerry_NJ]

Thanks, I have checked both cold (off) and hot (running in Park) and I just can't get a reading. As I may have said, I have always had trouble on the Subaru reading dip sticks, be it the auto trans, the engine oil, or the front differential - I haven't checked the last one lately but am ready to drain an refill that too.

I had the car in for a recall and the dealer did the usual and suggested I change the auto trans fluid, I assume they checked it, said (best I can recall now a few weeks later) that it looked dirty. I assume they looked on the dip stick and assumed it had never been changed.

I will go out now and check it cold and if I can't get a dip without the stick being wet about the full hot line, I think I will pump some out of the dip stick tube.

I looked up the ATF check and in my Haynes manual and it warns not to overfill as it can cause foaming and thus damage the transmission.

Edit: Just checked cold, about 40 degrees, and the dip stick appears to show fluid up well about the full "Hot" level. I took my Mityvac and ran a length of plastic tub down the dip stick tube. I pumped about 16 oz out of the transmission and then no more could be picked up, and I couldn't get the suck-tube further down the dip tube. The path bends at a sharp angle back toward the rear of the car, to the transmission housing. The dip stick still shows overfilled even for the hot marks. I will have to try a smaller diameter tube, maybe I'll tray to loosen the drain plug and let another pint leak out. Fail that, I could just drain again and watch the level as I fill - I planned to drain again at the next oil change as the transmission still has about 50% old fluid anyway. Makes me wonder how the transmission hasn't already been damaged. Best I can tell is shifts fine and has not shutter or slip that I can feel from the driver's seat.

[Uncle Scotty]

being under filled is worse than being over filled

[Jerry_NJ]

Thanks, and overfilled it was, or so it seems. I took a smaller diameter tube and was able to run it down the dip stick tube past the turn. Then with my "trusty" (happy I found a use for this thing, I purchased to facilitate brake bleeding/flush by myself, doesn't work well on that task) Mityvac and sucked more ATF out. I have now removed one quart and while the dip level is coming down it is still over for hot and I am measuring at cold (less than 40 degrees I'll guess, the fluid seems ticker at that temperature, I mean even to the "eye"). I am going to drive it again, heck, it has been driven with that much of an overfill for thousands of miles and there doesn't seem to be any transmission problems. I'll check it hot then and decide what else to do.

Given I dumped the pan and measured 1 gallon taken out, how in the heck could I overfill by a full quart when all I added was one, one gallon, bottle? Is it possible that an overfill results in an ability for the transmission to hold the overfill over a drain cycle? I'd assume if the full pan hold one gallon, an overfill would be the pan drain has access to all "free" fluid, that in the pan and that which is overflow/overfill. What the heck is going on?

EDIT: Hope what I am learning is helpful to someone else, or I am corrected if need be. It seems the idea of being able to check the transmission level cold is BS. As noted above I pulled out about a quart of transmission fluid and the cold DS still indicated over fill for the hot marks. I decided to go ahead and run some errands and would use that to get the transmission hot before taking any more out. I drove the car about 12 miles and pulled into the garage, going once into reverse first. Then with the engine running and the transmission in Park, I checked the level, it came out low, transmission hot, the level indicated below the cold okay level. I conclude that in spite of the dip stick having marks for cold level test, and the manuals I have read, including Haynes, say okay to check cold, better not, one can not check the level cold. I will put another 16 oz back in and check the level hot on the next use of the car. It must be that the converters (pumps, whatever) in the transmission take in at lease an additional 32 oz of fluid when running compared to what they have when off/cold - this is a wild guess based on my tests, if you get readings on cold it must be me. It seems the car was either not operated with a gross overfill or at most only a little over and that's why there was problems noted, this work was all about preventive maintenance, not corrective...although I ended up bouncing around trying to "correct" fill errors, of which there may have been none.

[AZP Installs]

Our rule of thumb is to put back in what came out. Which is what you did in the first place. It is great you are documenting this so others will be able to refer to it.

-mike


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[Jerry_NJ]

Thanks, I agree. I went back an cleaned up my previous post to make it easier to read (in my mind at least). I have now added the removed quart back and it reads full on hot. The only good aspect is I refilled with fresh fluid, so my first change resulted in a 4 quart change followed by a short drive, then a 1 quart change. So, the car now has more than 50% of the trany fluid fresh. Again, I plan to drain and fill one gallon again in a few months. I will know this time, in=out will not result in an overfill.

[Ernest]

I have done exactly what you have done and no issues for 85k and 9 years. Doing something right.