[Carmine]

Hi everyone. The transmission in my '71 GTO, a TH400, has leaked since I owned the car. I've tried to track down this leak numerous times, but I can't. I replaced the pan and a new gasket. The leak seems to favor the passenger side. The two fluid lines are not involved. I don't think the front seal in the transmission is the source either, but i could be wrong. I pulled the flywheel cover and it didn't appear unusually wet. The only thing left would be the dip stick tube and the modulator valve. It gets to be so messy under there, I can't determine the source of the leak. Everything get coated. It leaks so much that the force of the wind, drives the fluid down the tailpipe and a white smoke develops. The unburned fluid leaves like a thin brown honey consistency on the tailpipe. The cross member also has drips coming from it.
Yesterday, I went for a ride with my son who I gave my '67 GTO to. He lives about 10 miles away. On the way there, I drove normal. No WOT at all. Before leaving on our ride, I looked underneath. Not a drop could be found. Perfect. During our ride, I ran it hard a few times. WOT to 5000 rpm's. When we stopped for ice cream after about 45 minutes, my son asked me if I knew that I had white smoke coming from the area of the passenger tailpipe. No surprises there. I knew exactly what that was. Eventually returned home and put the car on the lift. Same old issue. Everything coated and dripping. Again, I tried to determine the source and couldn't.
I'm wondering if it was a coincidence or not, re. driving it normal in comparison to WOT at times. Would that extra pressure from WOT force the fluid out at it's weakest point?? Other transmissions also face WOT at times and I'm sure they don't act this way. There is a non stock converter in this car. Not my choosing, but I hope to eliminate it soon. I don't know the stall speed, but it does exist. Could this converter be wrong for the transmission?? Making too much pressure?? I'm not suggesting this transmission is leaking quarts of fluid. I know a little goes a long ways, but it does get to be a mess underneath. I really hate posting a question like this, because you would think that I could find the leak source on my own, but I can't. I've tried several times and have come up empty. Would anyone have experienced something like this?? Any suggestions or comments where or how I should be looking for the leak, would really be appreciated, Carmine.

[Mister Pontiac]

Carmine, I've had this too in years past, and I think you'll be happy to learn it's very simple to solve.

What you're describing sounds to me like the transmission is simply overfull, and it's throwing the excess out of the breather (passenger side near the cooler lines). They have a habit of doing that with a little too much fluid in them and then driving them hard. That's why you aren't noticing the leak when you drive it easier.

One good thing is that it will eventually self-adjust the fluid level (by puking out the excess) and stop the leak, but the best thing to do would be drain a half a quart or so and try again.

Hope that helps.

[Carmine]

Thank you very much and I hope you are correct. Just a brief update. I started the motor and let it run for about 25-30 minutes. It reached operating temp. I let it idle and also held it at 3000 rpm's for a brief period of time. Nothing higher and no WOT. I shut it off and put it back on the lift. Previously, I cleaned the entire area. Inspected the area again, and not a single drop of fluid. Nothing. No leaks at all. I had the flywheel cover off and felt up to the front seal which I could also see. Definitely nothing coming from there. So, it's presently on the lift and we'll see what happens over night. I never looked at or gave the breather any thought. The dipstick does say slightly above full, but I don't even know if it's the correct one or not. I will drain some out while I have it up. Many thanks, Carmine.

[Greg Reid]

I chased a tranny fluid leak for some time and changed the rear seal twice before I found that it was coming from a place that I did not even know COULD leak...the slip yoke. The end of the yoke is pressed in. http://forums.maxperformanceinc.com/...ight=yoke+leak
Of course in that instance, once I started looking frequently enough, I could see that the initial moisture to a very cleaned up area was behind the rear seal and pan. The first drop to show was on my u-joint.
What is the furthest point forward that gets wet?
In my case, it appeared to be where someone touched a welder to it and weakened a spot that later blew out.

[Pepper Judge]

I have experienced the TH400 leaking the excess out of the breather tube also, when the tranny is somewhat hot.

[Carmine]

Originally Posted by Greg Reid

I chased a tranny fluid leak for some time and changed the rear seal twice before I found that it was coming from a place that I did not even know COULD leak...the slip yoke. The end of the yoke is pressed in. http://forums.maxperformanceinc.com/...ight=yoke+leak
Of course in that instance, once I started looking frequently enough, I could see that the initial moisture to a very cleaned up area was behind the rear seal and pan. The first drop to show was on my u-joint.
What is the furthest point forward that gets wet?
In my case, it appeared to be where someone touched a welder to it and weakened a spot that later blew out.

The furthest point forward where I see the fluid Greg, would be close to the front of the pan. And these are drops that are kind of just dangling there, ready to fall. I know going 45-50 mph, with the wind underneath, the fluid would blow from the front to the back. I don't see how logically it could go the other way. It's just so puzzling. Everything else gets a fine coating so it's difficult for me to tell where the origin is. I did feel around the dip stick tube and that was also dry. So, I have to rule out that and the front seal. Leaves just the pan gasket, modulator and too much transmission fluid that blows out the vent under pressure. I'm really hoping that's it. I'm going to check this vent in the morning for any signs of being wet. Believe it or not, this morning there were drops of transmission fluid hanging from the rear end and also the muffler clamp. Both are a decent distance from the transmission. This might support my thinking of it blowing back by the wind, but where does it come from?? Just remarkable, Carmine.

[Greg Reid]

How about the governor cover?

Tranny fluid can take some weird routes forward if it's out of the airstream and above the pan boss. My yoke leak didn't go all the way to the front of the pan but there was enough on the pan to make me change the gasket in at least one effort to find it even though the leak was behind it. Had to clean it thoroughly and check it frequently to find the first drop. The first drop will tell where it is coming from.
If it's the governor it may not leak unless you are putting it through the gears. Drive and check, drive and check, repeat.

[Carmine]

Thanks Greg, I will check the governor cover, but I recently had it off, fooling around with changing the weights and springs in the governor itself. It seems somewhat dry around it, but then again, everything gets a fine coating, so it's so hard to tell. I agree with you about the fluid. It can take a strange route. I'm looking forward to locating the breather/vent tomorrow and see if there is fluid around it. With the amount that comes out, I would imagine that area has to be somewhat wet. My dipstick says a little above full, but I'm not even sure it's the dipstick for this transmission. An owner before me butchered some things on this car, so nothing surprises me. Just going to keep fussing with it until I figure out what it is. Maybe I'll get lucky and as previously mentioned, there is too much fluid in the transmission, Carmine.

[Mister Pontiac]

Originally Posted by Carmine

Thanks Greg, I will check the governor cover, but I recently had it off, fooling around with changing the weights and springs in the governor itself. It seems somewhat dry around it, but then again, everything gets a fine coating, so it's so hard to tell. I agree with you about the fluid. It can take a strange route. I'm looking forward to locating the breather/vent tomorrow and see if there is fluid around it. With the amount that comes out, I would imagine that area has to be somewhat wet. My dipstick says a little above full, but I'm not even sure it's the dipstick for this transmission. An owner before me butchered some things on this car, so nothing surprises me. Just going to keep fussing with it until I figure out what it is. Maybe I'll get lucky and as previously mentioned, there is too much fluid in the transmission, Carmine.

Carmine, you'll find the breather tube coming out of the upper right side of the transmission case, just above where the two cooler lines attach. The tube is very small in diameter (much thinner than a standard drinking straw) and only about an inch or two long. It usually has a slight curve downward.

I don't think you'll find much fluid around that area as it tends to blow out and down from the 'straw' (though you may see a drop hanging off of the tube). This is why the whole side and bottom of your trans is wet, and why it trails back to the crossmember, onto the right side exhaust, etc... (I'm betting the left side of your trans is dry and it's not hitting the left exhaust pipe, correct?)

You mentioned that your dipstick is showing a little over 'full.' Did you check it with the trans up to temp and the engine running? This is what you need to get an accurate reading. If you check it with the engine off, some of the fluid drains back into the pan and it will indicate over full.

I'm betting overfull is still your problem. I always start with the simple things first. Your description and symptoms line up perfectly with an overfull trans. Maybe drain a full quart, and take it for several more WOT blasts. Let us know what you find...

Hope that helps.

[johnta1]

Might try attaching a clear hose over the vent tube and run it back behind the tranny. (or even just below the tranny)
If it is coming from there, you will see it in the hose and where it ends behind the tranny.

[Mister Pontiac]

Originally Posted by johnta1

Might try attaching a clear hose over the vent tube and run it back behind the tranny. (or even just below the tranny)
If it is coming from there, you will see it in the hose and where it ends behind the tranny.

Great idea, John.

[Carmine]

That's a great idea, thank you, but I don't think I'll be needing to do that. At least not at this moment. I just finished conducting a non scientific experiment. I cleaned the bottom and around the transmission very well. I found the breather tube on top, right where someone said it would be. I could not only see it, but feel around it, and it appeared to be very dry. No residual of anything. Not even dusty. I used a coat hanger and attached a piece if paper towel to it. Inserted it into the tube about 1-2" and it came out dry. No signs of fluid on it at all. I wasn't feeling too optimistic at this point. Regardless, I went out for a ride until everything got hot. No more then 2800 rpm's under normal driving. No WOT. Returned home and put it back on the lift. Everything was dry and how I had left it. Took some advice from the members here, and drained about 1 quart of fluid, maybe a smidgen more, from the pan. Went for another ride. This time I pounded it several times. Probably 6-8 times, ran it through the gears to 5000 rpms. Similar to what I did this past Saturday. Ran great and shifted nicely. Returned home and first checked the transmission fluid. It didn't even register on the stick. Yikes. I thought I had lost a whole lot of fluid in addition to what I had taken out. . Put it back on the lift and couldn't believe what I saw; nothing. No drops. No drips. No runs. No signs of fluid outside the pan. I had a hard time believing this, but I know what I saw. The area and transmission were as clean as I left it before my first ride. Apparently, as suggested by members, there was too much fluid in the transmission and it constantly puked out what it didn't want under pressure. I guess what I removed and left, is a good level for the transmission. I also mentioned, that I wouldn't be surprised if that dipstick and tube, weren't correct for this transmission. Think I'll advertise in parts wanted for an original tube and dipstick. I would like to thank everyone who took the time to respond with suggestions and help. I really appreciate it. I don't discount what anyone says. I would have never thought about the transmission being too full. Thanks again, Carmine.

[Mister Pontiac]

That's great to hear, Carmine.

And you probably already know this, but just in case, keep in mind that the distance between "low" and "full" on the trans dipstick is only 1 pint, not a quart (like the engine). I think that is another reason they tend to get 'over-serviced' sometimes.

[Geoff]

The leak is probably going to be the o ring seal on the dip stick tube. O ring gone hard & lost it's seal from years of transmission heat. The later dip stick seal is better & interchangeable. Requires new dip stick tube & seal. GM changed the design for a reason.

[Old Goat 67]

Originally Posted by Carmine

When we stopped for ice cream after about 45 minutes, my son asked me if I knew that I had white smoke coming from the area of the passenger tailpipe.

Any time that happened to me, it was spitting out of the overflow and hitting the exhaust pipe. Leaks from elsewhere don't hit the exhaust pipe.

[Carmine]

Originally Posted by Mister Pontiac

That's great to hear, Carmine.

And you probably already know this, but just in case, keep in mind that the distance between "low" and "full" on the trans dipstick is only 1 pint, not a quart (like the engine). I think that is another reason they tend to get 'over-serviced' sometimes.

Thanks, but no, I didn't know that, but I do now. I always thought it was a quart or more. I'm wondering out loud here, but I thought the leak existed when I first bought the car. I'm not so sure. Can't really remember. But I do know, the PO had an aftermarket, very deep, transmission fluid pan attached, which I didn't like and I changed to a stock one. I looked in Summit and I think this pan held and extra 2 quarts or so. I do believe I put almost all the transmission fluid back in except maybe just a little. I know it came up and above the full mark, pretty quick on the stick, so I didn't add the full amount that I previously removed. Guess I really had way too much in it. But, it's all history now, as they say.
I just know that the dipstick and tube, moreso the dipstick, is not the original, stock one. Right now, with my previous removal of about a quart or better of fluid, nothing registers on the dip stick. Through a member, I was able to purchase an original dipstick and tube so that I can get an accurate reading. I also noticed that there is some type of substance around the tube where it is inserted into the transmission. Not sure what this is all about, but I'm going to wait for the parts to arrive, before I take it apart. At least now, I can drive it,

Geoff, you might be on to something there. I'll find out when I remove the old one and see what the substance is that surrounds it. Gives the appearance of trying to stop a leak.

Yes OId Goat, it certainly hit the passenger side tailpipe. I wiped drops from the bottom of the rear. I wouldn't believe it would travel that far, but I saw it myself. I just hope this issue is done, Carmine.

[Carmine]

Hi guys. Didn't want to start an entirely new thread, seeing how this question is related somewhat to the original posting. My original dipstick and tube arrived the other day (thanks Scott) for my '71 GTO. I took a look at the present installed one which is aftermarket, and saw that it was bolted by attached bracket, to I think the rear of the head?? From the bottom looking up, it looks almost impossible to get to the bolt as is. Very tight. Little to no room to work. Even if I got the bolt out, not sure how I would get a new one started. Haven't looked from the top down yet, but I don't see it changing any. I'm hoping someone has changed the dipstick tube and can share how they did it. Does the transmission have to be dropped down to get to that bolt?? Geez, I hope not. Thank you, Carmine.

[Greg Reid]

It's a hassle to get the bolt started from the top but it can be done. I had mine out and back in when I replaced my cooling lines.
Getting the bolt out is no problem but it takes some contortions to get it started since you can't really see the hole and plus, you are fighting the rigid tube which wants to go where IT wants to go.

[Carmine]

OK. Thanks Greg. I guess I'll struggle with it next week. I want to take my car to a really big car show this weekend in my area. I don't want to take it apart and not be able to get it back together in time, Carmine.

[Squidward]

I can't speak for an aftermarket tube and what it's attached to, but a factory style tube bracket bolts to one of the bellhousing bolts. It is a PITA, but doable. I'm not sure if undoing the crossmember and tilting the trans down a bit will help, but just a thought. Not necessary, tho.

Again, PITA, but doable. Take bolt out, wiggle tube, and Tetris it out the top and side.