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This will be a bit of a long post, so please bear with me.

I have a frustrating auto trans fluid leak in my 1969 GTO. This is a driver-quality, numbers matching car with a factory turbo 400 trans. I bought it earlier this year (see pictures below). The transmission shifts perfectly.
About a month after I bought it, I noticed trans fluid leaking on the garage floor – not a lot, but probably a couple of cups. So I took it in to a shop I’ve been using for a few other things.

First, some info on the shop. It’s not primarily a classic car shop, but they do promote on their web site that they work on classic cars, and in fact each time I’ve been in they’ve had one or two classics in their shop being worked on. They are AAA approved, won the AAA best shop in Washington State for 8 years, have Consumer Checkbook’s highest rating for quality and customer satisfaction, and a 4 ½ star rating on all the online review sites. I’m not a mechanic and so really did my research before choosing this shop. All indications are that they know what they’re doing. However, read on.

It looked like an oil pan leak, so they replaced the pan gasket. The trans has been rebuilt once and had a non-factory chrome replacement pan installed, which they said in their experience had more problems than an OEM part.

Drove it home (about 10 miles, 8 of which were freeway speeds), so the car was nice and warmed up. No leaks later that day. No leaks the next morning or the next night, but the second day home it was leaking again. About a cup the first day, and then maybe another cup a day after that.

So, took it back to the shop, where they were surprised, since it didn’t leak there after the repair. But they only had it for a day after and it didn’t leak for me until the second day home. They said they now suspected one of two things. First, when the trans was rebuilt they put an after market trans fill tube and dipstick and that may have been giving an inaccurate reading, causing the fluid to be filled higher than it should be and thus leak out.

The other possibility was something bad internally in the transmission, causing it to overheat and boil the fluid, making it leak out. I’m not a mechanic but said this didn’t make sense to me, since no fluid was leaking when I got it home and the trans had time to get plenty hot driving on the freeway, plus it shifts great. So I said to go ahead and put an OEM fill tube and dipstick in, which they did. It turns out that there was too much fluid in the trans, as indicated by the new dipstick. With the correct level in, they drove it, checked it that day and the next and no leaks.

I picked it up, drove it home, with the same result: no leaks after driving or the next day, but the second day once again about a cup leaked out and I’m pretty sure that will continue.
I suspect they will now lean towards the transmission being the problem, but in another post on the forum it was mentioned other possibilities: the modulator O ring, front seal, rear seal, or speedo cable.

The odd thing in all this is that it’s two days later before it leaks again. Has anyone else experienced this problem and if so can you recommend a fix? I’m no longer confident the shop I’ve been using is up the task.
Sorry for the long-winded post and I appreciate any observations/advice you may have before I go back to the shop.

Tom
 

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This will be a bit of a long post, so please bear with me.

I have a frustrating auto trans fluid leak in my 1969 GTO. This is a driver-quality, numbers matching car with a factory turbo 400 trans. I bought it earlier this year (see pictures below). The transmission shifts perfectly.
About a month after I bought it, I noticed trans fluid leaking on the garage floor – not a lot, but probably a couple of cups. So I took it in to a shop I’ve been using for a few other things.

First, some info on the shop. It’s not primarily a classic car shop, but they do promote on their web site that they work on classic cars, and in fact each time I’ve been in they’ve had one or two classics in their shop being worked on. They are AAA approved, won the AAA best shop in Washington State for 8 years, have Consumer Checkbook’s highest rating for quality and customer satisfaction, and a 4 ½ star rating on all the online review sites. I’m not a mechanic and so really did my research before choosing this shop. All indications are that they know what they’re doing. However, read on.

It looked like an oil pan leak, so they replaced the pan gasket. The trans has been rebuilt once and had a non-factory chrome replacement pan installed, which they said in their experience had more problems than an OEM part.

Drove it home (about 10 miles, 8 of which were freeway speeds), so the car was nice and warmed up. No leaks later that day. No leaks the next morning or the next night, but the second day home it was leaking again. About a cup the first day, and then maybe another cup a day after that.

So, took it back to the shop, where they were surprised, since it didn’t leak there after the repair. But they only had it for a day after and it didn’t leak for me until the second day home. They said they now suspected one of two things. First, when the trans was rebuilt they put an after market trans fill tube and dipstick and that may have been giving an inaccurate reading, causing the fluid to be filled higher than it should be and thus leak out.

The other possibility was something bad internally in the transmission, causing it to overheat and boil the fluid, making it leak out. I’m not a mechanic but said this didn’t make sense to me, since no fluid was leaking when I got it home and the trans had time to get plenty hot driving on the freeway, plus it shifts great. So I said to go ahead and put an OEM fill tube and dipstick in, which they did. It turns out that there was too much fluid in the trans, as indicated by the new dipstick. With the correct level in, they drove it, checked it that day and the next and no leaks.

I picked it up, drove it home, with the same result: no leaks after driving or the next day, but the second day once again about a cup leaked out and I’m pretty sure that will continue.
I suspect they will now lean towards the transmission being the problem, but in another post on the forum it was mentioned other possibilities: the modulator O ring, front seal, rear seal, or speedo cable.

The odd thing in all this is that it’s two days later before it leaks again. Has anyone else experienced this problem and if so can you recommend a fix? I’m no longer confident the shop I’ve been using is up the task.
Sorry for the long-winded post and I appreciate any observations/advice you may have before I go back to the shop.

Tom

RUN! Find another shop. Anyone who ever told me that trans fluid can boil over and cause the leak is not qualified.

The chrome pan could be a problem. The chrome surface on the sealing edge could be a little too slick to grab/seal the gasket. The seal itself could be a poor replacement - maybe not thick enough if the pan has any low/high spots. There could even be a stripped out bolt hole that is not drawing down tight enough - which all need to be torqued evenly.

The dipstick tube/O-ring can be a common leak, but it seems you got that fixed and have the correct fill level sorted - if they simply weren't using this as a line of BS.

The other items you listed could leak, but the trans shop should be able to ID that right away up on the lift.

My advice is to find another shop. Purchase a factory steel trans pan. Purchase a good quality cork gasket - not a cork/neoprene combo or anything else, just plain cork. Bring the pan/gasket to another shop and have them install it.

Here is a discussion on pan leaks on the Buick site: Help me with my quest to have a leak free turbo 400....can't get the pan to stop..... | V8buick.com
 

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Discussion Starter #3
Thanks, Jim. I will of course be looking for another shop. Disappointing, as their reviews were universally excellent. I live about 35 miles north of Seattle and unfortunately there aren't any other classic car repair shops anywhere near me except for this one, which is 10 miles away. I'm assuming any good repair shop can install the pan and gasket if I buy it, even though it's a 50 year old car - ??

Do you find it odd that both times it took two days to start leaking, even after a ten mile (almost) all freeway drive? I would have thought a leaky pan gasket would start leaking as soon as parked after that. But as I said, I'm not a mechanic - just a lover of classic cars - so what the heck do I know! LOL

I'll be interested in hearing from anyone who had a similar problem and was able to resolve it.

Thanks again,

Tom
 

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https://www.kirklandtransmission.com/

best tranny shop on the eastside ...

I lived in Kirkland for 30 years

been in the shop many times ...

and at NW differential 3 doors down

Tony the owner answers the phone 95% of the time
and his desk is right inside the door

very clean shop

Scott

front pump to case gasket,,,, torque convert to pump seal...

it needs to be power washed ... with the inspection cover off...

driven to his shop and put up on the rack for the weekend....

and then wiped down with a claen paper towel ... and follow the drip up hill

he just finished doing a power glide in my old land lords 57 nomad and I have a sweet th400 he built in my dually
 

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Discussion Starter #5
Thanks, Blk69Judge. I moved to Lake Stevens three years ago from Kirkland, where I lived for many years. I was in the Juanita area. It's a bit of a drive for me to Kirkand now, about 25 miles, but might be worth it if Kirkland Transmission works on classics.
 

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oh yah ,,, a th400 is a th400 basically

mine doesnt leak a drop and it been 45,000 miles shifts like the day it was put in firm

but not harsh ,,, harder you throttle it harder it shifts,,, NO clunky shift kit down shifts

brite red fluid still .. and 2 oil and filter changes since its rebuild .. chrome pans are usually infearior

go back to stock .....

I was living 1 exit south of you at the NE 85th exit ..on the west side of 405.. with a little lake view

my little 1925 2 bedroom rental house and property was 75x 135' sold for $1,080,000 for the lot

so I had to move ...

go visit him ..... have it towed ? AAA towing is a must with old cars .....
 

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Discussion Starter #7
Yeah, a 10,000 square foot lot in that area of Kirkland, with even a small lake view, would be well over a million dollars at today's prices. Pretty amazing, I know, for those of you who live in other parts of the country, but indicative of just how crazy prices in the Seattle have gone up in recent years. My guess is that the house was torn down and replaced by a very large one, with a selling price in the 2-3 million dollar range. That's what water views can command up here.

I can only afford to drive my GTO along the waterfront.....not live on it. LOL!

Tom
 

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Tom,
I’m not sure if what I’m about to tell you will help , but maybe. My ‘70 LeMans Sport Convertible also has a similar leak. I’ve narrowed it down to the tailshaft area around the speedometer cable. I bought a seal kit off of ebay and replaced the seals where the speedometer cable goes. I thought I was successful, drove the car on and off for a couple of weeks while the weather was good. We had a spell of rain so the car sat for 2 weeks and I had fluid on the floor again. Took it to a friend who has a shop told him him what I had done. He said there was an internal seal that I missed and maybe my ebay seals were not the best quality. He replaced all the seals again and I had the Same problem after getting the car home. So long as The car did not sit for more than a week, no leaks. When it sits for a couple of months over the winter, it leaks continuously (but slowly). I’ve learned to manage this, but it has been going on for over 5 years now. I’ve had this car since 1983. Before I got it the original owner had the transmission replaced with a remanufactured unit (Aamco or Jasper can’t remember and need to get under it and look), other than the leak it operates perfectly. I hope that there isn’t a crack in the casting somewhere in or around where the speedometer cable goes. Apparently it is common for the speedometer cable seals to leak so you may want to check that and it is a pretty easy diy repair. Good luck.
 

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Discussion Starter #9
Thanks for the info. I have it at another shop now and I'll tell them to be sure to check the speedometer cable seals. The really odd thing is that both times after the unsuccessful fixes, it didn't start leaking again until two days later. For the first two days, no leaks at all. This last time, I crawled under the car and looked until I saw the leak drip. It was coming from above and just in front of the front u joint, well behind the oil pan, so I know it's not the pan or the fill tube (which were the two fixes). I would guess it's the rear tranny seal, but that area looks dry. I can't follow the leak up because I don't have a lift and the car is too low to get completely under it. I told all this to the shop and hopefully they'll be able to nail it. Although the tranny shifts great, it leaks about a pint a day so it's not something I can ignore.

I'll post the results of the latest fix, which should be early next week.

Tom
 

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Discussion Starter #10
Thanks for the info. I have it at another shop now and I'll tell them to be sure to check the speedometer cable seals. The really odd thing is that both times after the unsuccessful fixes, it didn't start leaking again until two days later. For the first two days, no leaks at all. This last time, I crawled under the car and looked until I saw the leak drip. It was coming from above and just in front of the front u joint, well behind the oil pan, so I know it's not the pan or the fill tube (which were the two fixes). I would guess it's the rear tranny seal, but that area looks dry. I can't follow the leak up because I don't have a lift and the car is too low to get completely under it. I told all this to the shop and hopefully they'll be able to nail it. Although the tranny shifts great, it leaks about a pint a day so it's not something I can ignore.

I'll post the results of the latest fix, which should be early next week.

Tom
Well, got it back from the new shop and it didn't leak for them. They took it out for a 10 mile drive Friday of last week and then let it sit under a clean floor until Tuesday. No leaks and the trans fluid registers full. So, feeling like an idiot, I brought it home, since they could not find anything leaking. Let it sit two days, no leak. Took it for a 30 mile drive and two days later no leaks. So now it's a complete mystery and I guess all I can do is keep monitoring it. If it starts leaking again I'll crawl under and take a video, put the shop's suggestion. I still don't think they believed me that it leaked! The tranny shifts great, so I'll continue to drive it occasionally and take it from there.

Frustrating....
 

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Some additional input about TH400 leaks in this thread on a Chevy truck forum:

I had 2 leaks on the TH400 in my 68 GTO.
One was a cooling line adapter leaking at the case.The line was tight to the adapter, but the adapter was leaking.
The other was, as mentioned in that truck thread, a shift shaft seal that leaked even after replacing it with a new seal. I added a second lip seal and that stopped it.
 

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I do remember on my th400 in an 69 OLDS 442 w-30 I had that the speedometer housing needed replaced...
the speedo gear was all worn out.. 433 gears maybe it spun faster ? theres like 3? seals in that assembly...
 

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Discussion Starter #13
All - thanks for the additional info. This gives me some additional suggestions for the shop if it starts leaking again.

So far, still no leaks. Strange, but from all the threads here and on the Chevy truck forum, hard-to-find leaks are common on late'60s goats.
 
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