Turbo 350 leaks like sieve

[chev-obsession]

Hey guys, I bought this truck out of a field, truck sat for 20+ years and was started occasionally. Got the truck home, pressure washed under carriage, motor and trans to get a better look at everything. Trans was bone dry inside (no fluid on dipstick) filled trans with atf, leaked bad, Replaced the filter and shift shaft seal, cleaned trans pan well, verified pan was straight with no mushroomed bolt holes, used flanged bolts on pan and replaced pan gasket with 1 piece moroso silicone gasket and torqued pan to 10 ft/lbs with calibrated torque wrench. Filled the trans with fluid and now it leaks just as bad or worse. Last night before before I took this picture I washed the leak off with brake clean and stuck the cardboard under the trans and inspected first thing this morning.

**cooler lines, brass fittings are tight and tv cable are not leaking***

This is my first turbo 350 so any insight is great. To me it looks like the leak is coming from the pan or the two bottom bolts that hold the front pump together. Looking up into the front seal area it looks dry. It’s on clean/wet right around the pan area or slightly above the pan behind the converter.

Should I remove the pan and re verify it is straight or look at it closer? Any ideas at all? I’d like to start driving the truck but don’t want to with it leaking this badly.
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[AussieinNC]

Any auto trans that essentially sits still for that long is bound to have dried up seals and O rings throughout the trans.

That leak looks like the front pump seal has gone hard and is leaking, and or, the pump O ring seal has dried out and shrunk, allowing the oil to leak out.

You mentioned the trans was empty of oil when you picked it up. Sounds like it was parked due to the leaking oil situation and you have inherited the situation.

You seem to be mechanically inclined.
Have you ever resealed an auto trans?

TH350 transmissions are not difficult to rebuild, and there are several great threads on this forum to guide you along the way.

You could always send the trans to a local rebuilder and have them reco the trans.

BTW, other leak prone areas are the dipstick tube, the rear seal on the output shaft and the governor cover (the metal cover that looks like an inverted bowl on the side of the trans). All of which would be renewed on a rebuild (or at least on one of my rebuilds!)

Have you driven the truck? Does the trans shift and drive o?

[chev-obsession]

Originally Posted by AussieinNC

Any auto trans that essentially sits still for that long is bound to have dried up seals and O rings throughout the trans.

That leak looks like the front pump seal has gone hard and is leaking, and or, the pump O ring seal has dried out and shrunk, allowing the oil to leak out.

You mentioned the trans was empty of oil when you picked it up. Sounds like it was parked due to the leaking oil situation and you have inherited the situation.

You seem to be mechanically inclined.
Have you ever resealed an auto trans?

TH350 transmissions are not difficult to rebuild, and there are several great threads on this forum to guide you along the way.

You could always send the trans to a local rebuilder and have them reco the trans.

BTW, other leak prone areas are the dipstick tube, the rear seal on the output shaft and the governor cover (the metal cover that looks like an inverted bowl on the side of the trans). All of which would be renewed on a rebuild (or at least on one of my rebuilds!)

Have you driven the truck? Does the trans shift and drive o?

Thank you for the input!

Yes I’m fairly mechanically inclined but not sure if I’m confident to completely rebuild the trans. I was afraid the trans might need a rebuild. I have not driven the truck down the road to verify if it shifts into 3rd yet. The trans has Reverse, 1 and 2 though, I’ve drove it around the property as a little test drive but not on the road. I’ll call around today and see what a rebuilt trans will cost from a shop before I tear into mine.

[Accelo]

Any auto trans that essentially sits still for that long is bound to have dried up seals and O rings throughout the trans.
In my experience that hasn't been true. I have installed transmissions that sat for 15 years and they were fine. However, if it was overheated or ran hot for extended periods they do harden up. Worked on transmission for years and only encountered one leaking torque converter seal on a thm350. Very unusual to leak from this seal. However, the pump cover seals and bolt seals do sometimes leak.

This is an early THM350 as only the first couple years had the drain plug on the pan.

A 90% chance the issue is the dipstick tube seal. They are extremely difficult to identify. Requires a mirror and patience. Lots of cleaning. They leak and follow the pan seal around. One never know exactly where they will leak from. I can guarantee it's not next to the tube. The early ones had "O" rings and they were suspect from new. GM changed the seal type to what I call the top hat style and that cured the issue. To use the new designed seal it requires a matching dipstick.
Bet you find the issue there. Some factory tubes had two "O"rings. I would rule out the tube before moving on. Sometimes covering the cleaned up area with baby powder will help you follow the leak backwards.
Cheers

[chev-obsession]

Originally Posted by Accelo

Any auto trans that essentially sits still for that long is bound to have dried up seals and O rings throughout the trans.
In my experience that hasn't been true. I have installed transmissions that sat for 15 years and they were fine. However, if it was overheated or ran hot for extended periods they do harden up. Worked on transmission for years and only encountered one leaking torque converter seal on a thm350. Very unusual to leak from this seal. However, the pump cover seals and bolt seals do sometimes leak.

This is an early THM350 as only the first couple years had the drain plug on the pan.

A 90% chance the issue is the dipstick tube seal. They are extremely difficult to identify. Requires a mirror and patience. Lots of cleaning. They leak and follow the pan seal around. One never know exactly where they will leak from. I can guarantee it's not next to the tube. The early ones had "O" rings and they were suspect from new. GM changed the seal type to what I call the top hat style and that cured the issue. To use the new designed seal it requires a matching dipstick.
Bet you find the issue there. Some factory tubes had two "O"rings. I would rule out the tube before moving on. Sometimes covering the cleaned up area with baby powder will help you follow the leak backwards.
Cheers

Before firing the motor I replaced the original dipstick tube with the top hat style you are referring to. I’ve checked up there in that location and it is not leaking at this time

[67bullet]

It could be the washers around the main pump bolts,They are coated with a sealing agent

[Accelo]

Good move on the dipstick replacement.
I typically jack the rear wheels off the ground and with USA made jack stands. Clean everything up then I would run the motor and look for leaks with it running.
Well it was easier as I had a lift with a internal jack but you get the point.
Appears the leak is large enough to ID that way.
If nothing leaks shut it down and keep looking as the converter may be draining back and raising the oil level. Check the oil after setting over night to confirm.
A lot of things can go wrong. Lots of moving part and hot oil.
"Periculo tuo". Just because I did it I am not recommending you do it.
If it is the pump washers, the transmission will have to come out to fix the leak.
Confirm they are tight before pulling it.
Cheers

[chev-obsession]

I ended up pulling the trans after coming to terms with the leak not stopping. Took it to a reputable shop in town and had it rebuilt. They said it was a good thing I brought it in to be rebuilt since the clutches were flaking apart, also said the seals were very brittle and likely why it was leaking so badly. I’m sure they like having my money too

Going to install it sometime this week if the weather cools down. Thank you everyone for your input and advice. I will let you know how it goes.

[The Rocknrod]

Good to know, thanks for the follow up.