# Diagnosing TH400 Leak



## Praxis (Feb 23, 2014)

I am still having trouble with leaking TH400 in my 1967 GTO.
Here’s the story in a nutshell: Trans was rebuilt a couple years ago, good for high horsepower, then the car was seldom driven. It seems to always have left a drop or two everywhere it went, but a month or so ago while driving, after accelerating into a right hand turn, I suddenly noticed a trail of white smoke following me. It mostly cleared by the time I got home. Next day there was noticeably more trans fluid on in the pan on the garage floor. I took it to a shop that knows TH400’s (same shop that had rebuilt it, but they are gone, new owners now who do mostly trucks) and they found leaks at the usual places, pan gasket, governor cover, dipstick tube, modulator, old speedo mount (Gear Vendors OD now) and fixed all those. Then they filled the trans (deep sump) to the full mark on the dipstick At the final double check of the trans fluid level the dipstick broke in two where a rivet held a plate. I’ll come back to that in a minute.
Drove home and still leaking, but not from governor, modulator, speedo, or pan. Dipstick tube still leaks.

I took it to a race shop owned by a friend who reluctantly gave me lift time. They found the drain plug on the pan was a bit loose, and cleaned all visible fluid off of everywhere. We ran the car on the lift in gear for a while, no visible leaks. Shut car off and left it up on the lift long enough to replace the rear shocks, rechecked for leaks, found none. I drove home about 5 miles, no leaks when parked. I put clean cardboard under, three hours later, no leaks. Next day about noon, cardboard had a puddle of trans fluid, looked to be a few ounces. 

I started and ran the motor, backed out enough to replace the cardboard, pulled back in, and the leak was already still dripping. Over night it leaked another ounce or two. A week later, without starting or moving the car, reaching under as beat I could on the floor, drips still very occasionally dropped before my eyes, I put white towels to dipstick tube and yes still red fluid coming from there. Trans fluid seems to be running forward along pan gasket (Moroso high end gasket) and dripping off front right of pan. 

It is entirely possible the shop overfilled my trans. I don’t know whether the dipstick in the car at the time is a correct one. And now it is only the top portion to plug the tube. No one makes a correct stick for 1967 GTO. Except of course aftermarket.

Please, will some of you post photos of the trans dipsticks in your cars, specifically the lower part with level markings?

I have ordered a Moroso tube and stick for this car from OPGI, also the closest replacement stick and top hat seal. My plan is to drop the pan, fix the tube (either top hat seal or Moroso new unit) and with the dipstick in place, verify “full” mark is at the level of the bottom of the case. Right?

I think the dipstick tube is leaking enough to cause all this, plus possibly overfilling ( I really think the dipstick that broke was incorrect) but here is my next big question:

If it is a front seal, or pump seal, that is causing my leak, would the fluid end up dripping from the front bottom of the pan? Even if the car were not moving blowing it back? What is the best way to verify that? I do not currently have access to a lift so I cannot remove the inspection cover.

Believe it or not, I live on the Monterey Peninsula, home of Pebble Beach Car Week, and there are no shops here who will work on my trans. The car is too nice and they don’t want the liability for inadvertent damage. The shops that will work on it are bad, they are not talented and also usually break something else.

Thank you guys for any help you can offer. And if anyone knows of a good classic car shop on the Monterey Peninsula, please clue me in!


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## Praxis (Feb 23, 2014)

OH, I forgot another question: if it is a front seal, or pump seal, would it leak as I described? Still leak after days of not running? Not leak while running on the lift in gear and out?


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## armyadarkness (Dec 7, 2020)

I believe this is not the first time you posted this. As I mentioned last time, mine does the same thing, and yes, you blew your shift modulator when you passed a car, which is why your car smoked.

All Trans leaks from the front will go all over the entire underside of the car. So seeing fluid on the back of the pan does NOT mean that it's where your leak is.

As for "over filling"... Trust me, a TH400 doesnt need to be overfilled to leak, but if you fear youre overfilled... since you have a drain plug pan, drain the fluid and refill it with the appropriate amount, then verify it on the stick. If your stick reads full, then its good, if not mark it and now thats your new fill line.

All sealants are not created equal. Dont think that blue goo will ever stop a trans pan leak. Use Ultra Grey and do it well. Also seal the dipstick tube and all other orings or covers.

Trans oil is very thin, therefore small leaks look very big on the floor. DOn't assume that youre hemorrhaging, based solely on the puddle size. The dipstick doesnt lie.

A car can easily be jacked up high enough to remove an auto trans, and with a buddy it doesnt take very long to do. If you cant trust anyone to do the job, why not pull the trans and bring it to a shop?

Personally, I'd be bringing it to a reputable GM dealership, if possible. They have no problem putting $100,000 Camaro's and Corvettes on thier lifts, so why would they shy away from a GTO?


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## Praxis (Feb 23, 2014)

armyadarkness said:


> I believe this is not the first time you posted this. As I mentioned last time, mine does the same thing, and yes, you blew your shift modulator when you passed a car, which is why your car smoked.
> 
> All Trans leaks from the front will go all over the entire underside of the car. So seeing fluid on the back of the pan does NOT mean that it's where your leak is.
> 
> ...


Thanks for the dealership tip. Ill check.
My current problem is I don’t have a dipstick. The prior shop broke it while checking to see if it was full. It didn’t look like the drawing in the 1967 GTO shop manual, so I suspect its markings could be off. The current pan is a deeper sump than stock, so I can’t just add the recommended amount of fluid. I’m hoping to get the new dipstick I ordered soon. The shop that broke it was checking level frequently because of the deeper sump, but they seemed sort of surprised when they put what they thought was enough in but the stick didn’t register. That is why I am suspicious of overfill.

Would any of you be willing to send a photo of your dipstick, and note the distance from the seal that sits at the top of the tube in situ to the “full” line? That way I can at least get an idea of how full I am before dropping the pan.


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## armyadarkness (Dec 7, 2020)

Just to be clear, a deep sump pan uses the same stick... the OEM stick. 

Most deep pans add 2qts of capacity, so when you fill the trans with the recommended volume, it will not show on the stick until you add those 2 extra quarts. 

Im happy to send a pic when I can, but as I mentioned in the other thread... Inline Tube









64-79 Gm Transmission Trans Dip Stick and Tube Indicator Turbo T 400 With | eBay
 

Pn# - INL14390 - 1pc $39.00 Ea <br>1967-77 Oldsmobile (all Models) 442, Cutlass, W-30, 88, 98 & 1967-77 Buick (all Models) & 67-77 Pontiac (all Models) GS, Skylark, Electra, Tubro 400 Transmission Oil Dip Stick & Tube. Factory exact with rounded wire dip end, and welded on support bracket for...



www.ebay.com


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## armyadarkness (Dec 7, 2020)

And here is my pan, which you will notice adds 2 qts








B&M 20280 Finned Brushed Cast Aluminum GM TH400 Deep Transmission Pan +2 Quarts | eBay


B&M Part Number : 20280. B&M Hi-Tek Deep Trans Pan for GM TH400 Transmissions. Transmission TypeGM TH400. All these features combined help dissipate heat quicker netting lower transmission and fluid temperatures prolonging the life of your transmission, transmission fluid and corresponding...



www.ebay.com


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## BLK69JUDGE (Jun 10, 2010)

I have a real 67 68 69
GTO dip stick n tube spare set up .. it bolts to the head ,,,, not to the bellhousing

it has the riveted FAT tip that the early cars use, The full line is just at the end of the tube

the tubes like to crack at the "O" ring and leak ,,,, also I like using the CHEVY truck rubber sleeve





__





chevrolet th400 dipstick tube seal - Google Search






www.google.com





and a light smear of permatex black sealer after I wipe EVERYTHING down with brake clean,,,,

I usually have to drain some fluid out to make sure its below the tube level so it does not effect the sealing of the permatex
and make sure the hole in the case is suoer clean also,,,

seal into tranny first then tube into seal and then bolt to the head

let it set up overnite

Scott

2o6-4six5-9165


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## Praxis (Feb 23, 2014)

BLK69JUDGE said:


> I have a real 67 68 69
> GTO dip stick n tube spare set up .. it bolts to the head ,,,, not to the bellhousing
> 
> it has the riveted FAT tip that the early cars use, The full line is just at the end of the tube
> ...


Is this for sale?


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## Praxis (Feb 23, 2014)

armyadarkness said:


> And here is my pan, which you will notice adds 2 qts
> 
> 
> 
> ...


I knew it needed the stock stick, but I’m not sure how much more fluid my sump takes. I bought it a long time ago and don’t remember from where, might be a B&M like yours.


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## armyadarkness (Dec 7, 2020)

@BLK69JUDGE included his phone number for you to call him. I think you really should. If you bought your pan a long time ago, it's most certainly a B&M, because that wouldve been before the whole Amazon/ China/ Knock-off age. Your situation is likely much less complicated than it appears.


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## BLK69JUDGE (Jun 10, 2010)

sure ... its a spare ... my rigs are 4 speed cars ,, so no need for it

Scott
2o6-4six5-9165


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## O52 (Jan 27, 2019)

I found the best deep sump pans that don't leak are the factory units, installed on the 3/4 and 1 ton trucks from the 80s/90s.


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## Praxis (Feb 23, 2014)

armyadarkness said:


> @BLK69JUDGE included his phone number for you to call him. I think you really should. If you bought your pan a long time ago, it's most certainly a B&M, because that wouldve been before the whole Amazon/ China/ Knock-off age. Your situation is likely much less complicated than it appears.


Bought it!


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## armyadarkness (Dec 7, 2020)

Good. Step one is complete.


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