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  • How many places can a TH400 leak from?

    Howdy,

    Well I've been eyeballing my Riviera thinking what it will take to be able to enjoy it enough to take it to a cruise in or even to Denver. The most glaring problem is the huge transmission fluid leak.

    Best I can tell it doesn't leak while it's running, or at least not at the rate it leaks just sitting. I've straighted the pan as best I can, put on a high dollar and super thick Moroso gasket to try to seal it up and I've torqued the pan bolts to the recommended specs. I replaced the tailshaft gasket when I swapped the speedo gears and the rear bolts are tight.

    Where can I possibly be leaking like 1-2qts over a night from just letting it sit?
    Central TEXAS Sleeper
    USAF Physicist

    ROA# 9790

  • #2

    Re: How many places can a TH400 leak from?

    oh, lots of places.

    Dipstick tube O ring, especially if the tube is mangled like most of them are by now. Or the tube itself can be leaking.

    Shifter shaft seal.

    Kickdown connector or it's O ring.

    Speedometer gear housing to case O ring.

    Speedometer gear to housing seal. Or the gear itself can be worn, or the gear housing.

    Pump bolt washers or O rings.

    Pump O ring.

    Pump to converter seal.

    Output shaft O ring, if the yoke has a weep hole.

    You don't need a special pan gasket, you need to make sure the pan surface is flat and clean, and the case surface is flat and clean, then any old gasket will seal it.

    But if it leaks overnight, then it's something higher up than the pan gasket.
    Last edited by squirrel; December 11th, 2011, 08:52 PM.
    My fabulous web page

    "If it don't go, chrome it!" --Stroker McGurk

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    • #3

      Re: How many places can a TH400 leak from?

      I've seen shifter shaft seals drive people nuts when trying to find leaks........

      Or you can go buy yourself some of this....... pour it in and use a black light and see where the leak is coming from......


      http://www.ebay.com/itm/GM-TRANSMISS...ht_6770wt_1163
      Last edited by TC; December 12th, 2011, 09:00 AM.

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      • #4

        Re: How many places can a TH400 leak from?

        Jim forgot to mention a cracked case.
        BS'er formally known as Rebeldryver

        Resident Instigator

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        • #5

          Re: How many places can a TH400 leak from?

          they only leak between the bellhousing flange and the slip yoke
          Doing it all wrong since 1966

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          • #6

            Re: How many places can a TH400 leak from?

            the cooler fittings

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            • #7

              Re: How many places can a TH400 leak from?

              I stand corrected, they only leak from the radiator to the tail housing lol
              Doing it all wrong since 1966

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              • #8

                Re: How many places can a TH400 leak from?

                they only leak between the bellhousing flange and the slip yoke
                is that one end to the other.
                you narrowed that down helpfully.

                a leaker that badly..I'd put in a cooler once fixed.
                3...2....1....What happened!?!!

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                • #9

                  Re: How many places can a TH400 leak from?

                  ALL of my TH400's (and I've had several) ALL leaked at the converter to case seal - and no, replacing the seal doesn't fix it. I pulled the one from the Caddy for the second time and the trans shop guy took pity of me and fixed what was really wrong - the front pump has to come out and the converter/front pump bushing has to get fixed, then reassembly and a new front seal. He says you normally have to do this about every 100K miles (+/-, your mileage may differ). Now whenever I mess with a TH400 this happens before I install it.

                  No doubt Jim's list is more extensive and more accurate but that's been my experience.

                  Dan

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                  • #10

                    Re: How many places can a TH400 leak from?

                    all joking aside - the words "I straightened the pan as best I can" make me think it wasn't straightened enough - it's never enough (been there, done that, read the book, saw the movie, got the gun out to shoot the damn thing then thought better of it - I sold it to someone else). I know it's an expensive option, but if you can, get a cast aluminum pan to replace your current pan. Also, run a flat file over the edges in the longest stroke possible. I do a pretty good job of getting things sealed, but transmission seals, if I can, I take to the pros so when (not if) it leaks, it's their problem.
                    Doing it all wrong since 1966

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                    • #11

                      Re: How many places can a TH400 leak from?

                      all joking aside - the words "I straightened the pan as best I can" make me think it wasn't straightened enough - it's never enough (been there, done that, read the book, saw the movie, got the gun out to shoot the damn thing then thought better of it - I sold it to someone else). I know it's an expensive option, but if you can, get a cast aluminum pan to replace your current pan. Also, run a flat file over the edges in the longest stroke possible. I do a pretty good job of getting things sealed, but transmission seals, if I can, I take to the pros so when (not if) it leaks, it's their problem.
                      Unfortunately it's the early "shoe heel" pan, there are no aftermarket pans for it and going a flat bottom pan requires replacing the valvebody.

                      I've got the 4L80E in the garage already and once I get my hands on the rest of the wiring I need for it I think I'll try my hand at pulling the engine and swapping over. I'm pretty much done with trying to seal the existing transmission.
                      Central TEXAS Sleeper
                      USAF Physicist

                      ROA# 9790

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                      • #12

                        Re: How many places can a TH400 leak from?

                        Grrrr!!! still leaking and I've not even started it up after the external reseal.

                        All I can think of is the expensive Moroso steel core rubber gasket isn't working at keeping it from leaking around the pan or I've got a leak in the tail housing somewhere that's dripping down to the gasket rail and that's where I really see it. With full on the dipstick cold, is the fluid up high enough to get past a loose rear bushing and lip seal on the yoke seal?
                        Central TEXAS Sleeper
                        USAF Physicist

                        ROA# 9790

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                        • #13

                          Re: How many places can a TH400 leak from?

                          shouldn't be that high. And a rear seal leak will stop at the trans mount, not go forward to the pan.
                          My fabulous web page

                          "If it don't go, chrome it!" --Stroker McGurk

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                          • #14

                            Re: How many places can a TH400 leak from?

                            how many places can a turbo 400 leak? ALL OF THEM! but still --they are well worked out, pull it out and reseal it
                            Last edited by stage1scott; June 11th, 2013, 10:04 PM.
                            67 Fairlane 434 ci/464 hp/488 tq-RIP
                            05 MINI Cooper S Supercharged
                            08 MINI Cooper S Turbocharged

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                            • #15

                              Re: How many places can a TH400 leak from?

                              Mine leaks it seems from the front pump someplace. Ill get it redone one of these days.

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