Moroso TH350 Pan Gasket Woes. Can't Make it Stop Leaking!

Discussion in 'Transmission & Driveline Topics' started by slayer021175666, Feb 2, 2021.

  1. ChevyReb

    ChevyReb Veteran Member Lifetime Gold Member

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    How would you test the vent tube, and that's an excellent point BTW.
     
  2. camaro71/holland

    camaro71/holland Veteran Member

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    Make everything dry and put some talk/feet powder around suspected areas it will teel you where it's leaking from.

    Vent puking atf happens only when hot and overfilled, so it shouldn't leak when the car has been standing still for some time.

    I put a tab of RTV on every seal with a metal edge to be sure it won't leak.
     
  3. Cardinal

    Cardinal Veteran Member Lifetime Gold Member

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    Unless you can see the leak at the sump pan, I suspect that your transmission is leaking from some place else:

    1. Dip stick tube: we chased a leak at the dip stick tube with little success! We finally coated it with Permatex The Right Stuff problem solved.

    2. Shifter arm shaft seal. What a PITA that can be! Not real difficult to replace but it does require removing the shifter arm.

    3. Transmission vent. Another PITA. There are many reasons for it to vent fluid and most are not good. ASSUMING that your TH350 is in good shape, it could be overfilled especially if you have the wrong or an incorrect dip stick. The level should be check when the transmission is warm.

    4. Cracked case. It happens especially when people install solid transmission mounts! NEVER EVER use a solid mount on a transmission. Totally unnecessary!

    You say you bought an aluminum sump pan. ANY gasket should seal it to the transmission. Check the pan rails on the pan and the transmission with a (carpenter's) square and a flashlight. NO to little light should be seen between the square and the surfaces. I recommend using Permatex Right Stuff to and a gasket to seal it. The Right Stuff is AMAZING! EX: Moroso tin differential covers are NOTORIOUS for leaking! I put The Right Stuff on the 12 bolt cover in my 87 S10 drag truck then filled it with full synthetic diff lube. It has NEVER leaked a drop of fluid!
     
    Last edited: Feb 5, 2021
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  4. 70 1/2

    70 1/2 Member

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    How about the input seal?
     
  5. Zclassic

    Zclassic Veteran Member Lifetime Gold Member

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    It was already mentioned here in a post but wanted to share my experiences with the Moroso blue gasket. This gasket is awesome. It has fixed several stamped pan leaks I have had over the years even including a Ford AOD. Make sure the pan holes are not dimpled from over tightening and if they are deformed use the small end of a ball pein hammer as a form while striking with another hammer on a flat surface. Use blue locktite on the clean dry bolts and do not over torque. If this gasket does not seal your tranny there is another leak or overfilled IMHO.
     
  6. dickeymoore

    dickeymoore Member

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    I had this problem years ago turned out the transmission pump seal was leaking!!!!!
     
  7. accelo

    accelo New Member

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    I used to work on these a lot. Chances of it being the pan gasket are minimal, especially if the pan surface is flat and in good condition.
    90% chance it's the dip stick tube. It drips along the top and looks just like a pan gasket leak.
    Especially if you have the old tube style that takes an "O" ring.
    I would update the tube to the tube that takes the seal that looks like a top hat.
    easily identified as the end is tapered to expand the seal and no groove for the O" Ring.
    They are available aftermarket.
    Front seals rarely leak. I have even observed a crack at the converter hub cause this.
    Confirm it's not the easy stuff first. Sometimes a mirror helps identify the source.
    Best to dry the pan surfaces with brake clean before installation.
    Use the gasket dry, never use silicon and a gasket at the same time.
    Best to dry the surface with brake clean before installation.
    Best of luck
     
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  8. bullheimer

    bullheimer Veteran Member

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    agreed with the above and also with a rear intake manifold gasket leak to check. i used Moroso valve covers 30 years ago and they didn't fit like i wanted. needed a universal joint above the socket to tighten the top bolts. i don't like their products because of that. i have had good luck with ordinary dip stick tubes, just coated with permatex aviation gasket sealer prior to insertion
     

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