JJTL

Casual User
Jan 25, 2022
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Kitchener
VetteCoins
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1987 C4 Z51 Corvette
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Hi there folks!

Got a lot of work done on the Vette this weekend, doing a lot of drivetrain fluid changes. Unfortunately, it's become clear that I have a gear oil leak from the 4 speed transmission. Only took an hour or so and I had a small pool of oil under the car. Hoping if there are any Doug Nash experts here, they could shed some light on some common or likely causes of the leak that I can look into. I also have a FSM and that's given me a few ideas. When I drained the oil from the 4 speed transmission it was clearly low, so likely not a new issue. I've only had the car since January and haven't driven it at all yet. Below is a summary of the information I know, that might be helpful:
  • Leak is clearly fresh 4 speed gear oil (gold in colour, not red like the ATF in the overdrive).
  • In changing both the oils in the trans and the overdrive, so far I have no evidence of the oils mixing. Clear gold colour.
  • Oil is leaking somewhere in the area between the the 4 speed trans and the overdrive.
  • Oil is leaking down onto the top of the oil pan for the overdrive, and then rolls over it (see pictures below, drip rolls off edge of overdrive pan in the area of the large bolt).
  • Don't see obvious leaks from the seal/gasket between the trans and overdrive, but hard to see.
  • Test drive didn't reveal any concerning noises, grinding or difficulty shifting. In fact it was all quite smooth.
Thanks in advance for any help and input! Any common or well known areas where these leak would be helpful. Has anyone had any experience with a stop leak additive or anything similar?

Josh
 

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Mine leaked from the shift shafts, orings. And it leaked from the vent tube outlet on top of the 4spd section. Im not sure if there was supposed to be a grommet or a fitting where the vent tube (which runs up and pokes its head out between brake booster and distributor) inserts into case but the way i found mine was with rtv globbed around it. i removed the glob, cleaned with isopropyl alchohol and used fresh rtv.

Im not sure if you could get to it without dropping or at least lowering the rear end of the trans down. If you attempt this, be sure to remove the Distributor cap, so that it does not contact the firewall as you lower the tailshaft down to inspect.

a cheap cell phone pairing inspection cam might get in there to view it as well. i got mine off amazon for about $30
 
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Mine leaked from the shift shafts, orings. And it leaked from the vent tube outlet on top of the 4spd section. Im not sure if there was supposed to be a grommet or a fitting where the vent tube (which runs up and pokes its head out between brake booster and distributor) inserts into case but the way i found mine was with rtv globbed around it. i removed the glob, cleaned with isopropyl alchohol and used fresh rtv.

Im not sure if you could get to it without dropping or at least lowering the rear end of the trans down. If you attempt this, be sure to remove the Distributor cap, so that it does not contact the firewall as you lower the tailshaft down to inspect.

a cheap cell phone pairing inspection cam might get in there to view it as well. i got mine off amazon for about $30
Awesome! Just the kind of info I'm looking for. I've got a couple of places to start now. When I look up, I can see some rods and so on, on the driver's side of the 4 speed transmission, that have oil on them. Was certainly, suspicious to me...although not sure if they would some how cause a leak there that ran down between where the transmission and overdrive connect. I will definitely take a look on Amazon for an inspection camera.

I took the attached picture before I even knew about the leak, thought some of the oil up there looked suspicious. I wonder if the vent is near where the two units mate up, this could maybe cause a leak in the area I'm seeing. I'll check the FSM for the vent location. It's a pretty substantial leak, not like a it's just seeping from a gasket. You would have thought I overfilled it or something, looking at the pool of oil.

Thanks!!!
 

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those rods that have bends in tem come from the shifter base forward to short shifter shaft links. if you pull those off, carefully, then remove the side plate, there are orings on each of the two shift shafts. and nylon washers iirc.

and there are some sort of cam type things that have detents in them too i think. it was all very self explanatory for sure. all i did was replace the orings with some i matched from my princess auto oring kit. that solved that leak. and it also cleaned up the shifting smoothness.

i was wrong, the vent comes out of the OD. see the big black tube.

Here are a couple pics. Now that i think of it, i may have loosened the bolts and added some sealant between the case to stop gear oil from leaking. been a while.

youtube gearbox video 4+3 for a good breakdown.

1D19B23F-B719-4105-9501-15B77DEC1DD8.png


86C60799-CDFD-46D3-A106-490910B547AA.png
 
those rods that have bends in tem come from the shifter base forward to short shifter shaft links. if you pull those off, carefully, then remove the side plate, there are orings on each of the two shift shafts. and nylon washers iirc.

and there are some sort of cam type things that have detents in them too i think. it was all very self explanatory for sure. all i did was replace the orings with some i matched from my princess auto oring kit. that solved that leak. and it also cleaned up the shifting smoothness.

i was wrong, the vent comes out of the OD. see the big black tube.

Here are a couple pics. Now that i think of it, i may have loosened the bolts and added some sealant between the case to stop gear oil from leaking. been a while.

youtube gearbox video 4+3 for a good breakdown.

View attachment 95471

View attachment 95470
Thanks! I checked the FSM and there is a vent above the side plate on the T-10 4 speed, but it's separate from the vent line to the engine bay. Not sure a leak there, would drip down where it onto the top of the oil pan of the O/D. Hoping to get it back up on the Quickjack soon to try again at chasing down the exact leak source.
 
Howdy all! Been awhile, thought I would post the update/conclusion to this thread. The leak did end up being between the 4-speed adapter plate and the OverDrive unit. I had a local shop (Mr. Transmission Kitchener) do the work, as it required dropping the transmission and separating the units. Just not a job I had time to do myself. I can't say enough good things about the guys at Mr. Transmission here, they are real car guys and went above and beyond to learn the Doug Nash 4+3 to be able to work on it. They studied my FSM before doing the job as well. Highly recommend them.

Hope everyone' summer if off to a great start!
Josh
 
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