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The C4 Transmission mystery leak

Old 01-18-2019, 07:48 AM
  #1  
Chester_Kaup
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Default The C4 Transmission mystery leak

I have a 1966 mustang with a C4 automatic with a transmission leak that I cannot figure out.I let the car warm up, check the fluid level and correct if necessary. I then check under the car while idling in park and drive for leaks. All is dry. I will then take the car for a drive and bring it back into the garage. I then check for leaks and everything is dry. The next morning there is a puddle of transmission fluid under the car.I have tried about every pan gasket and gasket sealer known to modern man. My procedure is to clean the mating surfaces and dry them. I also make sure the pan flange is flat. I then install the pan and tighten the bolts to the recommended torque.When I inspect the outside of the transmission after it leaks it is dry around the case fill dipstick, transmission modulator, shifter shaft and overflow tube.Is there somewhere other than the pan it might be leaking from?
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Old 01-19-2019, 12:57 AM
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Gun Jam
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what about power steering?
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Old 01-21-2019, 10:06 PM
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imp
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Originally Posted by Chester_Kaup View Post
I have a 1966 mustang with a C4 automatic with a transmission leak that I cannot figure out.I let the car warm up, check the fluid level and correct if necessary. I then check under the car while idling in park and drive for leaks. All is dry. I will then take the car for a drive and bring it back into the garage. I then check for leaks and everything is dry. The next morning there is a puddle of transmission fluid under the car.I have tried about every pan gasket and gasket sealer known to modern man. My procedure is to clean the mating surfaces and dry them. I also make sure the pan flange is flat. I then install the pan and tighten the bolts to the recommended torque.When I inspect the outside of the transmission after it leaks it is dry around the case fill dipstick, transmission modulator, shifter shaft and overflow tube.Is there somewhere other than the pan it might be leaking from?
Cracked transmission case, worst scenario.
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Old 01-21-2019, 11:17 PM
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Urambo Tauro
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Something has to be wet where the leak is originating from. Front pump seal? Extension housing?

I don't know what your driveway looks like, but if you're parking it inside the garage, you should have a reasonably flat surface for leaking fluid to drop straight down.You should have been able to narrow down the general area by now, just by looking at where it's dripping on the ground. Slide some cardboard underneath there if you need a fresh surface to watch where puddles to start.
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Old 02-24-2019, 09:55 PM
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groho
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Common problem. The 66 c4 has a vent tube on the drivers side just above the linkage and neutral safety switch. Basically, your torque converter is draining back into the tranny through the pump and over filling the case as it sits in the garage. The problem is not when you're driving. Not a big issue, easy fix. You can add vacuum hose to the short 5/16 vent tube and route it upward so any excess fluid drains back into to tranny. The vent tube faces down, you might consider shorten it, or turning it forward before add a hose extension, otherwise fluid will just sit in the lower loop of the hose and never drain back.
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Old 02-26-2019, 01:49 PM
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Chester_Kaup
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It appears that the leak is around the bolts that hold on the pan at the rear of the transmission. Thanks for the suggestions and information.
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Old 03-23-2019, 10:46 PM
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Shifter shaft seal. Where the shaft goes through the case. Runs down on the pan.
​​
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Old 04-10-2019, 04:42 PM
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Chester_Kaup
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The leak has been fixed.
1. Removed the pan and cleaned the transmission and pan matting surfaces
2. Made sure the pan was straight and flat
3. Applied a bead of permatex #2 on the pan rail and around each bolt hole
4. Pressed the pan gasket gently to the pan and wiped off any excess permatex
5. Let set for 4 hours. The recommended time before placing in service
6. Applied a bead of permatex #2 to the pan gasket and around each bolt hole
7. Bolted up the pan putting permatex on the bolt threads of the bolts in the area that seemed to leak
8. Let set for 24 hours; the recommended time for a full cure.
9. Refilled the transmission
10. So far no leaks after 2 test drives a week apart

I know usually the gasket is put on dry but sometimes some old fashioned gasket sealer gets the job done.
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