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post #1 of 7 (permalink) Old 01-18-2019, 05:57 AM Thread Starter
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C4 Mystery Leak

I have a 1966 mustang with a C4 automatic with atransmission leak that I cannot figure out.
I let the car warm up, check the fluid level and correct ifnecessary. 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 thegarage. I then check for leaks and everything is dry. The next morning there isa puddle of transmission fluid under the car.
I have tried about every pan gasket and gasket sealer knownto modern man. My procedure is to clean the mating surfaces and dry them. Ialso make sure the pan flange is flat. I then install the pan and tighten thebolts to the recommended torque.
When I inspect the outside of the transmission after itleaks it is dry around the case fill dipstick, transmission modulator, shiftershaft and overflow tube.
Is there somewhere other than the pan it might be leakingfrom?
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post #2 of 7 (permalink) Old 01-18-2019, 09:54 AM
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Re: C4 Mystery Leak

Is it wet around the bell housing? It could be the pump seal.
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post #3 of 7 (permalink) Old 01-18-2019, 11:37 AM
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Re: C4 Mystery Leak

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Originally Posted by sixtyeightford View Post
Is it wet around the bell housing? It could be the pump seal.
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post #4 of 7 (permalink) Old 01-18-2019, 02:40 PM
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Re: C4 Mystery Leak

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Originally Posted by Chester_Kaup View Post
I have a 1966 mustang with a C4 automatic with atransmission leak that I cannot figure out.
I let the car warm up, check the fluid level and correct ifnecessary. 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 thegarage. I then check for leaks and everything is dry. The next morning there isa puddle of transmission fluid under the car.
I have tried about every pan gasket and gasket sealer knownto modern man. My procedure is to clean the mating surfaces and dry them. Ialso make sure the pan flange is flat. I then install the pan and tighten thebolts to the recommended torque.
When I inspect the outside of the transmission after itleaks it is dry around the case fill dipstick, transmission modulator, shiftershaft and overflow tube.
Is there somewhere other than the pan it might be leakingfrom?
When the car sits, the torque converter drains back into the pan, raising the oil level.

Let the car sit overnight (or however long it takes for the leak to show) and pull the dip-stick *before* you start the engine.

It should read high up the stick.

Now get under and check *really carefully* around these areas....

1. At the base of the fill tube/dip stick tube.
2. Linkage seal area
3. Front servo cover.

My money is on the fill tube O-Ring.

Here's what happens... when the car sits and the converter drains back as mentioned above...
... the fluid level gets above the fill tube O-Ring, and leaks past the O-Ring.
That leak then runs down and around the pan gasket, making it *look like* the pan gasket is leaking.

When the engine is running, the "Full" level on the stick is approx at the level of the pan gasket,
which is *below* the level of the dip-stick tube O-Ring.
So it never leaks while it's running.

When you inspect for the leak after the car sits... look very closely at the case area around and below the fill tube.
Use a good bright light and a clean white rag to wipe the area at the base of the fill tube.

I bet you find your leak.

If not that... then next likely place is the linkage seals.
These can leak in the same manner as the dip stick tube.

Edit;
As mentioned above, it could also be the front pump gasket or front seal, tho those "should" leak all the time.

Last edited by galaxiex; 01-18-2019 at 02:45 PM.
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post #5 of 7 (permalink) Old 01-18-2019, 04:30 PM Thread Starter
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Re: C4 Mystery Leak

Should i be looking at where the bell housing and transmission come together, the bell housing and the engine come together or take off the flywheel inspection cover? Then again all of these? I was wondering since I had the transmission rebuilt as a carry in to a transmission shop and when I got it back they gave me the transmission and the torque converter in a box if I could have damaged something installing the torque converter.

Thanks everyone for your assistance.
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post #6 of 7 (permalink) Old 01-18-2019, 11:37 PM
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Re: C4 Mystery Leak

I'll add , sure sounds like when converter drains back.. Check the kickdown seal and the breather. Also verify the dipstick is right so trans is not overfull.
Run the stick along the outside of the tube . As mentioned above the correct level is a bit above the pan rail
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post #7 of 7 (permalink) Old 02-05-2019, 12:20 PM Thread Starter
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Re: C4 Mystery Leak

Raised the car up all the way around so the transmission pan was level with the floor. Let the engine warm up and checked the fluid level. After running the car in gear for awhile it leaked. The only area that leaked was at the rear of the car. Since it was level I was thinking any leak would drip straight down rather than running any where. There may be a small amount of leakage between the pan and the pan rail. Most all the dripping was coming of the heads of pan bolts at the rear of the pan. Is it possible for it to leak around these bolts or the thread of the bolts? Thanks for your assistance.
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