1991 Buick Park Avenue - Trans fluid in the engine oil

I have got ATF leaking into engine on my park avenue 1991 3.8v6. transmission fluid level goes down and engine oil goes up and become red. How it possible? No one opening bonnet except myself.

The only thing I can think of is via the vacuum modulator in the transmission. But normally if that were leaking the transmission fluid would be drawn into the intake manifold and burn in the combustion chambers.

Sounds like a leak

First guess is you are adding transmission fluid into the wrong fill tube.

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I cannot figure out any way for this to happen. If you find out, we would all like to know.

Remove the vacuum hose from the transmission vacuum modulator. If transmission fluid leaks out of this connection, the modulator is leaking.



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Without a doubt. On vehicles with a vacuum modulator on the transmission the engine can draw ATF out of the transmission and into the crankcase. I’ve seen that happen on several cars and all that I specifically recall were GM models. The ATF would be drawn up above the intake where it would drain down through the PCV valve into the engine.

Did anyone but GM ever use transmission modulators? I have never seen anyone else’s cars going down the road spewing transmission fluid out the tailpipe. I didn’t know a significant amount of it could get in the crankcase.

I used to know some guys in the 50s and 60s who put a quart of transmission fluid in the crankcase of their old beat up GM cars.

These guys lived in stained and ripped jean and sometimes in their cars and were the “day late and a dollar short” type so I never paid any attention to their advice.

Yes. I had a ‘91 4cylinder Mustang, A4LD transmission. Doing This purely from memory, leaked fluid into the vacuum line then sucked into the intake and burned.

Benz used them for quite a long time

I had a 94 Silverado with a 350 that I used 1 quart of trans fluid in the engine oil at each oil change to free a stuck lifter. I did this on the advice of an old mechanic. I was skeptical, and it may have increased engine wear, but I only had about $500 in the truck anyway and it had already lived a hard life before I acquired it. If I didn’t add the trans fluid during an oil change, sure enough, the lifter would start to stick again on a cold start. So I’d add a little atf, and the lifter noise would subside. I drove the truck for about 10k miles, then sold it. I did disclose the lifter issue to the new owner. I wonder now if synthetic oil or maybe 0w30 would have worked instead of the atf. Ah well, atf is cheaper and it was a $500 truck. I miss that truck though! Should’ve kept it.

I would have tried marvel mystery oil instead, for what it’s worth

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I did use seafoam in the oil in that engine once. It worked too.

I always sort of wondered if Marvel Mystery oil wasn’t repackaged atf. Have never used it.

I think marvel has been around for longer than automatic transmissions. Remember, ATF is simply 10 weight engine oil with a LOT of additives, including aggressive detergents, anti-foaming agents, friction modifiers, and who knows what else.

a 91 trans does have a vac modulator? so it was agreed to change it first? and see what happens?