1997 Olds 88LS 3.8l with 4T60E trans. Tranny totally rebuilt recently. Problem happening before then and still is. Trans fluid level drops and motor oil level rises. It'* happening at a faster rate now that they put a new pump in the tranny. Vacuum module appears dry. All the mechanics I've spoken to including the trans shop say it just can't happen but I am seeing it. Anyone with an idea how?

You are delusional.

Like how fast and how much? Like the transmission is a quart low and the engine is a quart high? How long (in run time) does it take to move a quart from one to the other?

Good check, however, if transmission fluid was getting through the vacuum modulator then it would get burned as part of the engine running.

Sorry to say I agree with them. There isn't a direct nor an accidental path between the two that wouldn't also involve transmission fluid and engine oil gushing all over the place.

I understand the skepticism; I've been hearing it all along. Latest test was: I had my transmission shop add fluid to the exact position where it was at the top hole in the trans dipstick--double checked level. I changed the motor oil and filter, let car run for a couple minutes then marked the level on the motor dipstick; double checked that too. Went for a drive of only 50 miles--mixed freeway and hills secondary roads. When I checked the dipsticks again; the trans mission level had dropped by 1 hole on the dipstick and the motor oil level had risen by ~ 1/4". I would love to be wrong but this is similar to 2 prior tests only it seemed to be happening at a faster rate which I think may be because the new trans pump is creating more pressure. So-if I'm delusional explain this.

I agree with Cathedral. There is no PHYSICAL way for trans fluid to migrate from the trans to the engine crankcase.

If the 3800 had some sort of external oil cooler that was somehow mistakenly cross tied to the trans oil cooler, than I could see something like this happening. I've never seen a stock 3800 with an external cooler.

Verify you don't have coolant disappearing into the engine which is LIKELY going to happen because its a 3800. Continue to monitor the trans. If the trans level continues to drop, then I would seek a physical external leak.

so I've been told over & over. Still doesn't explain what I am actually seeing. Also, I'm not a novice at cars; been working on them 50+ years.

I have stage 4 cancer and have to unload my toys.

Have you eliminated every possibility that the trans does not have an external leak?

A tiny couple drops of motor oil up front but no trans oil. Even bottom is clean. tranny has only about 100 miles on the rebuild

It sure is purdy! And with only 100,000 miles on it, it has lots of life left in it!

Sorry to hear of the cancer. It has taken way too many of my family. I know it is a tough road to look forward to.