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Transmission fluid is disappearing, no leaks evident

I have a '96 Buick Skylark with a 3.1L V-6. It has been using a quart of transmission fluid in less than 100 miles. There is no smoke to indicate that it might be getting sucked into the vacuum modulator like in the old days. (Does it even have one?) There is also no evidence of it leaking onto the ground or the underside of the car. Where is it going? I’m spending more on transmission fluid than I am on gas.

Your trans should have a modulator, remove the vacuum line going to it and see if it is wet inside or any fluid is leaking out of the modulator. The only other place it could be going is in the radiator from a ruptured cooler.


The coolant looks normal. I looked at it the other day. I’ll take a look at the hose when I can see the car later and let you know what I find.

Why is there no smoke?

Is the modulator a simple bolt on item like they used to be?

I looked at the car again this afternoon. It had been parked in the same spot since last night. There was no fluid on the pavement under it, or evidence of leaking on the underside of the car. The line to the modulator is clean, no transmission fluid is in the line or the modulator.

Does anyone have any other ideas?

There is no other place for it to be going other than the cooler. There are only 3 possibilities. An external leak, a ruptured modulator allowing ATF to be sucked into the engine, or leaking into the radiator. There is no other place fo the atf to hide.


Another thought is an odd seal leak that only leaks under driving conditions. One way to check for this is to clean the bottom of the transmission, especially around the inner cv joints as best as possible. Drive it around for a bit of time, then reinspect the bottom of the transmission. With a leak this large, you should be able to see fresh fluid staining near where the leak is.

Alternately, you could try a dye tracing kit like this

are your buddies at work the practical jokers I think they are?

BustedKnuckles, that was it. No leaks on the pavement, but I was looking at the front of the car for traces of fluid. Had I been looking toward the rear I’d have seen lots of it. The right inner seal is the culprit. It will be changed tomorrow.

Thanks for all of your input.

I think I may have this same problem. I have a 92 LaSabre with 78k originial miles. A month or so ago I had my oil and filter changed at a quick change place. They came in and showed me my trans fluid on a paper towel and said it was bad but they didn’t recommend doing anything as that could cause the trans to fail. I never had a problem with the trans ever. Now a month or so later my trans fluid keeps dropping. I wonder if they could have done something to the inner seal you speak of? Is this inner seal difficult to replace yourself? Is it expensive? I would appreciate any more info. Thanks

Don’t put much faith into what a fast lube facility tells you; at all.
If the fluid has never been changed then yes it should be changed now. Not ever changing the fluid is usually what causes the transmission to fail; not changing it after the fact.

If you’re losing fluid then it’s due to an external transmission leak, a faulty vacuum modulator, or a leaking transmission cooler. No matter which one is behind this is needs to be repaired promptly as an automatic transmission can be damaged very very quickly due to a low fluid level.

As to price, the most serious cause of a leak is the torque converter seal, which requires removal of the transmission. After that the next most costly repair is a leaking trans fluid cooler and this requires replacement of the radiator or the addition of an add-on fluid cooler. My preference would be the add-on cooler as it’s cheaper and works far better than a radiator unit.
Other leaks are generally not major at all.

Thank you OK4450. I am going to crawl under it tomorrow and see if I can see a leak near the cv joint as was the case with the gentleman that started this thread. This is the wife’s car and she drives only a couple days a week teaching piano. I will keep a daily check on the fluid level until I can get this sorted out and fixed. I saw at Advance Auto that the cv seals are only a few bucks and one feedback that was left said they were easy to replace. I am trying to find some directions on how to do this just in case. Three weeks ago she overheated when the hose from the waterpump to the heater inlet broke. A five dollar part I replaced. It was after this overheating that this problem started with the trans fluid. Before there had never been a problem and I check her fluids weekly.

At Advance Auto I saw some stuff called Barz Transmission Stop Leak that supposedly rejuvenated seals. Would it hurt to give this a try? What would you recommend with regards to this product.

I do appreciate your help. We are retired on pretty much a fixed income and right now Do It Yourself is my first choice. LOL

Thanks again for your fast reply to my plea for help.

I’m not a fan of additives except in certain situations. If the loss of fluid is due to a leak then it’s unlikely that an additive will cure a leak that leads to fluid loss this severe.

Considering the overheating you should make sure the radiator fluid cooler is not leaking.
With the engine cold remove the radiator cap after the car has been sittng all night and make sure that trans fluid is not present in the radiator. This will often appear as a tan muck rather than red fluid.

To check the modulator do as transman mentioned earlier and note if there is any fluid in the small vacuum hose. If the modulator is bad that’s also an easy fix.