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Transmission fluid empty, instead in engine oil compartment

I have a 2004 Toyota Avalon XLS, automatic transmission with 166k on it. I have never swapped out the transmission fluid at sound advice from loved ones with self-maintained vehicles that have aged well over the decades. I recently noticed the shifting doing two things at a frequency of about once per week: (1) not engaging as quickly as it used to when changing, and (2) getting stuck and revving up the RPMs unexpectedly. So, I figured it was time for a change (NOT a flush). My boyfriend lovingly offered to do the job for me, as he had done it before on his Previa without a problem. I did not hover over him and only came into the garage when he said the job was done. We started up the car and it would not shift into gear. We ran the car for maybe about 10 minutes, occasionally shifting through the gears. Still, no motion in the ocean. We took a step back and I asked him to show me what he did. When we got to the step of him adding the transmission fluid, he showed me that he funneled all 4 quarts of transmission fluid into where the engine oil goes! SO…we ran the car with an OVERFILLED engine oil compartment (likely at a 1:1 oil:ATF ratio) and an EMPTY transmission.

What kind of damage am I looking at due to the empty transmission and overfilled oil?
How best do we clean the oil compartment? Change it several times in short succession? Use an additive?

Thanks all for your help!

If you did not drive the car there’s hope! The transmission holds about 12-14 quarts, so it did not self destruct if you did not drive the car. If your boyfriend put the oil plug back in, your engine was overfilled by 80% or so with transmission fluid. Again, if you did not drive the car, not much damage was done. It should all be drained and refilled with the right oil. Then after driving a hundred miles or so you (after properly refilling the transmission) can change it again to get any residual fluid out. Transmission oil is also a lubricant so it won’t damage the engine.

This mistake is often made on Subarus who have two similar plug side by side.


I don’t think you did much damage but he owes you an oil and filter change and more trans fluid before it leaves the garage. Likely the trans fluid cleaned the engine out a little. This one might be worth it though for the long term ability to hold it over his head. “Yeah sure, remember when you changed my transmission fluid? Haw haw haw.”


You did zero damage to the engine, just do a normal oil change. The ATF probably did some good in the engine, in the old days, we used to dump a quart into engine just before doing an oil change to clean it out.

As for the transmission, it uses the ATF to provide hydraulic pressure to do things, without the hydraulic pressure, things don’t get done, so there is probably no damage either.

After your boyfriend changes the oil and fills the transmission, give him a dope slap.

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I might add this should be the last time he works on your car

You got lucky, in the sense that there is probably no damage, and nobody got hurt

Just imagine if he made a “simple mistake” while working on your brakes

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Going 166k miles without ever changing the transmission fluid is not sound advice.


done the same thing many years ago. Drained trans fluid and, without much thought, pored trans fluid down the oil filler tube. Even drove it around the block…to check all is great only to start wondering later why the transmission fluid tube was on top of the valve cover. That’s when I woke up. Changed the oil, pored the right amount of trans fluid down the correct tube and all was well.

Just drain the trans fluid/oil and fill her back up with the correct oil. I suggest you change the oil filter also. And…don’t forget to add trans fluid.


I’m a little surprised that the responses so far are so positive. With four extra quarts of fluid and the engine running for ten minutes, there’s a good chance the oil got whipped into a foam, which doesn’t pump or lubricate well. However, if there is engine damage, there’s nothing you can do about it now, so change the oil and keep your fingers crossed.

Toyota tranny fluid is synthetic with Toyota specific additives. I think it’s made by Mobil. I would only use Toyota fluid. New transmissions tend to do funny things when the wrong fluid is used.

Not so much at idle speed.
Presumably the engine didn’t make any bad sounds?

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Thanks, everyone for all your advice, and admonition. The bf was quite contrite and humble about the oops.

No frothy fluid came out when we drained the oil-transmission combo. We did the oil change (with filter change), filled the transmission with the right fluid and she’s running very well. We’ll do another oil change in another couple hundred miles…unless anyone would advise sooner or later than that? And we’ll do another transmission fluid change in the near future.

I will let him touch my car again with some safeguards in place to avert potential disasters. I’m okay with self-care, as long as it’s done right - better than going to a quick lube or getting hosed by a dealer. :wink:

I wouldn’t bother with an early oil change.
The 1/2qt or less of remaining trans fluid won’t hurt anything.

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+1 to circuitsmith’s comment!
Save yourself some money, and just do the next oil change at the next scheduled interval.

I wouldn’t bother with another oil change, but I would suggest another transmission drain and fill in about a thousand miles. Don’t need to do the filter again, just drain and fill with the correct ATF.

BTW, have you checked your coolant? If it is getting cloudy, do a drain and fill of your radiator. I would recommend against a flush, just a simple drain and fill with a major brand universal antifreeze and distilled water, or premix.

If it happened to my truck, after first getting the proper fluids where they need to go & at the proper level, and replacing the tranny’s filter and a thorough cleaning of the bottom interior of the tranny pan looking for metal debris, I wouldn’t use any special additive, but I’d probably change the engine oil & filter three times over the course of a week’s driving, while watching the tranny fluid level for anything unusual. If the engine ran ok, and the tranny shifted ok after that, I’d guess the problem was solved. If not, the engine runs poorly or the tranny shifts poorly, well, some permanent damage was probably done and will require more fixing.

As mentioned above, if you want professional-results, suggest to use a professional-mechanic to correct this situation and for ongoing repair and maintenance. You and your bf? While the car’s being worked on, take his car & go to an art fair together :wink: You’ll be a much happier couple.