I'm looking for help.
I've read that overfilling the transmission can lead to problems.
When doing a transmission flush, the normal strategy is to fill the transmission up and then run the engine until it reaches a certain temperature range (between 104-113 degrees Fahrenheit) and opening the check plug and letting the excess fluid drain out.
While that makes sense, while the ATF warms up, it's going to expand inside of the transmission pan. So, before the check plug is actually opened, the fluid is temporarily "overfilled".
So, is there anything wrong with LEAVING the check plug OPEN while the ATF warms up?
This makes sense to me, as doing a flush in this manner ensures that the ATF NEVER expands past the limit of where the check plug level is.
I only ask this because I've never seen this recommended before.
If you D.I.Y. your own transmission flushes, I'd love to hear your opinion of this.
Thank you!

The transmission is intentionally overfilled before setting the fluid level as per Toyota doc.
If you pull the plug before it's time to, you'll lose the extra fluid right off the bat AND any additional fluid that has risen as the system warms up.
I see a system that's now low on fluid when the fluid temp is at the desired point for pulling the plug that was pulled a ways back.

Not to cut hairs, but the fluid level set is not a flush.

I overfilled mine during the flush procedure by a quart or so.

Nothing exploded.

Still works fine.

Temporary overfilling by a qt or 2 is not a problem.

What's the hack? I just follow the normal procedure and never had issues.

Yeah I am going to go ahead and stick with the way the most reliable car manufacturer on earth designed it to do.

Yeah you're over thinking it. I followed the Toyota procedure ie overfilling, and I have 135k miles with zero issues.

Sounds like a hack job

Thank you. I'm just nervous to do my own transmission flush for the first time.

The pan is over the level when the engine isn’t running. I assume you’re not talking about pulling the plug before it’s started up but before it’s completely warm. I just don’t think you’ll get the proper “trickle” that’s noted in the manual. It’s not too hard of a procedure but I was nervous like you to get the level set correctly. My problem was I couldn’t get it in test mode with TechStream and after finally resorting to just using the temp readout it was too close to over. I had to wait almost 2 hours to try again. Then I was severely under filled, about a qt. So it took me 2 more tries filling 1/2 qt at a time. I finally got the damn trickle, then I forgot to replace the level plug gasket, so one more time.
Don’t forget to cycle through the gears.

Thank you!

Good for the OP to float an idea before he tried it.

Yeah.

Perhaps a bit more extreme, but awhile back someone at tacomaworld.com quoted a post from a Mustang forum. The OP of the thread claimed he had stuck a garden hose down into the valve cover, removed engine oil pan drain plug, then started the engine to flush with water.
Afterwards, he was wondering why his engine was now running poorly.