I picked up an E63 that had a minor front end accident, previous owner said crushed radiator leaked out transmission fluid and car doesn't move anymore and had to be towed. He told me he ran it for 5 minutes with it slipping and a neighbor pushing the car to move it into backyard.

I towed it to my house, replaced core support and installed new radiator, hooked everything up, and filled the transmission with the appropriate Titan 4134 oil and used overflow method at 45C using a Fluke 62.

The problem: transmission works fine when it's cold, and even at 45C it works great and hooks good. But as soon as you try to drive it on the streets and the temps get hotter, it starts slipping up to the point it won't go anywhere anymore. Turn the car off for a few minutes and turn it back on, and it'll move again.

I've researched for a few hours about this and to my understanding it seems that it sounds like this isn't a torque converter problem and instead, to my understanding, the transmission has bad seals due to the excessive heat of the previous owner trying to move the vehicle hardening them and when it's cold the fluid is thicker so it moves fine, but when it's hotter it gets thinner and it slips past the hardened orings/seals and causes pressure drop?

I am an experienced mechanic in everything else (all engines, body work, etc) but don't have a lot of transmission experience other than rebuilding a few manual transmissions. I would like to tackle this on and perform the repair myself. If my understanding is correct, I have a few questions if you may assist me.

1) Does it sound like my research is correct?

2) Would it work if I purchased a used transmission and swapped over my valve body?

3) Do you require any special/expensive tools to disassemble a 722.9?

4) I see there are a few "Overhaul Kits" and "Master Rebuild Kits" for the 722.9, the overhaul kit having the gaskets, seals, and sealing rings. Can I currently assume I only need the overhaul kit and don't need the master rebuild kit?

5) Is there any advice/tips/experiences anyone can share that's had a similar issue with their 722.9 and/or performed internal work?

Any help would be greatly appreciated, this is a pretty big disappointment and headache.

You are better off spending $200 for the dealer to evaluate it before spending the cash on a rebuild/refresh kit. Maybe as simple as a valve body or conductor plate.

These cars are finicky and often need to be hooked upto SDS to reset them into proper operation. Good buddy of mine is a mechanic so I get to hear a lot of these oddities. For example, I know after you swap a used trans into these, you need the dealer to program it to your car even though it may have come out of the same exact year and model. They'll do odd things....like lock out 1st gear and work well in all others.

Get it properly diagnosed. The oil pumps in the 722.9 transmissions have been failing for some time now.

Change the filter if you have not. Taking the pan down and seeing whats inside is always helpful. Filters get clogged up and that is probably the cause but that is a symptom. You can swap the transmission as long as you keep your tcu from your car.
Aftermarket banner kits are mostly garbage. And you are pretty safe getting a low mileage used transmission.

Thanks for the insight, I highly appreciate it. I went ahead and decided to invest in a Star Diagnostic system since I plan to keep this car for a while.

I will update with what trouble codes I'm having when the system arrives.

Smart move! I just purchased it too through Shardul here on the forums and am impatiently awaiting its arrival 😁

Hooked up Star and only error code was that it was slipping. Everything looked good as far as torque converter/TCU/conductor plate was concerned.

Pulled off oil pan and it had a good amount of clutch material and silver shavings. Swapped oil out twice with no change in symptoms so final diagnosis is tranny swap needed.

Picking up a used one tomorrow.

RIP