This is going to be a wordy post.

About a month ago my water pump started weeping. I brought it to a shop and had the water pump, timing chain, gallery, gaskets, etc done. Cost me about $2k USD. Pretty fair. The day I picked it up and drove home, it started shifting really poorly. I limped it the last 5 minutes home and it was spewing coolant from the engine bay. So I had it towed back to the shop. They said it was a blown tranny line. They fixed it, did a coolant flush and a tranny drain and fill. All has been good until Saturday. I have taken it on quite long drives as I travel for work sometimes.

Saturday I drove on the freeway for about 1.5 hours and all the sudden there was a loss of power and it wouldn't shift right. I thought, great another blown tranny line? Nope. I pulled off and put in park, the idle was all over the place accompanied by a not so good metallic sound. Then THUNK! It shuts off. Had it towed the 114 miles back to the shop that did the recent work.

They're telling me that they found transmission fluid in my crankcase along with my coolant. And my engine is seized. They know that these cars have a tranny cooler and if it burst inside the radiator that would cause the fluid to mix with the coolant. However they do not know how it could've gotten into my oil. They even suggested foul play, that someone may have put a quart of tranny fluid in my engine because it is almost exactly one quart over filled.

No one, not even me (as far as I know) has touched my car since their shop. And I don't have any enemies. I drive to work and drive home usually. Car stays in my garage and I live in a secluded area with no neighbors.

Any ideas on how tranny fluid could get into my crankcase otherwise?

 

This is a weird one, The only way I can think off doesnt apply to G37 unless it uses an oil cooler on the engine oil. Radiator bursting will cause ATF/Coolant to enter anywhere oil does so usually it's transmission.
Another weird perspective is that Oil and ATF can mix and not separate so it's weird that they were able to obtain an exact quart from the crankcase.

One suspicion is that the engine overheated a part of the block/head from lack of cooling which caused the HG to blow in that area, this then caused this coolant/oil mix to enter in the oil drain side of the block (if it didnt start smoking)

If your car came equipped with an engine oil cooler then that cooler might also be another culprit.

 

Occam's razor- someone at the shop sabotaged you to get more $.

 

This makes sense. Radiator fails allowing coolant and transmission fluid to mix. The failure then causing the overheating, and subsequent heat gasket failure and limp mode. The shop checked the dipstick and saw the milky oil and assumed coolant as trannie fluid would be hard to notice when mixed with oil as it would not separate as cleanly.

What I'm wondering about is the initial diagnosis of a blown transmission line based on coolant spewing everywhere? The transmission fluid and coolant are separate flows inside the radiator. Why would a blown transmission line cause an overheating situation? Seems that this would have tripped the transmission pressure switch and caused limped mode. Am I missing something? Maybe there were two issues initially, only one was resolved. But then why were you able to drive the car with no issues for a period of time as the mixing of coolant and transmission fluid should have caused failure more quickly?

 

So my car never overheated... at least not from the temp gauge in the cabin. They found transmission fluid in the engine oil, not coolant. I looked at it as well. It wasn't milky, it looked like just used engine oil, but it had a red color to it.