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Join Date: Jan 2021
Car: M235i
Default N55: Coolant draining directly from expansion tank to oil pan...HOW?? - 01-24-2021, 08:20 AM

Hey guys. First post, as I am new-ish to the N platforms (plenty of M and old S experience though).

So here's a crazy situation I am currently facing with this N55:
I got my hands on a 2014 M235i with a bad head. The car ran and drove and even built boost, but cylinder 3 was toast.
Got a replacement head, rebuilt, machined, flow tested, etc. Replaced the head, got everything buttoned up. Started up the car for roughly 45-60 seconds, with oil only (no coolant) to make sure it's timed correctly and running right. All is well at this point. Shut the car down.
Go to pour the coolant into the expansion tank; a gallon goes in right away (that's about what I drained before taking the original head off); the expansion tank is still empty. I proceed to pour in another gallon. It eats it up. Expansion tank is once again empty. This is all with engine OFF. At this point I know something is wrong.

Turns out all that coolant went directly into the oil pan! I removed the oil drain bolt and all that coolant came rushing out, followed by all the engine oil.
To test, I left the drain bolt off and proceeded to pour a half a gallon of clean distilled water into the expansion tank again. Sure enough, 5-10 seconds later it all comes gushing out of the oil drain pain.

How is this possible? Where could it possibly be draining directly into the oil pan?
I checked all the oil and coolant lines that were taken off, and everything seems to be reconnected correctly. There really is only one way to get every line on to fit properly. There are no external oil or coolant leaks either.
It's like the fluid goes down the expansion tank hose, and magically finds a route into the oil pan.
I spent 3 hours soaked in coolant under the car, trying to find an obvious culprit. Nada.
Any ideas or suggestions are more than welcome!

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