We always called it a milkshake.
His milkshake brings all the boys to the yard, and their like.... Your head gasket is F'd... Or it's WAY worse, like a cracked block.
There are multiple things that could get coolant into the oil:
Headgasket, or the more notorious one I've fixed is a casting plug that is in the side of the head in the timing housing. Once popped out you get a fantastic mix of all glory.
I had a similar leak when I first bought mine (pre-owned, of course). Coolant seemed to just evaporate -- no signs of a leak. The overheating warning will probably happen more often as the leak gets worse, so get it fixed asap. That's what happened to me.
It turned out to be a gasket. I'll look through my receipts and see if I can find out which gasket. Over the four years I've had it, my R56 CooperS has leaked just about everywhere possible.
If the car isn't leaking it, and it isn't migrating into the crank case, you need to pressure test the system and find where the evaporation is happening from.
A failed expansion tank cap can cause evaporation of water from the coolant, and is a cheap part. Have you tried to slowly open the cap after the car has been up to operating temperature? Does it just come off, or does the coolant "race" to the expansion tank?
I would start with refilling the system to proper level, pressure testing, then if no results are found keep a close eye on the coolant level.
Is this an MC or an MCS? The Coopers are notorious for radiator leaks that are slow and hard to see without being able to get under the car.
I couldn't find the receipt for the leak I mentioned before. But, the coolant only leaked when the car was running. It stopped when the engine was off, so there were no puddles left in the garage.
I'd expect a pressure test should reveal it, since it should work with the engine running or not. Right?
