BMW Repair: bmw 325i overheating, cylinder head gasket, blown head gasket

Question
car overheated, checked the following day and I got some sludge almost milky looking.What do you think is my problem.This sludge was when I opened the oil cap.

Answer
Sludge is mixing of oil and coolant. Usually a blown head gasket, cracked cylinder head, or cracked engine block. Or any combination of the above. If you do the cylinder head gasket, release and retorque the head bolts to make sure the threads in the block will hold. Check the block for straightness and cracking. Send the cylinder head out to a machine shop to check for cracks and straightness. You also need to find the source of the overheat. (thermostat, electric fan, water pump, etc.) Buckle up, this one could get expensive.
-Evan