Uncovering the Causes of Oil in Coolant Reservoir and How to Fix It

The only thing that can cause it is a head gasket leak. You will need to have the engine pressure tested to see if the head gaskets are leaking. It is likely the oil or transmission fluid is a higher pressure than the coolant, it will likely fill the radiator with oil, and not water in the transmission ... Worst case is blown head gasket. There's a kit which attaches to the cooling system. It contains a dye that reacts with hydrocarbons. Check your transmission dipstick for possible coolant contamination, also. That "film of oil" in your coolant overflow tank may not be that much ... A leak between the oil containing part of the block and the cooling part is the only way that you can get motor oil in the coolant. Typically it's an oil cooler failure. If the cooler wasn't properly torqued last time, it could have lead to the premature failure this go ... One tip, is to double check the oil cooler itself for trueness against a straight edge just to be sure that it is not slightly warped. Not ... I have asked an "Audi specialist" garage for an opinion on the oil in the expansion/coolant tank. They believe the oil cooler to be the most likely fault. E60 Discussion - Oil in coolant reservoir - So after taking my BMW to a local INDY he replaced the oil cooler and oil filter housing did 3 ... The oil filter housing has a one peice gasket for coolant and oil, if this gasket is bad enough it can cause the mixing of oil and coolant.