Oil cooler failure
Oil coolers sometimes perish and the result can range from a bit messy to catastrophic. Some vehicles have an oil cooler for the engine, whilst others have a second cooler for the automatic gearbox. Failure of the cooler, allows engine (or gearbox) oil to enter into the cooling system. If this is caught early enough, the fluid in the expansion tank will seem slightly discoloured, perhaps with only a few blobs of oil drifting on top of the water and coolant mixture. When undetected, more oil will seep into the cooling system and eventually the coolant level warning light will come on, and the engine will start to overheat.
The general image when opening the expansion tank, will be a creamy consistency, ranging from a runny milkshake-texture, to a thick mayonnaise-type mixture. It is imperative to stop driving as soon as you become aware of this situation. The more the engine runs, the worse the situation becomes. What was previously clean water and coolant, has now carried the oil into every nook and cranny of the cooling system. This includes the heater radiator, thermostat, water pump, expansion tank, all the coolant hoses and engine radiator. The longer the oily mixture is in contact with these components, the more damage it can cause. Rubber hoses, seals, gaskets and o-rings swell over time and perishes. The radiator and thermostat may be too blocked to degrease and reuse.
We do not do replacements of oil coolers, but recommend that you have your vehicle towed directly to a BMW dealership. They will replace the oil cooler, and quote on the visibly damaged components. A tedious degreasing process is started to remove as much of the oil mixture as possible. But please be advised that removing absolutely all the oil is near impossible. A good analogy is when pasta is cooked. Oil is added to the pot, and then the water is poured in. When the water heats up, some of the oil comes loose and drifts to the surface of the water. A similar situation will occur in the engine’s cooling system. Even after the repair has been completed, and faulty components replaced, it may happen that over a few months (we know of a case where it took a year), small blobs of oil will surface in the expansion tank.
It’s advisable to check the water level once a week after the repair and use a clean cloth to dab the oily blobs off the surface. Be sure to add a 50/50 mixture of water and BMW or MINI coolant, should the level drop too low. The water/coolant level should be checked first thing in the morning, before starting the engine.