Oil in coolant
I have a 2005 Renault Trafic dci 100 with 136,000 miles on the clock. The cooling fan has decided to run all the time, so I have taken out the fuse. The engine never overheats and I have never noticed it running before, even when stationary. I can see there is a thin layer of black engine oil on top of the coolant in the header tank, but there is no trace of coolant in the oil and no evidence of head gasket failure.
I thought it may be the oil/coolant heat exchanger so I removed it, but could find no leaks. I replaced it and fitted a bypass for the coolant, then cleaned out the cooling system with washing-up liquid and ran it like this for a week, but the oil is still collecting in the header tank. There is about a half-centimetre layer after two or three hours of driving. Other than this, the engine runs OK.
The only other reason for this that I can think of is a problem with the head gasket, a cracked head or possibly the turbocharger. Are there any other common problems that could cause this, and how could the problem be diagnosed without pulling it all to bits? Bruce Knight
My first move would be to fix the cooling fan problem. Although the engine is not overheating, running it without a cooling fan could lead to trouble. The most likely cause of the continually running fan is either a faulty coolant temperature sensor or a broken wire between the sensor and the ECU. This would cause the ECU to trigger the cooling fan.
Moving on to your main worry, you have already followed the steps I would have taken with regards to the oil cooler. Oil can be very difficult to remove from the cooling system, but with the steps you have taken I would not expect it to still be building up such a level of oil if the cause of the oil entering the cooling system had been stopped.
The other way oil may be getting into the cooling system is, as you suspect, via the head gasket or through a crack in the cylinderhead. This may have failed between the oil feed and the coolant gallery. The high pressure of the oil may be forcing its way into the coolant via a breach too small for the coolant to enter under a lower pressure. This would not cause any typical head gasket failure symptoms, as the gasket would still be complete around the cylinder, preventing any escape of combustion gas.