Oil in coolant
Q My son has a 2007 Audi A4 2.0 TDI (140) S line saloon with engine code BRE, which he has had since 2014. I have serviced it annually and it has served him well. The last time I serviced it I noticed evidence of oil in the coolant expansion bottle (dirty black inside) which I tried to clean out (with limited success). There was no evidence of coolant in the oil, which was drained and the filter replaced as usual. As the engine was running fine, we decided to leave it and see how it went.
This week, the low oil level light came on but when I checked the oil level with the dipstick it was fine (at max). After doing this the light went out. I checked the coolant again and there was definitely a bit of an oil slick on the top. I topped this up as it seemed to have dropped slightly. The light apparently came back on the next day.
So, is the low oil level light being triggered by contamination of the oil perhaps? Also, do you think we have a head gasket problem or is there any other cause of oil mixing in the coolant on this engine? I think we are going to have to take it into our local garage but it would be useful to know what to expect.
It is beginning to show it’s age after not being used so much lately (my son working from home) with a bit of visible smoke sometimes from the exhaust (probably wants a good blast down the motorway or maybe the EGR valve cleaned!). So, we’ll need to decide whether it’s worth the expense, although it does run very well.
Your thoughts, as always, much appreciated.
Chris Humphreys A
The oil contamination in the coolant may be from either a failed head gasket or a failed oil cooler. It is difficult to say exactly where the oil may be entering the cooling system, but if no engine performance problems are present, then the oil cooler would be the most likely.
Cross contamination of the water into the oil, will rarely occur with a failed oil cooler, as the cooling system runs at a much lower pressure, and the failure of the oil cooler is normally very minimal requiring the full oil pressure to breech the failure point.
If the failure is in the head gasket area, then this may allow coolant to enter the sump, and if this is the case it will affect the low oil level sensor.
I would consider removing the sump drain bung, to check if any coolant is present in the base of the sump. Provided this is not the case, then I would suspect that the oil cooler may be the source of the problem.
Whatever the source of the oil contamination proves to be, this should be cleaned out as soon as possible, as the thick oil in the cooling system will begin to block up the coolant channels and could cause a progressive problem with the cooling of the engine. This may then allow further problems to develop.
As the amount of oil showing in the header tank is still minimal, then if when removing the hoses from the oil cooler a large quantity of oil is present, then this could confirm the source of the leak.