Recently we had the dubious pleasure of cleaning out the entire coolant and oil system after a customers BMW 525d M-Sport (E61) burst an oil cooler and the component liquids combined to form a mayonnaise like sludge. It was one of the worst examples we had ever seen. However, coolant in the oil is relatively common in combustion engine breakdowns.
This week though, we have had two examples of a different kind of contamination. In both cases the engine is a BMW N57D30. One car was a 2011 X5 40d (E70), the other a 530d GT (F07).
Both owners had experienced an ‘Oil level too high’ warning, and neither had recently been serviced or had oil added to the system. Our first port of call in both cases was to remove some oil and set a level around halfway between Min and Max on the dipstick indicator. 24 hours later after 50 miles or so we checked levels again. In both cases the levels had risen, between 250ml and 500ml. The oil was clean and there was no sign of emulsification. In both cases the diagnosis was clear. Only a leaking high-pressure fuel pump adding diesel to the oil could possibly be responsible.
When the oil was drained, it had no viscosity at all, and was rather like black water, or ink. The main risk of course is damage to the bearings due to the lack of viscosity and thickness of the oil, although many other areas would suffer too.
Interestingly, both cars had different fuel pumps and the repair job was hampered in the X5 where there was barely any room to work, even once the inlet manifold had been removed.
If you have rising oil. Beware!