[jrobbins70]

A friend with a w211 E3504matic (2006) recently showed me his coolant. It was the original yellow coolant (never changed) and it had a nasty oil film inside the overflow tank. I've never seen this scenario before. How bad does this seem? Where is a good place to start? Drain/fill coolant and see if the oil returns?

J

 

[Noodles]

You could also do a combustion gas test. Most of the big box parts stores loan out the equipment.
You would ave pay several dollars for the fluid needed to run the test.