W140 500SEL gunk in coolant overflow tank
 

I recently went to check coolant in tank and found a tan colored foam around the lip of the opening. I reached down in the tank and found a peanut butter color film on the inner top of the tank. I wiped out what I could but I still need to know what caused the gunk.

Thank you

1992 500SEL

Most likely engine oil mixing with coolant. Very likely a bad head gasket. I would take it off the road ASAP. If the engine oil has any froth, don't start the engine until a knowledgable tech checks it.

Sixto
95 S420
87 300SDL

As Sixto says, have it . . .
 

checked asap. It's not so bad to get oil into the coolant but that also means the you're getting water into the oil. Now that's really bad!