So, I've recently acquired an '87 300D with the 603 engine from a trusted friend who had it for at least 5 years. He replaced th radiator several years ago due to a leak, but has never had any overheating issue.

I've put some miles on it over the last 2 months, and never experienced any overheating or the dreaded signs of a cracked head. (yeah - it's a 14 and no idea if it's ever been off). No residual pressure in the cooling system, pressure builds slowly as it warms up, no weird coolant level variations.

I was concerned to find dirty oil sludge in the expansion tank, though. Not a lot, but definitely a film building in the tank. The coolant looks very clean... I removed the tank and thoroughly cleaned it out with dish detergent and hot water. 800 miles later I have a film building again. I did not flush the entire cooling system when I did the tank, but probably will try that next. Some water and non-sudsing dishasher detergent should get any oil residue out of the system. I'm still not sure if there was a problem in the past, and maybe a head gasket was done but oil film remained in cooling system?

Another test I have done is the combustion gas detector for cooling systems. It passes that test just fine, and they are pretty sensitive. So, if it is leaking its only oil...

Any thoughts? Other simple tests? Experiences of 3.0 liters leaking just oil? Seams lke I've read the 3.5's are prone to this but not th 3.0?

Thank you

Any engine can have head gasket problems. It takes a lot of cooling system cleaning to get rid of all the residual oil after a head gasket failure. A leakdown test might tell you more but I'd go through a dozen or couple dozen cleaning cycles first. If there is a head gasket problem, you'll know soon enough.

Sixto
87 300D