Hey all, I honestly don't know tons about the cooling system. This new to me 1985 300D retains pressure in the system for hours after being shut off. When I twist the cap off the reservoir I can hear the pressure releasing. The longest I've probably tried this after shutting off the car is 12 hours. So, I searched and I have some questions. Obviously, this scares me as I read a lot about coolant and motor oil mixing due to a cracked hear or gasket. I've looked and the coolant doesn't look like there is oil in it. The coolant is not boiling in the reservoir even when really hot. This car in fact almost never goes above 75 degrees Celsius on the temp gauge. It will go up to 85 when I am idling after extended driving or going up mountains. I believe there is a very small coolant leak. When I first got the car I saw some drip below the car, but I don't lose enough to have to fill up the reservoir. Oh and the radiator hoses get firm when hot and then they release when cool.

So what does everyone think? Could it just be a faulty cap? Could this all be explained because of the leak? Or is something worse in play here?

Thanks,
David

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1985 300D. 179,000 miles.