Cooling system pressure...
 

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

I see no problem here other than the small coolant leak you mentioned.

Sounds ok I wouldnt worry about it unless :

1. you see bubbles in your radiator / reservoir when engine is running

2. with the cap on your hoses pressurize instantly when engine is started

3. you blow coolant out of the overflow while driving

These were the symptoms I had when my head gasket went . It wound up blowing between the piston and the coolant passage.

no oil in the water and not bad enough to leak into the piston.
It would allow piston compression to pressurize the cooling system though




.

pressure builds up because of the heat going through the system, but when the system cools, it should go down. 12 hours is too long. I'd beware of a headgasket issue. It sounds like that to me - exhaust to coolant...

Don't worry, even overnight I still have slight bit pressure(not much at all) in the cooling system on both of my cars its normal.