I have an 87 300sd. the problem is as soon as the car starts it exceeds the pressure on the radiator cap, maybe 25psi. It does this almost immediately. I pressure checked the coolant system and can find no external leaks. the car has about 160k on it. It starts a little rough but runs. I was thinking head gasket. Is there any thing else it could be. Any help would be appreciated. Thanks

 

nothing else comes to mind. try runinng the car wit the cap off, if you get air comping out of it, than you got a head gasket problem. has the car ever over heated before

 

My daughter drives it. She says no, but who knows. I was planning on doing a compression test next. It seems funny that I can pump the radiator pressure up and no leak down, but upon starting the radiator builds pressure immediately.

 

well you test the cooling system with no more that 17psi. but the compression pressure can go over 200psi so it could be a tiny hole on the gasket or even a tiny crack on the head

 

Sounds like a leaking Head Gasket or a Crack in the Combustion chamber. I cannot think of anything else that could casue your issue.