Your description sounds like a blown head gasket. The pressure and heat are from the combustion gasses displacing the coolant. There are a couple tests that can be done to determine if this is the case. One is a coolant system pressure test using a special pump and adapter to put pressure into the system. You then watch the pressure gauge to see if it drops, which indicates a leak. The other test is a combustion gas test. It uses a fluid in a cylinder which changes color if there are combustion gasses in the cooling system. There should be none as the cooling system should be sealed.
The tools for both tests are usually available as loaner tools from some of the auto parts stores like AutoZone, Advance and others.
https://www.2carpros.com/articles/head-gasket-blown-test
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Wednesday, January 17th, 2018 AT 4:28 AM