The size of the radiator and its condition determine where the temperature will stabilize once the thermostat opens. A clogged or corroded radiator will let an engine overheat no matter what the thermostat setting is, or even if there's one installed at all. Another source of overheating would be an exhaust gas leak into the coolant- - - -leaky head gasket, cracked cylinder head, a pinhole in a cylinder wall, etc. It doesn't take much exhaust gas at 1,000+ degrees and hundreds of pounds of pressure to overwhelm even the best condition cooling system. Compression/combustion pressure alone can force coolant past a pressure cap, even without the coolant itself getting over the boiling point. "Test- - - -don't guess!"
Jerry
