[Ak Sourdough]

190 is not overheated. Even plain water is fine up to boiling at 212 degrees. The problems occur when the coolant boils and creates pockets of steam where the metal temperature is momentarily much hotter than the surrounding area which causes unpredictable, uncontrolled and rapid expansion and contraction in small areas sometimes causing cracks.
A pressurized cooling system or antifreeze raise the boiling point higher giving you an even larger safe zone, but boiling the coolant is what you need to be concerned about.