OPEN A HOT RADIATOR CAP
Engines operate at high temperatures, which means their cooling systems must work within this range as well. Only problem is, coolant can boil in that heat, which makes for inefficient thermal exchange. So just as a pressure cooker raises water’s boiling point, the cooling circuit is pressurised to do the same.
Assuming your engine is working properly, the coolant is under between 0,7 and 1,0 bar of pressure when hot. Opening the cap to atmospheric pressure causes rapid depressurisation of the hot coolant, and that liquid converts to dangerous steam. Cars with over ow tanks keep pressure sequestered with a spring-loaded valve, but it’s better to be safe, and let the engine cool before opening the system for service. When in doubt, place a damp towel between your hand and the cap.