A cooling process for an internal combustion engine comprises the steps of mechanically pumping a boilable liquid coolant having a saturation temperature above about 132.degree. C. at atmospheric pressure from the engine coolant jacket through a radiator and back to the coolant jacket, continuously removing by substantially unrestricted convection through at least one outlet in the highest region of the head portion of the coolant jacket substantially all gases other than those that condense in the coolant jacket, conducting gases from the outlet to a condenser, and returning condensate from the condenser to the coolant jacket. Cooling apparatus comprises a liquid cooling circuit and a vapor discharge and condensation circuit adapted to carry out the process.