A cooling system for a liquid cooled internal combustion engine having coolant passages and a heat exchanger selected to operate the engine cooling system in the region of nucleate boiling. A sensor detects the presence of nucleate boiling and a variable volume chamber has a flexible diaphragm urged on one side by controlled air pressure and responsive to the sensor maintain the coolant system pressure at a level maintaining optimum nucleate boiling to increase heat flux from the engine and reduce overall size of the system.