In order to obviate vapor locking and subsequent cavitation of a pump which returns coolant vapor condensate from a radiator to the coolant jacket of an evaporative type cooled internal combustion engine and maintains the highly heated structure of the engine immersed in a predetermined depth of liquid coolant, the load on the pump is sensed by determining the amount of electrical current the pump is drawing and in the event that the load is at a level indicative of pump cavitation the connection between the radiator and the pump is interrupted and communication between a reservoir and the pump is established so as to introduce cool liquid coolant into the induction port of the pump.