Abstract: An internal combustion engine is provided with an engine driven pump forced cooling system that is closed to permit operation at elevated pressures and temperatures. A protective device is provided to stop or otherwise modify normal engine operation upon a failure of adequate coolant flow. The protective device is made responsive to the difference in coolant pressure between the pump inlet and outlet to eliminate any effect of cooling system pressurization upon the indication of coolant flow. Variations in normal coolant flow with engine speed are accommodated by additional means responsive to pressure in another engine fluid system, such as the air box, where the pressure varies with engine speed, such that reduction of the differential coolant pressure across the pump below a predetermined value relative to engine speed permits actuation of the protective device to stop or modify engine operation.