of the electric pump 18 is controlled by a control unit 62 which is disclosed in japanese patent application no 58 22816 filed dec 2 1983 in the name of yoshinori hirano with ths control the coolant level in the coolant jacket 12 is kept substantially at the predetermined level during normal operation of the engine 10 it is to be noted that the entire amount of coolant used in the cooling system is so determined that when the coolant jacket 12 of the engine proper 10 contains therein the coolant 26 to the level of the coolant level sensor 60 the lower tank 16 is allowed to contain therein a predetermined amount of remaining coolant as may be understood from fig1 a temperature sensor 64 is disposed also in the coolant jacket 12 to detect the temperature of the coolant therein receiving signals from the temperature sensor 64 and the other sensors not shown such as engine speed sensor throttle opening angle sensor and fuel supply rate sensor the control unit 62 controls the operation of the electric fan 20 so as to allow the engine proper 10 to have a desirable temperature that is the temperature of the liquid coolant in the engine in accordance with the operation mode of the engine under operation of the cooling system the system as a whole is hermetically closed so that changing the pressure in the system induces variation in the boiling point of the liquid coolant contained.