218 is stopped and valve i condition to produce flow path b at step 1015 a command to stop the operation of fan 248 is issued and the program recycles to step 1003 as will be appreciated while the temperature of the coolant is low viz below 80 u00b0 c the system is held in an open state however upon the temperature of the coolant entering an acceptable range the program will recycle between steps 1004 and 1003 until such time as the goes above an upper limit which varies with operational conditions of the engine thus in cold climates wherein the heat exchange efficiency the radiator need not be particlularly high by way of example as soon as the temperture of the coolant enters the above mentioned acceptable range the system will be placed in a closed state even if the radiator is still partially filled with liquid coolant this state will be maintained until such time as the inclusion of atmsopheric air or the like induces the situation wherein the temperature exceeds the optimal temperature by 2 u00b0 c under such conditions the level of coolant in the lower tank 214 is determined if excess coolant is found to be still contained in the radiator 210 steps are implemented to firstly maintain the coolant jacket level at h1 then pump an amount of coolant out to the reservoir 224 however if the coolant level in the radiator 210 has been lowered to the minimum level viz h2 then it is deemed that air rather than excess coolant is the cause of the elevated temperautres and accordingly a suitable control routine is entered until such time as the temperature of the coolant drops.