and 18 with continued reference to fig2 a water pump 70 draws water from the body of water 54 and circulates it through the heat exchanger 68 the heat exchanger 68 as is generally known to those skilled in the art contains a closed loop coolant passage 72 and a water coolant passage 74 an engine coolant such as ethylene glycol is recirculated through the upper portion of the circuit shown in fig2 and through the engine coolant portion 72 of the heat exchanger 68 water drawn from the body of water 54 is circulated by the pump 70 through the water side 74 of the heat exchanger 68 and returned to the body of water 54 the basic concepts of the present invention are similar in both embodiments shown in fig1 and 2 but fig1 shows an open loop cooling system and fig2 shows a closed loop cooling system fig3 is a slightly more detailed schematic representation of a cooling system of a marine engine made in accordance with a preferred embodiment of the present invention fig3 shows an open loop cooling system which is generally similar to the more simplified schematic shown in fig1 as described above the water pump 20 draws water from the body of water 54 and conducts it along two streams 24 and 28 to first and second cooling systems 10 and 18 the second cooling system 18 includes the cylinder head cooling system 12 and the cylinder block cooling system 14 which are shown connected in series fluid communication with each other the pressure responsive valve 40 is configured to be.