Abstract: A two cycle or four cycle outboard motor is equipped with a closed circuit cooling system having a coolant pump, a heat exchanger, an expansion tank, a series of coolant passages in the motor and some external piping to complete the circuit. In one embodiment of the invention, a conventional outboard motor is modified to include the closed circuit coolant system with the conventional water pump being converted to the coolant pump. In this modified embodiment, the thermostat seals have to be modified, the pump has to be sealed, and several bypass holes have to be plugged in the engine to isolate the flow of coolant.