Yamaha outboard motors are equipped with a thermostat that limits the quantity of cooling water allowed into the motor until the motor reaches its normal operating temperature. It prevents the water from starting to cool the motor too early in that process, to increase engine performance and reduce emissions. The thermostat components are mounted in a cooling passage, under an access cover that is sealed with a gasket or an O-ring.
Twist the engine cover lock and remove the engine top cover. Locate the small rounded thermostat covers at the top of each cylinder head.
Make a line that extends from the small rounded cover positioned at the top of each cylinder head to the adjoining cylinder head with chalk. Working on one thermostat at a time, remove the two bolts that secure each cover with an adjustable wrench. Tap around the outside of the cover using a plastic mallet to help loosen the seal and pull the cover from the engine block.
Check to see if the seal or gasket came out with the cover. Make sure all traces of gasket and sealant material are removed from the cover and the powerhead mounting surface. Take note of the direction in which the thermostat is facing before removal.
Visually inspect the thermostat for obvious damage including corrosion, cracks/breaks or severe discolouration from overheating. The cause of a malfunctioning thermostat is often foreign matter stuck to the valve seat. Inspect the thermostat to make sure that it is clean and free of foreign matter. Make sure any springs have not lost tension.
Install each of the new thermostat components from the thermostat replacement kit in the reverse order of the old parts' removal. Replace any gaskets, seals or O-rings. Pay close attention to the direction each component is installed.
Install the thermostat housing cover using a new gasket. Retaining bolts must be lightly oiled with clean two-stroke oil before installation and tightening. Tighten the bolts until snug. Tighten the bolts alternately and evenly to 100 inch-pounds with a torque wrench.