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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. The thermostat 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 sealed with a gasket or an O-ring.
Items you will need
Thermostat replacement kit
Clean 2-stroke oil
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 to remove all traces of gasket and sealant material from the cover and the powerhead mounting surface. Take note of the direction the thermostat is facing before removal.
Inspect the thermostat for obvious damage including corrosion, cracks/breaks or severe discoloration 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 of each component.
Install the thermostat housing cover using a new gasket. Lightly oil the retaining bolts with clean two-stroke oil before installing and tightening them. Tighten the bolts until snug. Tighten the bolts alternately and evenly to 100 inch-pounds with a torque wrench.
- "Yamaha Outboard Repair,2 -250 hp 1-4 Cylinder, V4 and V6 Models 2- and 4-Stroke Models, Including Jet Drives"; Seloc Marine; 2007
Will Charpentier is a writer who specializes in boating and maritime subjects. A retired ship captain, Charpentier holds a doctorate in applied ocean science and engineering. He is also a certified marine technician and the author of a popular text on writing local history.