Replacing the thermostat on Yamaha's F115 requires little more than opening a cover, removing the old thermostat and replacing it with the new one. One problem you might encounter, putting the thermostat and relief valve in backward, is resolved by taking careful note of the way the retiring thermostat and relief valve, sometimes called the poppet valve, are seated. Installing the thermostat and relief valve incorrectly causes the thermostat to remain closed and bar the passage of cooling water to the motor.
Locate the thermostat housing on the top-front of the starboard side of the powerhead. Remove the flywheel cover. Remove the five bolts and the gasket sealing the thermostat cover with a 1/4-inch socket.
Tap the outside edges of the thermostat cover using a rubber or plastic mallet, to loosen the cover's seal. Lift the cover from the powerhead. Remove the old thermostat and relief valve, noting the direction in which each is installed.
Set the new thermostat and the relief valve into the openings beneath the thermostat cover.
Ensure all traces of gasket and sealant material are removed from the cover and the mating surface on the powerhead by gently scraping both with the edge of the blade of a utility knife. Install a new gasket around the thermostat cover and apply a bead of gasket sealing compound to the gasket.
Thread the five bolts that hold the cover back through the cover and into the powerhead. Tighten the bolts alternately and evenly to 60 inch-pounds with a torque wrench
Some variations of the F115 have a thermostat/relief valve cover that appears to have matching ends, rather than a large cover segment for the thermostat and a smaller cover segment for the relief valve. Make a "match mark," a line that extends past the edge of the cover, on to the powerhead surface, if necessary.