Yamaha Outboard F15A F9.9C, FT9.9D F15 Service Manual, Page 250Get this manual
OIL FILTER, THERMOSTAT AND EXHAUST COVER
SERVICE POINTS
Thermostat inspection 1Measure: Valve operating temperature Valve lift Out of specification Replace the thermostatWater temperature Below 60 (140 F) Above 70 (158 F) Valve lift mm (0 in) mm (0.12 in)