1986+ Mercury 6/8/9.9/10/15HP 2-stroke Factory Service Manual, Page 180Get this manual
POWERHEAD
Thermostat (If Equipped)
68Work 10 (1999 And Prior) 68Work 10 (2000 And Newer) 9.91015 No Thermostat 120 Thermostat 120 Thermostat
1Wash thermostat with clean waterUsing thermostat tester, similar to the one shown, test thermostat as follows: 2Open thermostat valve, then insert thread between valve and thermostat bodyAllow valve to close against thread3Suspend thermostat (from thread) and thermometer inside tester so that neither touches the containerBottom of thermometer must be even with bottom of thermostat to obtain correct thermostat opening4Fill thermostat tester with water to cover thermostat5Plug tester into electrical outlet6Observe temperature at which thermostat begins to openThermostat will drop off thread when it starts to openThermostat must begin to open when temperature reaches (5 F) above designated stamping on bottom of thermostat7Continue to heat water until thermostat is completely open8Unplug tester unit9Replace thermostat, if it fails to open at the specified temperature, or if it does not fully open