Mercury Mariner V-250 V-275 Outboard Service Shop Manual 1990, Page 129Get this manual
Thermostats 1Inspect thermostat covers and cylfnderhead covers (thermostat opening) for cracks and corrosion damage that could cause leakageReplace parts as necessary
hReplace thermostatif it fai ls to open at the specified temperatureor if it does not fully open NOTE: BE SURE that water in thermostat tester is allowed to cool sufficiently [below 130 (56 C)l before testing the other thermostat
2Remove and discard gasket from each thermostat3Wash thermostats with clean water4Using thermostat tester, test each thermostat as follows:
aOpen thermostat valve, then insert thread between valve and thermostat body Allow valve to close against thread