Mercury Mariner 4 and 5HP 4-Stroke Outboards Service Shop Manual 1999, Page 152Get this manual
CYLINDER BLOCK AND CRANKCASE
Measuringrresting Thermostat
1Inspect thermostat, replace if stuck or damaged
2Use Measurement Test following to measure: aValve opening temperaturebValve lift
MEASUREMENT TEST
1Tie piece of thread (or other non-conductive material) onto the thermostat2Suspend the thermostat in water3Heat water slowly while observing the thermostat