Mercury Optimax Models 135, 150, Direct Fuel Injection., Page 287Get this manual
COOLING
Troubleshooting
Thermostat Test
1Inspect thermostat covers and cylinder head covers (thermostat opening) for cracks and corrosion damage that could cause leakageReplace parts as necessary2Remove and discard gasket from each thermostat3Wash thermostats with clean water4Using thermostat tester, similar to the one shown, test each thermostat as follows: aOpen thermostat valve, then insert thread between valve and thermostat bodyAllow valve to close against threadbSuspend 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 temperature of thermostat openingcFill thermostat tester with water to cover thermostatdPlug tester into electrical outleteObserve temperature at which thermostat begins to open(Thermostat will drop off thread, that was installed in Step "a", when it starts to open Thermostat must begin to open when temperature reaches 140-145 (60-63 C)fContinue to heat water until thermostat is completely opengUnplug thermostat testerhReplace thermostat, if it fails to open at the specified temperature, or if it does not fully open