Mercury Optimax Diagnostic Guide., Page 14Get this manual
OPTIMAX DIAGNOSTIC GUIDE
Procedure 11
ENGINE TEMPERATURE
Correct engine temperature is important to the proper operation of the OptiMax systemA new 54 (130 F) thermostat and new thermostat cover have been developed that will help maintain higher engine temperature and reduce the chance of debris holding the thermostat openA thermostat held open by debris would reduce engine temperature causing spark plug foulingInstall thermostat kit (885599A1)This kit includes all of the covers, thermostats and gaskets to upgrade one engine