1985 Mercury Outboard V-300 V-3.4L Shop Service Manual, Page 115Get this manual
Thermostats 1Inspect thermostat covers and cylind er head covers (thermostal ope ning) for cracks and corrosion damage tha could cause leakageReplace pari as necessar y2Remove and discard gasket from each thermostat3Wash thermos tats wilh clean water 4Using thermostat lesler, si mila 10 Ihe one shown in Figurelest each therm ostat as foll ows: aOpen ther mostat valve, th en inser th read between va lve and thermostat 'bodyAllow valve 10 closeagainsl thread bSuspe nd therm ostat (from threadand thermometer inside les ter so ,hat neith er touches th containerBou of thermometer must be even with ha llom of thermostal lo obtain correcllemperat ure of therm os tat peningcFililh ermostal les ter wit water to cOlier th ermostat, dPlug tester into electrical oulle!eObserlle temperature at which thermostat begins to open(Thermostat wi ll drop off threadIhat was in stalled in Step "a", when it starts toopen) Th er mos ta mu st begin to open when temperature reaches 140 145 F' (60-63qfContinue to heal water untillh er mos tat is complet ely opengUnplug thermostat testerhReplace thermostat, if it fail 10 open al the speci fi ed temperature, or if it does not full ope nNOTE: BE SURE Ihat water inthermostal lester is al lowed 10 cool sufficiently [below 130" (56 e)1 before tesling the other thermQ5lal