1985 OMC 65, 100 and 155 HP Models Commercial Service Manual, PN 507450-D, Page 190Get this manual
Checking Motor Temperature All Models The Markal Thermomelt Stik, heat sensitive stick similar to crayon which melts on contact with surface at specified temperature, is used to measure powerhead temperatureThe motor is best checked when operating on boatIf this is not possible, run the motor in test tank for at least five minutes at maximum speed of 3000 rpmMark the cylinder water jacket with the stikThe mark will appear dull and chalkyWhen the surface temperature reaches the temperature rating of the stik, the mark will melt becoming liquid and glossy in appearanceOn some painted surfaces on which the stik will not leave mark, it will be necessary to hold the stik against the surfaceBefore checking the motor temperature, check temperature of inlet water to the water pumpFor the following motor temperature valves, inlet water to the water pump should not exceed 260C (800F)Warmer inlet water will raise the lower temperature valveTwo Thermomelt Stiks are necessary to check motor52 (125 F) stik and 730C (163 F) stikDO NOT use the stik on the center of the cylinder headThe center of the cylinder head on the motor will normally run slightly hotter than 730C (163 F)When performing the test, be sure steady stream of water is being discharged from the overboard water indicatorWith motor operating under 1000 rpm in forward gear, motor should melt 52 (125 F) stik and should not melt 730C (163 F) stikIf the lower degree stik mark does not melt after reasonable length of time, the thermostat is stuck open and the motor is running too coldIf the higher degree stik mark also melts, the cooling system is not functioning properly allowing the motor to overheatCheck for worn pump assembly, leaky water system, or restrictions in the cooling systemRun engine at 4000 rpm or more for several minutes and hold 730C (163 F) Thermomelt Stik on cylinder water jacketIf 730C (163 F) stik melts, the engine is running too hot