I have an original 2GM20F installed in my 1986 30. I also have a water temp gauge installed and it has been rock steady at ??? for years when I am underway. That is the problem. It has been so rock steady I’ve lost track of what the heck the temp should be. My gauge has a mark at 120 degrees and another at 180 degrees. There is a third un-numbered mark between the two. Presumably 150 degrees. The engine has historically run just below the 150 or 180 degree mark but I can’t remember which is. I have a twin diesel engine powerboat that runs right at 175 degrees so I have totally confused myself.
I have both the complete Yanmar service manual and the owners manual and can not find reference to the actual operating temp., although I did find conflicting specs on the operation of the thermostat.
You indicate that you have freshwater cooled engine. Then running temperature should be close to 180 degrees. If you had raw water cooled engine then temperature should stay close to 120. Engines are acctually identical exept additional cooling circle in freshwater cooled engine. Thermostats are different also. Thermostats are set for different temperatures, raw water engine runs much cooler to prevent salt buildup. Hence conflicting information you could have found.
Thanks much. 180 or just below sounds right and the issue with freshwater vs raw water probably accounts for my confusion re the thermostat operating temp.