After a short one week trip over the fourth of July, I had a slight overheating problem on the way home. ( Yanmar 3GMD ) Did not overheat, and I made it home, but temp was up to about 30 degrees above where it usually runs. Didn't take too long to find a problem. One vane broken on the impeller, and lodged in the output of the raw water pump. After I replaced the impeller, had a really hard time getting the pump primed and pumping. Then got it done and all seemed normal. Five or six days later I went to the boat, cranked the engine up to check some other stuff. It ran at normal temps. for about an hour. Then jumped up to 160. Normal temp. on the guage is right at 130. Immediately shut it down, and walked away. Later I pulled everything from the thru hull intake, to the thermostat housing. Checked the thermostat, and found it to be fine. Removed and cleaned and re-sealed the raw water strainer, and replaced all the hoses. Put it all back together and it ran nice and cool for over an hour. Put it in gear tied to the dock, ran the engine up to about 1500, for another hour. All fine. The only thing I found that could possibly lead to the second overheating was a little bit of corrosion on the back of the thermostat. Not much. What I think was happening, is that the thermostat by pass is open, when the thermostat is closed. When the thermostat opens, it closes off the by pass. Apparently the small amount of corrosion was not allowing the thermostat to open fully and close off the by pass. Can find nothing else abnormal.
The second supposed problem, was there was no water coming out the exhaust, until after the thermostat opened up. The way the 3GM is plumbed, the by pass is open before the thermostat opens, and water should be pumping through the system and out the exhaust whether the thermostat is open or not. Even though the engine did not run at all hot, it bothered me to not be getting any water flow from the exhaust untill after the thermostat opened up. What I found was a very tiny pin hole in the muffler. Over a period of a few days, the muffler would drain out into the bilge. It just so happens that it takes approx. as long to fill the muffler when empty, as it does for the thermostat to open.
The second supposed problem, was there was no water coming out the exhaust, until after the thermostat opened up. The way the 3GM is plumbed, the by pass is open before the thermostat opens, and water should be pumping through the system and out the exhaust whether the thermostat is open or not. Even though the engine did not run at all hot, it bothered me to not be getting any water flow from the exhaust untill after the thermostat opened up. What I found was a very tiny pin hole in the muffler. Over a period of a few days, the muffler would drain out into the bilge. It just so happens that it takes approx. as long to fill the muffler when empty, as it does for the thermostat to open.
