I just replaced the exhaust manifold and since I had it all apart I replaced the thermostat too. The housing was kind of corroded so I went ahead and got a new one. The new one is a much lighter shade of grey. I don't have temp gauges so we always check the engine temp at the thermostat housing with an IR temp gun. It's not an exact science but we have it down pretty good and we always get the same reading, basically about 165. The new thermostat is only reading 125. We are a catamaran so I have two engines so I can compare temps. The valve covers read the same temp. The exhaust manifold reads 10deg cooler on the engine with the new thermostat. I assumed I had an air bubble still in the system but after checking numerous times after running the engine, even squeezing the thermostat hose to burb some more, I'm pretty sure their is no more air in the system. My only questionable possibility is this engine has the water heater on it and maybe, maybe there is air in that line that isn't getting to the coolant cap.

After running this way for a week I decided maybe I had a bad thermostat so I put in a new one, new and old one from Yanmar. Didn't change a thing. Still reading 125 at the housing. Thermostat has 76.5 Celsius stamped on it. So it seems to be the right one. Man I wish I hadn't thrown out the old one that was working. Any ideas on what to do next?