Overheating

[Capn]

I have a 1982 Pearson with a Universal Diesel 44 HP. It is a Kubota block. I recently installed a new heat exchanger. I can run the engine at normal cruising RPM for 2-3 hours and it never overheats (runs about 160 degrees), but when I stop for an hour or so and restart the engine, the temperature will spike to 200 or so. If it sits overnight, it won't spike the next morning when I restart it. I think I am getting an airlock in the system but don't know where it is coming from. Any thoughts or ideas?

[Nailhead432]

Several things could cause this issue, but my first hunch would be to check the thermostat.  However, during a diagnostic procedure, I first check fluid levels, then raw water operation.  Check seas strainers, raw water pump impeller and make sure the seacock can open and close freely.  If all that is good, then I start checking fresh water components.  160 seems a bit on the cool side to me, and I'm comfortable at 200.  220 is when I start getting concerned and 240 you are doing damage.  But the quick and easy, see if your thermostat is stuck open.  Good luck and fair winds.