That's probably it, but how hot is hot? Does your temp gauge give an actual number? The important thing is to be sure the engine
isn't over loaded by being over propped. At full throttle, then engine should reach full rated RPM
, preferably a little more (say 5%) more. You should confirm that and re-prop if necessary. Overloading a diesel
can significantly shorten it's life.
Oh, and another thing to confirm is that the exhaust
is being properly cooled. An infrared thermometer is a quick way to check the surface temp to look for hot spots.