Just in case you need a HX source:
http://c34.org/bbs/index.php/topic,6920.0.html
IIRC, your OP said you have had this boat three years? If so, now's the time, in addition to us being able to help you here, to get a good book on diesel engines if you haven't already. I put off buying a larger boat for too many years 'cuz I was "afraid" of diesels. When we bit the bullet, we bought one with an engine that the PO said was prone to overheating. I did a lot of reading, and ended up taking off the HX, found a lot of salt precipitation on one of the ports, cleaned it out and it's been running fine ever since. I also read a lot and learned to bleed the engine - I put if off for a year, 'cuz I was afraid I wouldn't be able to get it to restart.
Diesels are pretty simple, other than complete teardowns and rebuilds. If you need to learn more, I heartily recommend that you do so, 'cuz none of us was "born" diesel mechanics, we, too, all had to learn all this claptrap!

Good luck. Let us know what else we can do.