Thanks again, it sounds like I need to check the water thermostat.
The starter had broken once before (About 6 months ago) due to, trying to get the motor started with ether. I had been having some problems with starting and a mechanic got it started with ether once. Well, somehow, it was too much, and cracked the starting casing. on that starting effort. (Now, don't laugh too loudly at my ignorance). I am just learning diesels, for the first time.
So, that was the reason the starting cracked. The motor has run fine, (for some reason I cannot explain, and started on first turn of the key ever since; except for the recent episode of getting hot, at idle. (The motor did, once, have the warning heat alarm sound off a month ago), when my wife got confused on which way the thru haul raw water intake valved turned. She turned it off, and thought she had turned it on). So now, I have drawn a picture beside the thru haul valve for the raw water for ON and OFF positions. But, on this occasion, the warning heat sensor alarm did not sound off. I am wondering if a plastic bag might have been sucked to the the outside of the raw water intake. But, now, it could be the thermostat needs checked. I have since, ran the motor for over 20 hours, with over 15 different starts, up and down the ICW in Florida, travelled over 70 miles, and have not had the least bit of motor problem. But, I was glad to have ordered the extra starter (since I switched it out, in 20 minutes and was going again), without needing to call for a tow.
Is it much work to pull the head? I have never done it. And I guess the pattern of re-tightening the headbolts and the torque is in the manual? Right? And, what does scarring look like on the cylinders? Are there any pics on youtube that show a scarred cylinder. Wouldn't I have to pull off the block in order to replace the rings? Does a compression test simply tell me if the rings are shot? It still starts easily now on the first turn of the key.
Is that a good indicator of good compression? The rating of the 3GMD is 20 HP at 3400. The RPM goes to 3600, when running at full throttle. And I move at 7.3 knots. Haul speed is 7. Thanks so much!
If it's running ok now maybe you dodged a bullet. Keep a close watch on oil consumption. Diesels will always need a top-off every 20 hours or so but any more than that and you should suspect some significant cylinder scoring. The next time you're not out cruising, maybe pull the head and check it out. Scoring looks like thin lines in the walls. The engine will still run with some scoring but you should see a lot of smoke if it's in need of immediate repair. As mentioned by dem above, you might want to re-ring and hone the cylinder walls. A danger is that the pistons and rings may have been damaged which can result in catastrophic failure and trashing the engine entirely. Also, the bearings may have overheated and become out of round slightly but this is probably not true unless you really baked it.
Gasket sets and rings for these are not terribly expensive. I recently re-ringed my 3GM30F for around $400, including a complete gasket set with a lot of extra gaskets that may be useful in the future. Before I bought this boat, the new engine had sat for 6 years. The rings never seated correctly. I was able to pull the pan right in the boat. If you can get the pan off with engine in place, it's not a difficult job. You can mount a mirror in the bilge so you can see what you're doing. Mark all the rod caps (they are probably numbered already), make sure you put them back in the same direction they came out so the oil weep holes are in the right direction. When in doubt, label it. Taking pictures before you take stuff apart with a dig. camera is also often helpful. The objective is to have no leftover parts.
A ball-end hone is recommended but I like an old style stone hone. Both are fairly cheap. You will also need a ridge reamer if there are a lot of hours on the engine.
If you need a ridge reamer then you ought to check as to whether you need oversized rings. Also check all the journals to see if you need new bearings. I would only leave the old bearings if the engine has very few hours on it and the old plastigage as ok.
To do this kind of work you really need the shop manual for this engine as well. All that said, if you have never done this kind of work and don't feel confident about doing it, it would probably be better to yank the engine and bring it to a shop.