Though I have no interest in joining or fanning some of the arguments here, I would like to add a story about my experiences in a relevant situation:
When I was examining my current boat for purchase I noticed that the Universal 5432 diesel wasn't firing on the #1 cylinder. You could hold the injection line
and clearly feel that it wasn't pulsing. The motor looked like it hadn't been cleaned or maintained, and the owner was clearly unaware of a good maintenance schedule. On the test run the boat would not come up to it's rated cruising RPM of 2200 and ran at a very cold 160 degrees for a fresh water cooled diesel. This raises serious concerns about the combustion of the engine and the carbon build up from running so cold.
As it was not my job to diagnose his diesel problems, but to buy the boat, I negotiated the price down a full $10,000 on the belief that I would have to replace the diesel. He had every reason in his mind to believe the diesel was in need of at least a major overhaul, if not replacement.
I suspected otherwise, but it wasn't my job to tell him. After purchasing the boat, I immediately pulled the injectors and took them to a great diesel repair
shop where they told me they were in perfect condition mechanically and were simply gummed up with carbon and needed cleaning.
That solved the #1 cylinder problem. When I hauled the boat out, I saw that the prop was a 3 blade 16X12 and research told me that for my boat and that engine a 15X10 would be a better fit. Had the prop re-pitched and she reached hull speed easily in flat water at her rated cruising RPM of 2200.
I then replaced the thermostat with a 190 degree version and she ran at a steady 185 in our cold NW waters.
The outcome of all this? I have a 25 year old diesel in my boat that has 800 total hours and is in superbly good shape, uses .68 gallons an hour average over 100 hours, burns completely clean, uses no oil, and has no leaks. The previous owner thought it was trashed and needed rebuilt or replaced.
The cost of everything I did to put it into excellent shape? Something less than $250, and most of that was for the prop repitch. I've done many other preventative maintenance tasks since then, but nothing near a boat "unit".
If there is a point to this story, it is just that there are a great many variables with engines, and that it is entirely possible to salvage a seemingly dead diesel if the owner becomes aware of basic repair
I wish all of us luck with our motors...