The usual suspects to look at when a diesel has a hard time starting is, #1: air in the system. You then have to bleed the fuel system by opening bleeding screws along the fuel tree all the way to the injectors. When the bubbles cease and fluid is present with no bubbles, the engine should start. #2 Water in the system, which could be from either condensation or rain water seeping around the deck fill cap which needs a new rubber 'o' ring. #3: Dirty fuel or something that is clogging the system which means you may need to clean out your filters, etc. If these don't take care of the problem, then you can start checking injectors. Pull them and crank the engine and spray the stream on to a napkin. The pattern should be conical with a nice oval spray. If this appears ok, then I would suspect the rings at the pistons. If the boat sits a long time, rings can rust and stick which inhibits compression essential to diesel engine starting and operating. Compression creates the heat required to fire the pistons in the combustion chamber much like a spark plug does on a gas engine. By putting in a little oil atop the cylinders, you create a temporary seal which gets the engine going. Sometimes that will do the trick, but if you have to repeatedly do this, then you need to find the culprit.
