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In regards to the care of vehicle i would say - do proper tire inflation, tune-up, oil change and please don’t leave your vehicle idling for long periods of time and fuel conditioner. Every one can Increase Fuel Efficiency ++ Reduce Carbon Footprint if they are using good fuel conditioners.

If there are no glow plugs, then it is likely a Cummins or similar design. The only maintenance for a diesel is oil and fuel filters, oil changes, and adjusting valves, Very few have timing adjustments. (mostly older, non electronic injection) Also check you belts and hoses. Especially if the boat has ever seen salt water. Remember, the surrounding water is used in the cooling system. So a thermostat and cooling passage flush may also be a good idea.

As long as the engine is getting clean air clean fuel and has a clean oil filter and fresh oil that is all you need to do. don't even think about adjusting the injector pump unless you know what you are doing and without sounding rude it doesn't sound like you do,

glow plugs....