Fallard I was told all Diesel engines will have black oil after an hour or two of run time on a new oil change? "Always...
1. Don’t baby the engine. Diesels don’t like to idle in neutral, or even in gear at low speeds; they do like to work hard under load. Properly matched to its boat, a diesel engine can run at cruising RPM (the “sweet spot”) for hour after hour, day after day. What’s cruising RPM? Generally, the sweet spot is approximately 75-80% of the maximum RPM as defined in the owner’s manual. However, the sweet spot will vary from one engine brand to another. Volvo- Penta, for example, recommends that some of its larger engines be run at 200 RPM below the “obtainable maximum RPM.”
Running at the sweet spot, after just a few minutes of initial warm-up, the engine should move the boat at reasonable speed with minimal noise and very little harmonic vibration. The sweet spot also maximizes fuel efficiency and longevity of the engine. Example: My 2-cylinder Yanmar has a rated maximum RPM of 3,400. I routinely run it at 2,400 RPM (75-80% of the max), and at that speed it moves my 8,4000-pound sloop at 5.8 knots, consuming approximately 1/2 GPH.
Especially avoid idling the engine for long periods. At idle speeds, fuel combustion is incomplete (compared to cruising RPM under load) so excessive idling wastes fuel. Excessive idling also accelerates wear and tear on the engine, leads to gradual build-up of detrimental varnish on the cylinders, and deposits soot and carbon on the engine’s valves and in the exhaust system, particularly at the manifold injection elbow where raw cooling water exiting the engine mates with the exhaust gases.
So, don’t baby your engine. Run it hard. However...after running at cruising RPM for several hours, a brief cool-down at idle speed, with no load, is beneficial. A few minutes is enough
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