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Onan genset spewing black smoke

Professor DieselQuestion: I have a 4-kW Marine Onan genset with 600 hours on it. If I let it run for a lengthy period it produces so much black smoke that a residue discolors the hull around the exhaust port. A year ago, I cleaned the heat exchanger and installed a new raw-water pump. There are no overheating issues. Do I have an injector or rack problem? —Charlie Carpenter, Morristown, New York

Professor Diesel: Are you overloading your genset? You can’t pull 5 kW out of a 4-kW unit. Remember that most maximum ratings are valid only for an hour or less of operation; the continuous rating is typically 70 to 75 percent of the maximum rating. If you’re pushing your genset too hard, try reducing your boat’s power consumption.

Another possibility is high ambient temperature. During normal operation, the air temperature surrounding your genset rises. If it rises too much, the air becomes less dense, producing an excessively rich mixture and black smoke. Next time your genset begins smoking, open its compartment hatch to let cooler air in. If the smoke disappears, you should increase air flow by installing an extra blower and/or more vents.

Another possible cause is “wet stacking,” which occurs when you run a genset under light load for extended periods—say to charge a battery orpower a few lights. The result is incomplete combustion that creates a glaze on the cylinder walls. As this glaze thickens, the piston rings fail to properly seal, causing incomplete combustion and/or excessive oil consumption. Putting your genset under 70-percent load for about two hours each month will create cylinder temperatures high enough to remove any cylinder-wall glazing. Also, with 600 hours on your genset, it’s possible your exhaust elbow, muffler, or exhaust system is plugged, causing excessive backpressure.

Professor Diesel is Larry Berlin, director of Mack Boring’s Training Services division.

This article originally appeared in the December 2010 issue of Power & Motoryacht magazine.

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