Onan generator will not run - carbon buildup

Mike Ondra


Some have asked to hear the rest of the story.


So after verifying compression, changing fuel filter, rebuilding injectors, having the oil tested (all being in the OK range) a diesel head buddy said it sounded like an exhaust side blockage. Checked from the mixing elbow all the way to the thru-hull. All OK. Still no start. Time to call in the professionals.


Mechanic loosened the bolts holding the exhaust manifold to the engine…started right up. Removed the mixing elbow from the exhaust manifold. The opening between them was 80-90% blocked with hard carbon. 2 photos posted. Replaced both and the Kubota starts swiftly and purrs nicely. Total bill $2700 parts and labor.


Generator has about 2000 hours on it. We have always been aware of running the main engine hot from time to time to burn out the carbon in the exhaust system and turbo (TMD22) but never thought about the Onan. It is usually very lightly loaded, usually just to recharge batteries.


Mechanic suggests loading it up from time to time, maybe with the air conditioning. Also recommends decarbonizing the engine every 1000 hours or so. He injects an oil/solvent followed by water. Googling on decarbonizing diesel engine yields and interesting array of solutions.


Would be interested in others experience with generator maintenance to keep it in viable operating condition.


Mike Ondra

Aletes SM#240




Upon startup this spring after 6 months or so idle, generator started up fine but within seconds went to BLACK smoke and soot on the water. After shut-down and checking things, started again, BLACK smoke, and quit. Now cranks but doesn’t start.


Diesel mechanic after 3 hours is stumped. Replaced fuel filter and fuel seems to be getting to the injectors. Glow plugs seem to be working. Oil tested, comes up clean, thinking internal moving parts OK. He says its not getting to starting RPM. Ignoring the prior startup and black smoke, he says either starter bad or there is load resistance in the system somewhere preventing reaching starting RPM.


My current theory, which mechanic is not buying, is that over the winter some fuel (after the filter) went bad and fouled the injectors. Black smoke coming from lack of atomization of the fuel and no start due to blocked injector.


Anyway, being away from the boat, can anyone give me the Kubota engine model for the Onan generator circa 1999. We are hull #240. I am thinking the next step is to replace the injectors when I can ascertain the exact model number.




Mike Ondra,







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