Ask SAIL: Smoking Diesel Engine

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Q: I have noticed black smoke from my diesel under high rpm, over 2,000, and suspect unburned fuel to be the cause. I have checked the air intake and fuel filler gauge, made sure of good flow from the exhaust and use a bio-additive in my fuel. The oil level has not dropped. The engine starts and runs normally except for the black smoke. When I back off the throttle the exhaust clears. The engine is an 11hp Universal. Is it possible a dirty prop is the cause?

John Iannacone, via



Black smoke at higher engine loads is quite common with older engines, especially when accelerating. Unless there is some kind of injector failure, which I doubt in this case, it is the result of excess fuel entering the combustion chamber and not being fully burned. The most likely causes are either overloading, to which the engine’s governor responds by pumping in more fuel than can be burned, or some kind of an obstruction in the air and exhaust flow through the engine.

If it’s the first scenario and you have a foul propeller, you would see a substantial reduction in boat speed for a given engine speed. However, you could have a rope or something wrapped around the propeller shaft that might overload the engine without affecting boat speed much.

As to the second scenario, you say you have checked the air inlet and exhaust, but you may need to look more closely. On older engines, especially on sailboats where engines run long hours at light loads, an exhaust can become fouled up with carbon deposits. Break the exhaust hoses loose to see if this is the case. While you’re at it, see if there are any other potential obstructions in the exhaust system. For example, older hoses sometimes soften and partially collapse, or there may be a kink in the hose.

Nigel Calder is an author and expert on boat systems and diesel engines

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September 2015


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