Internal combustion engines act like air pumps: They breath air in, mix the air with fuel, make a fire, then exhaust out the smoke. Diesel engines, with their higher compression, rely even more heavily on clear inlet and exhaust passages than gasoline engines. So when my buddy Jeff started complaining that his 30-foot sailboat’s Yanmar 2GM diesel would start and idle but had no power and would only rev to about 2000 rpm, accompanied by lots of black smoke and unburnt fuel in the exhaust, a closer look was warranted.
First he removed the air filter and found it in pieces, with some of the pieces missing. You guessed it, the engine had ingested parts of the filter. But even with the filter removed it still wasn’t running any better. We discussed the possibility of clogged injectors, but before embarking on this line, due to cost and time, he was advised by Diesel Mike, Oldport Marine’s engine guru in Newport, RI, to check the exhaust. We found the exhaust elbow completely clogged, so it was a no brainier to get it replaced. With a new air filter and exhaust elbow, the engine was breathing easy and running great again.
Mike explained that exhaust elbows rust profusely on the inside and eventually trap carbon buildup. When they get to a critical point they can create so much back pressure they will destroy the air filter via the exhaust port, which is what happened to Jeff’s engine. The small pieces of foam filter actually burnt up in the cylinders. Mike also advised that if you see a little rust on the outside of the exhaust elbow, the inside is most definitely rusted. So if your diesel has the symptoms of low power at higher revs and is spitting black smoke and water, check that the engine is breathing properly, in and out, at both the air filter and exhaust elbow.
It turns out that Jeff had replaced the exhaust elbow about 12 years ago when he first bought his boat, so that should give you a reference on how long these things last with normal seasonal use. Now that the engine is running smoothly, we’re all breathing a little easier.