Black smoke and low rpms

On Saturday I headed out for a solo sail and just after I got the mainsail set I noticed black smoke coming out the exhaust. Yikes. So began a weekend of troubleshooting. I quickly put fenders back out and prepared to dock bow in since I wanted to minimize maneuvers in the marina. Came in bow first at low speed in the hope that if the engine shutdown I could make it into my slip and wouldn’t hit the dock too hard. Fortunately, the engine didn’t quit on me and docking was fine, although I must say I impressed myself with my speed estimate.

Now began a weekend of troubleshooting. I got on to Mr. Google and began searching. Here are the symptoms:

Max rpms in forward 2.2k, in reverse ~3k, in neutral >3k some black soot or un burnt diesel in the exhaust.

Eric, who does boat cleaning and waxing, was working on the boat next to me. He offered to dive to see if there was anything on the prop. Prop was clean which was confirmed by the ability to turn the prop manually with engine off and in neutral.

I changed primary and secondary fuel filters. They looked clean, but a new set wouldn’t hurt. No change in performance.

Perhaps the fuel injectors were dirty. I purchased some seafoam fuel additive to clean the cylinders and injectors. No change in performance.

Maybe one injector was bad so I measured the temperatures of the 3 cylinders. All the same and the engine sounds fine.

Rick who works on boats in the marina dropped by and spent time helping me diagnose.

Back to Mr Google. It seems that the only option remaining is that the exhaust mixing elbow was clogged with carbon. The mixing elbow is also referred to as the Achilles heal of the Yanmar 3YM30 diesel engine. I tried to take it off but didn’t have the right tools. Time to call in the troops

I spoke to several people in the marina to find a diesel mechanic – they are hard to come by. Howie in the marina office suggested calling Steve’s shop in Charlestown which repairs Yanmar’s. I spoke to Brian on the phone and he said likely an injector or mixing elbow problem. Within 24h he came to the boat and confirmed the mixing elbow was at fault and replaced it.

Looking forward to going over tonight to give her a test drive and to race in the final Wednesday evening race of the year.

Thanks to all for your help and advice. It’s a great community in the marina.