So we got off to a late start this year and the boat (2000 H340) has only been in the water for a week – clean hull, clean prop and shaft. We were motoring out to a favorite anchorage on a windless day when after about 20 minutes of running time at our usual cruising setting of 2,900 RPMs and about 6.5 knots I noticed that our usually reliable Yanmar 3GM30F started to briefly decelerate about 100-200 RPMs and then come back up. My wife was sitting in the stern seats behind me and commented that there was black smoke coming out of the exhaust. I turned to see puffs of black smoke, not a billowing cloud but definitely black smoke. I throttled back and decided to turn around to head back to the slip. If that was not bad enough, my wife soon called up from down below with the dreaded words “Honey, there is water all over the floor down here. She took the wheel and I went down to find water spilling out of the port side engine access panel. Looking at the top of the engine, I could see that the plastic fitting on the engine cooling water vented loop had broken off and lots of water was squirting all over the engine and compartment. I managed to stop the leak with many wrappings of electrical tape and we headed back in at a slower speed. Curiously, the black smoke disappeared after that and the exhaust was virtually invisible even when I brought the RPMs back up closer to 3,000 and in gear. The RPMs stayed constant under power and the engine temps also stayed constant and normal throughout as per my digital gauge. BTW, the exhaust mixing elbow, engine flange and coupling are only 12 months old and these Yanmar parts were installed by a certified Yanmar mechanic and performed well for 40 hours of engine run time last season. For 12 years I have never had any black smoke and my transom is always clean at the end of each season. Back at the slip, I was able to replace the entire vented loop assembly that shunts water from the heat exchanger to the exhaust mixing elbow. I could not get the engine to repeat the black smoke or wavering RPM problem in the slip although I did not run the engine under load. The following day I removed, replaced and checked both primary and secondary fuel filters and found no hint of water or sediment in the filter housings or in the pre-filtered fuel poured into a clear glass jar. While changing the filters, I noted that there was a good flow of fuel from the tank. The prop and shaft turn easily by hand from inside the boat. From what little I know about diesel mechanics, I cannot figure out how the vented loop problem could be related to the black smoke and wavering RPM problem or is something else going on. Does anyone have any thoughts or ideas as to whether these symptoms are or are not related?
