Yanmar 3YM20 problem

[Esther-May]

My Yanmar 3YM20 seems to have the same problem. I had it installed three years ago and has done 340 hours. Today it started blowing thick black smoke with carbon in the exhaust water at 2500 revs but still runs smoothly and cleanly at lower revs. All servicing up to date except for exhaust elbow which has not been inspected, fuel filters clean, air cleaner clean and engine compartment well ventilated. Boat used for cruising and has solar panels for charging so motor is mainly used for motor sailing and not battery charging. Will have the exhaust elbow checked next week, but any comments appreciated!

[Minnewaska]

Originally Posted by Esther-May

My Yanmar 3YM20 seems to have the same problem. I had it installed three years ago and has done 340 hours. Today it started blowing thick black smoke with carbon in the exhaust water at 2500 revs but still runs smoothly and cleanly at lower revs. All servicing up to date except for exhaust elbow which has not been inspected, fuel filters clean, air cleaner clean and engine compartment well ventilated. Boat used for cruising and has solar panels for charging so motor is mainly used for motor sailing and not battery charging. Will have the exhaust elbow checked next week, but any comments appreciated!

Two random inputs. Could mean a restriction in air intake, unburned diesel in exhaust. Could mean excessive crankcase back pressure pushing oil mist into air intake that won't fully burn. In both cases, the air filter might be a problem. Did you check it? Guessing......

[Esther-May]

Thanks for the suggestion. Checked today but the foam air filter and the passages immediately behind seem clean. I last cleaned the foam filter in warm water on the last service 8 months ago. I also dived under the boat today and found the prop and shaft clean and the shaft rotates easily. I ran the motor again today also. The motor runs smooothly and sweetly with no detectable roughness, vibration or unusual noises apart from sounding a little more laboured than usual when under load. When not under load it produces a little sooty black smoke and when the revs are dropped back from about 2k to idle there is a burst of thick black smoke and black exhast water - there is no sign of any oily sheen on the water and no sign of unburnt diesel when holding my hand over the exhaust. Under load there is considerable black smoke and black water as the revs build. Again no obvious sign of unburnt diesel and just a slightly more laboured note to the motor. Will not go over 2400 revs under load - usual maximum revs just over 3100 with usual sweet cruising revs 2600-2800. I have done all the regular servicing myself on time (I'm a handyman but not a mechanic!) apart from the first post instalation service which was done by the agent, but have NOT checked the exhaust high rise elbow. I'll have the elbow checked by the agents next week.