I had this problem with a generator. The genset seemed to run just fine except that it looked like a tire fire in back of the boat and the water had an black soot over it. After two generator companies came out and checked the injectors and everything else you can think of they said we needed a new genset, this one was just wore out.(1750 hrs) After puting the thing back together again (2ND time) I just happened to check the air filter screen and it was completly closed with exhaust soot! So I cleaned the filter assembly and it was perfect! No smoke, no soot , just clean water out the exhaust. I guess you don't always get what you pay for. These guys weren't cheap. So I learned something new.The companies you hire don't always know more than you and make sure the filter screen is clean.