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I have Yanmar 6LYA STP engines, with 335 hours. The boat is a 31 FBC, absolutely clean bottom. The fuel is dirty and I have been changing the primary filters every 30 hours (Racor 10 micron). The boat had a full tank of fuel when I bought it in Sept. 06, and I have reduced the fuel load to about 60 gallons, and anticipate having the tank cleaned and fuel polished.
The port engine is pumping gray raw water, and a whisp of white smoke.
The starboard engine is pumping clear water, and not a sign of smoke. I changed the primary and secondary fuel filters today, have not run the boat. I guess I'll call the mechanic in the morning. Does any have a guess as to what the problem might be. I'm turning 22x24 wheels with prop pockets. Thanks guys for any input. Jim Clark

A stuck exhaust valve?

start with the easy..........filters (all of them)........maybe the turbos.......had a friend with a similar problem and he changed the turbos and the problem was gone.....

Check coolers as well.

I say Exhaust valve. I had my heads and injectors redone a few years ago and that engine from that time on ran grey water out the end. Stuck Exhaust valve was the problem Smoked a bit black that cylnder ran hot finally blew something lost oil pressure and now re-powering with a reman.
Don't know what blew yet maybe rings and valve. Won't know until I get her out.
Check the temp of that cylnder if you can.