Join Date: Jul 2000
Thanked 171 Times in 160 Posts
Rep Power: 17
Yanmar (3GM 20hp) Blue Smoke Problem
Indications are possible compression ring problems. Suggest you do a compression test and an air pressure test of the combustion chambers.
Problem could be simply that the rings are sticking in their piston grooves. My 3QM30 used to do this and was solved by soaking the crankcase with "Marvel Mystery Oil" (super high detergent/solvent based oil - available at automotive speed shops) ..... running at NO LOAD until warm, let set a week, drain and replace with regular oil and 25% MMO thereafter - will help to keep the piston grooves and valve stems, etc clear of debris. Some mechanics use a mix of transmission oil and regular diesel grade oil for the same effect.
To check the breather tube, simply open the oil fill port and see if there is oil smoke coming OUT, if coming OUT - blocked breather; if suction IN - OK.