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I have a 1983 Irwin 34 with a Yanmar 3GM 20hp diesel. I have started noticing that at about 2000-2500 rpm''s I start to get blue smoke exhaust instead of clear. I don''t get it while operating with/without load under that rpm, or over 2600-3400.I just purchased a Yanmar shop manual and I''m more perplexed than ever. Could it be the breather tube assembly, and if so, why? Any suggestions? Thanks for the help!
 

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Farr 11.6 (Farr 38)
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I believe that blue smoke (vs white,black or grey) is lube oil burning. It is usually a sign of bad valve seals or piston rings. What perplexes me is that it only occurs at a certain range of RPMs.

Jeff
 

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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.
 
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