Oil consumption

[Brian S]

Recently had yanmar 2gm20f rebuilt in Mexico. I'm losing about 1/3 cup oil in 2 hrs. Operation. It's not leaking, some blue smoke at startup, but none while running. I think lots of blow by at filler cap, but not sure how much is to much. I'm using 10 40 would additive or thicker oil help.
Is it rings or scored cylinders. Or something minor. Any ideas?

[a64pilot]

It's blow-by, sounds like rings haven't seated.
Best way to seat rings is to go out and after she is fully warmed up, run the snot out of it, I mean full throttle - wide open run it. Obviously watch for overheating, but if after an hour or so of running her hard, if the rings are going to seat, this will seat them.

10W-40? You sure that is Diesel oil? What brand of oil? what two letter designation is it?
Does it start with a "S" or a "C"?

[Brian S]

My mistake it is 15-40. Delo 400 ApI. CI -4 plus/SL Lubricant for Diesel

[ontherocks83]

Typically the short answers are if you are getting air out of the filler cap it is a valve issue, if you are getting air out of the dip stick tube it is rings, if you are getting air pressure built up in the cooling system (provided it is not raw water cooled and uses a heat exchanger) then it is heads. Where you said noticeable blue smoke, I would start with rings

[a64pilot]

OK blow by, is blow by, pretty much all of it is past the rings.
You just don't get significant blow by anywhere else. A little blue smoke on start up may well be valve guide seals, oil leaks past when the engine is off and burns off at start up. Annoying I guess, but nothing to be real concerned with, but excessive blow by means rings are not seated, or are worn out, cylinder walls glazed etc.
Assuming it was in fact "rebuilt" which is a term that means different things to different people, it should mean nothing is worn out, leaving rings that didn't seat.
Without disassembling the engine the only real way you can seat them is to run it hard, this build high cylinder pressure, which forces the rings out against the cylinder wall which hopefully will make them "wear in" or seat.
Do not change to a thicker oil, your using excellent oil, that's not the problem. Go to a single grade if you like, but you may either try to seat the rings, or just live with it, or take her apart and find out what the problem is.


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[ontherocks83]

Pulling an injector and using a high quality bore scope might make you feel better but if you are burning oil then your options really break down to keeping extra oil on board and checking religiously or doing a rebuild. If you are really lucky and the cylinder walls aren't glazed there is a slight chance that they didn't stagger the compression rings when they did a rebuild and all of the gaps are lined up causing you to burn oil. Most likely though the walls need to be honed and new rings put on.

[Brian S]

Ok lots of different ideas and suggestions, from everyone. Thanks to all replies. Being in Mexico and new at working on engines I appreciate all responses, now I have to figure out the best way to go. Smooth sailing to all. Thanks Brian