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My 318 onan engine started blowing white smoke. It was blowing oil through the carburetor. I checked out this forum and since then I have cleaned the breather (it's the plastic breather stuffed with steel wool) and I made sure there's no leaking around the top or bottom of the dipstick. It has quit blowing through the carburetor but now after about 30 or 45 minutes of mowing it starts blowing oil through the breather. Any suggestions? The engine had a thorough rebuild 4 years ago and I am very diligent in keeping it maintained.
 

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Dumb things first, is it over filled with oil? My next thought would be bad piston rings or possibly a crack in the block. Do you have the ability to do a compression test or leak down test?
 

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Did you check the reed valves in the crankcase vent under the carb on the top of the engine?
 

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It's not overfilled. Where do you find the reed valves? I looked under the breather when I had it out, are they under the carburetor?
 

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Did you check the reed valves in the crankcase vent under the carb on the top of the engine?
This.
When people mention checking the breather, they generally mean the entire crankcase ventilation system. So, youve got part of it with the breather filter itself, but that doesnt work without the reed valve in the actual breather working properly.

Now, on some engines with overhead valves, a blown head gasket leaking in the area where the pushrods go can do this as well, but the Onan isnt an overhead valve engine.

Ill see if I can get a screen shot of the manual and show where it is.

OnanBreather1.jpg
OnanBreather2.jpg
 

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Discussion Starter #7
Finally got a chance to work on it. The reed valve was fine. Checked the valve adjustment while I was there and the exhaust valves were a little tight so I adjusted them. Did a compression test both sides were good, 120 and 115. Put it back together went out to mow. Same thing, ran great for 20 minutes then started blowing oil through the breather. I'm stumped, the only thing I have done differently this year is I switched from Castrol 20/50 which I have used for years to Mobil 1 20/50 synthetic. Surely that can't be the problem?
 

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I doubt it is the oil change either. I would double check dipstick but you state you've been there. I would recommend making a little crankcase vacuum tester out of some looped clear tubing and see if you have negative crankcase pressure. Not sure why it would take 30 minutes of running to develop positive pressure? Check it cold start, then hot after running.
 
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