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2004 18 hp 4 stroke Compression blowby

N

NimrodAK

New member
I have a 2004 MFS-18B2 with very low hours. When it is running and I open the oil filler cap there is a substantial amount of compression/ air flow. I would suspect a rings or head gasket issue. How much blowby gas in the case is normal for these engines? The outboard starts and runs fine and I have not noticed any issues with performance. Any suggestions on how to proceed are appreciated.
 
Elvin is right. I would add that there is a fair amount of crankcase pressure (quite noticeable with the cap off) under normal operating conditions. If you overfill the oil, even a little, that pressure will go through the roof.
 
I have seen many Nissan and other Japanese 4-strokes "make oil", and to a considerable extent. They do this because they're not properly or completely broken-in. When people get a new 4-stroke and drive it really slow all the time the cylinder walls glaze causing a poor seal and allowing fuel to blow by into the oil dilluting it. More often than not when I check the oil in a engine like yours it has too much because of this condition. There is nothing you can do about it except try to avoid trolling for extended periods of time and run the engine in the top end more. In time you should counter act this condition. It may take all season, maybe two. So to answer your question - Yes, fuel blow-by is acceptable in these engines, not ideal but acceptable. If the engine runs well you probably don't have a problem. Keep an eye on the oil and see if the level rises. Check you breather hose too and make sure it's not obstructed. Under normal conditions, if you were to take the cap off with the engine running you would get "wind" coming out. If you were to drive it without the cap it would likely "puke". I'll bet your motor is fine either way. Don't be afraid to drive it harder, it was made for it, and made well.
 
Forgot to put the cap on after an oil change(in a hurry to get on the water).Started out on the river,wide open.Heard a
unusual noise.Stopped and took the cover off,what a mess,oil every where.They will blow the oil out with the cap off.I do
alot of trolling so my engine"makes alot of oil".I use a turkey baster to suck some out when the level gets up.
"Haste does make waste"and a big mess too.
 
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