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69stang

69stang

Member
OT, what would cause enough pressure to build in 12.5 hp B&S engine on lawn mower to blow oil into air cleaner and out fill hole when cap is loose. Started suddenly...today.
 
PCV valve is going bad on it and it is what keep crank case pressure down and if it goes bad it can and will cause that problem
 
I think any of them will blow out the oil cap if loosened while running.

Check the oil level, it could be gas contaminated and overfilled.

Like Old said, check the crank case valve where the vent tube comes out of the block.

In severe cases, excess blow-by will cause oil blowing. Is this an overhead valve? Check the head gaskets.
 
Old is right. It's your PVC valve. The rubber gasket at the bottom of the dip stick tube has nothing to do with it.
 
I would say vent is clogged up. Most common. Of course there are the normal things. Seals,rings and others. Onan's would blow oil everywhere if the vent clogs up.
 
Hello 69stang,

A really clogged air filter can make the engine burn oil and push it out the exhaust as well,


Guido.
 
is this an overhead valve engine? If so, that series is sorta famous for blowing the head gasket between combustion chamber and pushrod passage.
It then pushes pressure into the crankcase and causes the exact symptoms you describe.
 
Someone else touched on it: IF its overhead valve, you probably have a blown head gasket. Those overhead valve single cylinders tend to blow the head gasket between the pushrods. Then you leak combustion pressure into the crankcase and the crankcase ventilation system can't handle the extra pressure.

If it has been backfiring when you switch it off, that is probably when it blew the gasket. Don't throttle it down before turning off the switch.
 
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