[Masteryota]

If there is pressure inside the crankcase, either the vent tube from the valve cover is blocked, or you have blow by, meaning the combustion pressure is getting by the piston rings. Don't fret, these engines are easy and cheap to rebuild.

Next time you run it, do it with the oil fill cap loose and see if you feel air pulsing out of it, that will be your tell tale sign, or do a compression test with the gine hot.