As an engine runs, the gasses inside the combustion chamber leak past the rings. I don't care what motor, who built it or what ever, but NO engine is 100% sealed. These gasses travel down into the crankcase (where the crankshaft and stuff is). The PCV valve allows the engine to suck in a moderated amount of the gasses to return to the cumbustion chamber to be burnt. If you've ever shaken a PCV, you'll hear it rattle. It ain't broke, it's supposed to do that. There's a 'ball and spring' type set-up in there that meters how much gass goes by it. A stuck open valve could create a 'Rich' condition while a stuck closed valve could create a 'Lean' condition. Now my fingers are hurting, so thats' enuf of that.........