Pcv?

[Projectbadass]

What is the Pcv valve for?
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[king-918]

Positive crankcase ventilation. It stops pressure from building in the crankcase and blowing out seals
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[gchemist]

It's also a way to get moister out of the oil and crankcase.

[GCncsuHD]

As mentioned, the PCV system and an old school crank case breather serve the same purpose, to evacuate pressure and moisture vapor from the crankcase. Obviously there is oil particles bouncing around and mixed with the moisture vapor. Old school engines vented directly to the atmosphere, then they started putting breathers with built in filters to capture the oil, but let the moisture and air pressure vent through. The PCV valve routes the oil, moisture, and pressure back through the intake to be burned, and therefore cleaned through combustion. The carburetor is designed to run on a set amount of a "vacuum" leak caused buy the air coming in from the PCV, without the valve there to meter the air it would cause too big of a vacuum leak and run poorly or shut off. It is designed to meter the air, less at idle, and more at higher rpm and not let any fuel/air mixture backfeed into the crankcase, hence the rattler inside. A faulty PCV can let too much or not enough air through, or a system not big enough for the engine or excessive blowby could coat the intake with oil. A properly functioning PCV doesn't affect performance, just reduces emissions, but a faulty one can have adverse effects.