Positive Crankcase Ventilation (PCV): Also known as a PCV valve. These became common around 1967. Simply put, it’s a one-way valve that keeps engine ‘blow-by’ gases from entering the outside air.
All engines, no matter how new or how efficient they are, allow some unburnt gases past the piston rings and into the oil pan. This creates pressurized blow-by that needs to go somewhere.
Before 1967, most engines just had ‘drag tubes’ that exited the valve covers to let these gases get sucked out and blown into the outside air. Not only did gases come out, but a microscopic mist of oil did as well.
On a PCV system, there is a PCV valve, usually on one valve cover. It’s fitted in via a grommet most of the time. The other end is connected to an intake manifold vacuum port. Coming out of another point of the engine is a tube connected to a small filter that allows the exchange of gases with the valve. This small filter was usually found inside the air cleaner assembly. The overall arrangement allows unburnt gases to come out of the oil pan and get burned inside the engine. This process saves a great deal of hydrocarbons from getting to the atmosphere due to these gases being mostly unburnt fuel and discarded motor oil.
Believe it or not, this is actually a good system to have. The extra vacuum inside the crankcase is said to help with piston ring sealing. This means more reliable performance and less oil being burned.