[59toaster]

Well it is a spring loaded valve that once there is some vacuum applied that it will open and suck blow by gasses and water vapor out of the crank case. Those gasses are then fed into the engine and burned.

Some of the symptoms that a problem with the PCV would create is idle problem, poppin up the dipstick. It may also cause the oil to blacken up a little faster if they are stuck closed since the crud that would other wise be air born condences and gets back into the oil.

I replace mine about every 9K (every third oil change). Typically if the rattle when you shake them they are usualy ok but as cheap as they are...replace it. An engine with high miles may have more blow by and require a more frequent change interval.

There will be a second filter in the air cleaner where the crank case will get the freash air that should also be changed regurlarly as well. It will be into the side of the air cleaner assembly and have a pipe down into the valve cover oposite of the PCV.

The rubber gromit that those fit in on the valve cover tend to get hard. If yours is like that most parts stores have them in the "Help" isle for a couple bucks.