Basically, all a PCV valve does is allow a certain amount of airflow through the crankcase of an engine to scavenge blowby gases and relieve crankcase pressure. The PCV is usually located on a valve cover. V shaped engines are easier to explain so I'll use that for an example. On one valve cover, you have the PCV valve with a vacuum line that goes to the intake. On the other valve cover, there is a larger diameter line that also goes to the intake; this is the fresh air side of the PCV system. The PCV side creates more vacuum than the fresh air side so the vacuum from the engine pulls air through the fresh air side, on through the crankcase, through the PCV valve, and into the intake manifold and then into the combustion chambers where the vapors can be burned. When vacuum is high (throttle closed or partly open at higher engine speeds), the PCV valve partially shuts from the high vacuum because not very much flow through the system is needed. When your accelerating hard, there is no vacuum, and the valve stays open all the way and allows maximum flow through the system when it is needed. The system is in place because blow by gases/moisture from combustion will form acids in the oil which equals sludge/oil degradation. Also, crankcase pressure, if unrelieved, will blow out oil seals and cause a power loss. Back in the old days, there was just a vent to the crankcase to relieve pressure, but a closed loop PCV reduces emmisions and pulls the acid forming vapors from the crankcase. Whooooooo, my fingers are tired now!
[ April 16, 2005, 10:38 PM: Message edited by: Drew99GT ]