positive crankcase ventilation, usually found on one of the valve covers,will have a 3/8 hose running to a manifold vacume source, main pupose is the feds say we can't vent crankcase fumes to the atmosphere, but the upside is it pulls a slight vacume on the crankcase and keeps crankcase preasure from blowing oil out of seals and gaskets.... one is used but you have to have a breather somewhere with a filter as it has to pull clean air in (usually on the opposite valve cover)......Hope this helps.....