Neither the front or the rear cover should be drawing air in! The purpose of even having vents on the valve covers are to vent positive pressure in the crankcase from blow-bye from the combustion chamber, they should always be pushing air out (even at idle it pushes air)! unfortunately when they do make a vent oil vapor will also be escaping and the manufacturers use the intake to draw the the oil vapor in to be burned off by the engine for 1) lazy people/less maintenance 2) Lower Emissions.
Remember how confused I was about this last year? Well, I have been studying up since then.
Take a look at the OE air filter box for our 3.0 engines. See how the back valve cover vent is connected, by a hose, to the air filter housing?
The purpose of positive crankcase ventilation,
is in part, to flush out accumulated combustion gases that build up in the crankcase from blow-by, with fresh filtered air drawn in through a corresponding PCV system intake vent. The other important reason for PCV, is the reduction of crankcase pressure, which builds up from blow-by during on throttle operation.
Our 3.0 engine, under low or no throttle condition of increased manifold vacuum, draws fresh air in through the air filter in the OE filter housing connected to the back valve cover vent, by that manifold vacuum created by piston down stroke. This fresh filtered air is then drawn through the crankcase to the forward valve cover, then into the PCV valve at the lower intake and down into the combustion chambers.
How could our cross flow crankcase ventilation system work, if fresh air could not be drawn in at idle and throttle let up, to replace those gases? If a PCV system does not allow fresh air to be vented in, at idle and throttle let up, then the manifold vacuum through the PCV valve could only try to suck blow-by gases into the crankcase when the engine is idling or at throttle let up, when there are fewer combustion cycles and hence little or no blow-by.
When people put a air filter on the back vent (yes I did it) it was to catch most of the oil vapor before hitting the atmosphere which in time just soaks the filter and makes it useless, i'm just avoiding filters all together now and replacing PCV valves every so often.
Remember that oil vapor did vent out though.
At idle and throttle letup, when manifold vac is high, crankcase gases vent from back to front and out through the crankcase to the PCV valve. When the throttle is open and manifold vacuum drops, blow-by crankcase gases vent from the crankcase, being drawn by the vacuum at the throttle body. Well at least it is supposed to, and that is why the back valve cover
vent was connected to the the OE filter box. The on throttle function of PCV, prevents the crankcase from pressurizing, which impedes piston down stroke and stresses seals, .... and other bad stuff.
Those engineers know what they are doing Roguetrip.
But I didn't know what I was doing when I first wrote this post. Now edited for accuracy, you can see the original mess, which Clayton quoted below. Oh the shame!
..... LOL!
