I have a 4.0. When I bought mine, the PCV system was not intact and must not have been so for quite some time. I bought new PCV ports, hoses and gaskets, then took off old valve cover............ And was encountered by what looked like a compost pile of carbonized oil. In fact, it was so full of hard oil that it maintained the look of the inside of the valve cover even after I removed said cover.
I chipped away as much carbon as I could, trying to keep any from going down valves. Then I emptied te oil from the crank case and filled it up to the brim with diesel fuel, letting it sit for about 24 hours.
Next day, I emptied the diesel fuel out and had ordered a brand new valve cover because the PCV ports were so clogged with carbon, it was trashed.
Installed new cover, ran engine for a few minutes, then changed oil again. Now she runs like a gem...
Just my experience.