I don't know where to start looking to fix my vacuum issue. Truck runs like crap and there is so much vacuum in the valve cover that's it's extremely difficult to pull off the oil cap with the truck running..and when I do finally get it off the truck stalls. Can anyone get me started in the right direction? 2004 5.4 with about 140k miles

 

Crankcase breather/pcv valve stuck closed?

 

When you pull the oil cap the truck stalls? Jeepers, I'd say, PCV as well...
What rpm does your truck idle at with this condition? Are you getting any codes?