None of these Mopars use a "vented" cap. The later ones using the evap emission controls use "pressure vacuum" caps, but those are not meant for an "operational" vent.
However it certainly could be that the system has a problem with the vent IE plugged
Rubber connector hose tank to supply tube? Rubber connector hose tube to pump?
pickup plugged in the tank?
OR EVEN JUST as simple as a defective pump. "New" parts don't mean "good" parts
There's a procedure in the service manuals for checking the pump, pressure and vacuum, as well as estimating flow.
