E320 fuel pressure regulator is in engine compartment.
Front , left side , 2" fowardward of the dip stick handle.
It has a vac hose connected to it . [ a bad regulator usually has gas coming out the vac hose ].
6" rearward of the dip handle is a fuel pressure test port. That will tell all fuel pressure conditions with a gauge attached.
You are looking for :
46-52 psi at idle
54-61 psi at idle w/vac disconnected
43 + psi after 1/2 hr. rest . [ pressure hold test]