So after multiply attempts at multiple different things, still cannot get my p38 4.6v8 gems to run smoothly, found that by clamping off the fuel return line the car would run mint, few readings, fuel pressure ignition on, 37psi then drops to 34psi, and stays, once started, it’ll drop to 29psi then climb back up to a max of 32psi, if you blip the throttle it’ll climb to 35psi no more, if you apply throttle steady it’ll drop down to 25psi and slowly climb and start to miss fire again, with the fuel return line clamped off it runs mint and smooth but fuel pressure with climb to around 60psi which is far too much, passenger side o2 sensors read 0.3v and drivers side read 5.0v with multimeter, on the scanner they both say around 0.14-0.60v ish replaced fuel pump, filter, pressure regulator, o2 sensors, along with a whole list of Other bits, only other thing I can think is maybe the injectors, any idea on a way to test them correctly? I’ve had them out and checked for clicking and seem to be okay, and tested loom with a noid light and again all are blinking as they should be

You need real time diagnostic data to check the O2 sensors. They should be switching voltage up & down 0-5V if I remeber correctly.

What are the fuel trim values ?

Fuel pressure should be 34-37psi so dropping to 29-32 would explain why it runs better with the return clamped. If the pressure rises with the return clamped, the pump is supplying enough pressure and the filter isn't clogged, so that would point to the pressure regulator.

Lambda sensors, if working correctly, should show 5V for lean and 0V for rich. However, a dead sensor will show a permanent 5V.

use vise grips to just restrict the fuel flow to around 37psi instead of closing it off and see if the issue stops. you may need a new fuel regulator. also remove the vacuum line and check that its dry .