The pressure reading is coming from the PCM (pretty much everything in that OBD-II port is going through the PCM, it's not like all the sensors report directly there or anything) and, again, when unplugged the PCM kinda makes up
Yes, you should be seeing positive voltage numbers from the ICP (like 0.85v at about 500psi) and the fact you don't, has to mean either it's passing No Voltage OR it IS and the scanner just isn't recognizing it. This would be hard to figure-out if the engine just cranks & cranks but doesn't start. BUT you DO start - right? So, my thinking is, it must be...
The PCM has certain "rules" for starting (really before it activated the IDM) and when these rules aren't met, these parameters are out of limit, it will Never run. They are things like;
100rpm min on the starter
above 8.5v in the system (the older trucks, like yours, actually need like 10.5v)
it needs to "see" ICP pressure (seen as that voltage like 0.85v)
I'm not positive, never tried really
, but with a Zero reading from the ICP I don't think it would even try to start (let alone the fact that if you actually had no pressure, the Injectors couldn't inject fuel)
So, I have to conclude you MUST have HiPres Oil being made & distributed to the injection system for it to start & run (as evidenced in the IPR %'s). Gotta be that scanner isn't reading it properly...