Thanks. That makes much more sense.
So to run 99-01 computer, I need to obtain the fuel rail, regulator, and injectors from those years, correct?
I wonder, as a rock crawler, if going to a returnless system is a poor choice. How does it purge air? Time for some more research on my end.
The Explorer returnless system is not a true returnless system. The fuel pressure regulator is mounted in the tank directly above the pump and purges the excess off before it leaves the tank.
So, assuming your pump and lines are capable of 65 PSI, all you would need is an adjustable fuel pressure regulator mounted on your existing fuel rail. Crank it up to 65 psi, remove the vacuum line, and you're good to go. I think it would be fine for a crawler, there should be about zero chance of vapor lock with 65 PSI.
The 99-01 injectors use a different style plug (commonly referred to as USCAR) than what a 96-98 harness has (commonly referred to as Jetronic).
Injector plug adapters are a dime a dozen so that is easy to overcome. However, the 99-01 uses a 2 wire cam sensor and your harness is setup for a 3 plug cam sensor. Since I still don't know if changing the cam sensor type in the tune works or not I would run a 99-01 harness if you are going to use a 99-01 PCM.
If you go with a 99-01 harness I would suggest getting 8 USCAR injector pigtails from a Chrysler donor car. I have yet to see a 99-01 harness that didn't have the majority of the injector connectors broken and the Chrysler design seems like it may hold up better.