Problem with Chevrolet 4.3 Vortec fuel pressure / oil pressure relay

I need some information regarding a situation with a 4.3 Vortec engine in
a Chevy Pick-Up. The engine started hard, which was an indication that
the fuel pressure relay was faulty or weak, and as a result, would not
achieve a speed of over 50 MPH on the road. This seems peculiar due to
the fact that the oil pressure relay is supposed to act as a backup for
the Electronic Fuel Ignition system in this particular set-up. Once the
oil pressure achieves a safe level, the oil pressure relay has the ability
to take precedence over the EFI. The fuel pressure relay has since been
changed; the problem that I have is that I have been told that a faulty
fuel pressure relay can render and EFI system useless. I find this hard
to believe due to the fact that the oil pressure relay is able to function
as a back-up in this situation. Any suggestions?
were is the oil preesure switch located?