Chevrolet Repair: fuel delivery 305 chev 1500, 1997, oil pressure switch, fuel pump relay

Question
Van will not start unless primed by putting fuel thru air cleaner. First noticed if Van set without being started for a week or so it would not start unless I fully charged battery, even though the starter would turn engine properly. Replaced battery, but now it won't start without priming it. Once started van runs perfect and will start if it does not get cold. Should mention that van has a add on theft deterent system, but from all I can find out it is not connected to fuel system.
Thanks for any info Dan

Answer
Hi Dan,
Sounds like the fuel pump relay is not working, and the pump is running by power from the oil pressure switch, which is a backup system for the relay. As long as there is 4PSI or more of oil pressure, the pump will get power.

That deterant system could dissable the pump relay. I don't know.

Van