Swap keeps running with key off
 

5.3 in a 98 chevy truck. It ran great for two weeks, no problems. Now it keeps running when I turn the key off. I disconnected the ignition power wire from the truck to the pcm and it kept running, so it's on the pcm/engine side, not the truck side. Any ideas where it's getting feedback power from after 2 weeks of running properly?

Disconnect the plug to the alternator and see if it still does it, not the big wire just the plug

I did. I found that unplugging the oil pressure sending unit kills it and everything works fine with it unplugged. I need to figure out how to keep the stock 98 wiring from enabling the fuel pump when there is fuel pressure, I'm controlling the fuel pump with the new setup anyways.

Jesse, IIRC, the 98 and earlier truck harnesses killed the fuel pump with loss in oil pressure so that may be what your duplicating, (but probably not the cause), although it's been a long time since I've touched those. As far as it running on I've had electric fans do that if not wired correctly, but that is easy to fix.

Thanks for the insight Joseph. It turned out to be the opposite. The truck is set up so that in the case the relay fails, the fuel pump will still be enabled if there is oil pressure. I cut the two non-tan wires to the OPS and left the tan one that goes to the gauge. Now it works perfectly.

That's the other thing I was thinking since I made that post, it was a backup setup. I remember having both the relay and sender go bad on a few older trucks causing a no-start.

It makes too much sense to cut the fuel pump if oil pressure is lost.

does it keep running, or does it cut off after about 10 seconds.
could be an electric fan keeping the system going for a few seconds.

It keept running. The fans are isolated from the system, when I first started it up I simply wired the fans up to stay on all the time for test driving and it would run till they slowed down, but that has long since been fixed since I wired them up permanently using a controller.

It is the oil pressure system causing it.