Chevy 350 TBI starts but won't run

Throttle body injection problem

1987 Chevy 350, TBI runs but quits after 15 to 20 secs, must turn back ignition switch (key) and it will start again with same results. Something is cutting the power to the fuel pump after 15-20 secs. I put 12V to tan/white wire to the fuel pump, and the pump runs, van will keep running with 12V there – not the fuel pump.

Replaced fuel pump relay, and no change. Thinking about the oil pressure switch, also part of the fuel pump circuit.

Is there a way to diagnose this problem? I don’t want to randomly change out sensors and modules if they’re not broken.

Any help would be appreciated.

It isn't storing any codes either. When it starts and runs for the short period of time, it has full engine power and runs smoothly.

One other thing, when it's going to die, clearly from fuel starvation, I can flip the key back and forth, and the pump will run for another 15 to 20 secs. Therefore, each time the key is flipped on and off, the pump will run 15 to 20 secs. Seemed to me like some kind of relay. Although I beleive the ECM will power the pump for a short period automatically, and after that will look for signals from the sensors/distributer etc. - which it doesn't seem to be getting.

Also, anyone know what the fuel module does? Could it cut power to the pump as well?

I found a loose wire behind the fuse panel under the steering wheel area. It's the wire leading to the fuel pump fuse coming from the ignition. It powers the fuel relay and oil pressure switch, and keeps the pump running after the initial 15 to 20 secs, now that it's live. Now it keeps running.

The problem is with the MAF sensor, try unpluging it and the engine should run, but it will set a code due to it not being connectted.