98 chevy 1500 starts and then shuts right off have replaced fuel pump and filter ,crank sensor,cam sensor ,oil sen. coil,ign.mod and PCM

On some chevys the oil pressure sending unit closes the circuit to continue the fuel pump to run after start. In the event of a wreck ,the car stops running thus no oil pressure thus turns off power to the fuel pump .A safety to help prevent fires. When you first start the car, the computer turns on the fuel pump for about 20 seconds just enough time to start and allow the oil pressure sending unit to close the circuit to allow it to continue running. when the car car has NO oil pressure or the sending unit fails ,it will start then shut down..as many times as you try to start. replace the oil pressure sending unit and secure a good conection. Dan