I have 1992 GMC G3500 5.7 and will crank but not start why?

I have 1992 GMC G3500 5.7 I have changed fuel pump, Ignition module, idle
air control vale, temp sender, oil sender, ECM, throttle position sensor, plugs
and wires, no codes just 12 letting me know it's working, it doesn't have a
crank sensor on this engine, it's getting fuel when it cranks you can see it
with the air cleaner off, it has started a few times after I have added new
parts but last two times it idled a minute or two then I tried to give it throttle
and it stalled,  and won't restart what else could be wrong??