1997 Chevy k1500 no start-fuel injectors

Hello. My father's truck has a 5.0L, CFI engine, with 120,000 miles. He says it progressively got worse in starting up, sometimes taking three tries. It finally quit starting. We replaced the oil pressure sender. I have also swapped out the PCM, BCM, ignition lock cylinder, and ignition switch wiring harness, all with no luck. I have tried the Passlock re-learn procedure with the key. Monitoring fuel pressure results in 55 psi after a few cycles. It has spark, and will run briefly on starting fluid. Fuel is reaching the injectors, but something is keeping them from opening. If it is the Passlock system, what else should I be looking for? A separate module, perhaps? The keys have no resistor pellet (originals, I believe), so I am not even sure which version, if any, it has. Any help is appreciated.