4.8L Vortec decent idle but NO power

[Wiregrass mechanic]

Quick backstory:
Customer Swapped 4.8 vortec to replace 4.3L in a 94 C1500. Brought to me in pieces.

Computer has been flashed all unnecessary stuff removed (including theft)

When you attempt to push throttle no matter if under load or not. Motor does not rev very well and sputters and cuts out. I am leaning towards injectors not firing properly to supply additional fuel.
Before I start tearing into it figured I would ask if anyone had a similar issue and see if it ended up being a wire or connector not allowing computer to properly control fuel or EGR, MAF, or TPS etc?

Thanks in advance.

 

[Wiregrass mechanic]

Here are some things I've found while diagnosing.

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[Wiregrass mechanic]

So another update. Truck basically revs up to around 1500RPM then just cuts out and drops back to idle even if you continue to press gas pedal. Feels almost like a rev limiter or fuel cut.

Thoughts? Suggestions?

 

[Wiregrass mechanic]

I unplugged EGR and it made no difference: No better or worse literally no change. I repaired the purge valve.

My thoughts:
1. Some issue with EGR or broken EGR
2. Problem with ECM wiring
3. Throttle position sensor
4. Injector(s)
5. Problem with coil packs? (Has new plugs and wires)

Other possibilities? Or thoughts? Any help greatly appreciated

 

[Wiregrass mechanic]

Unplugged throttle position sensor no change so replaced it with a known working one still the same issue.

It's almost as if the computer isnt making adjustments to fuel and or spark when you press throttle. If you hold throttle to flow it revs up 200-300 rpms then just cuts out drops back to idle then repeats

 

[gearheadE30]

Did the customer put a proper fuel pump in there that can handle the higher pressure that the LS engines use?

Is the cal set up for an electronic throttle or something, but physically equipped with a cable throttle? Those are the first things that jump to mind.