3.4 swap backfire and stumble issue - help!

[Telly]

My son and I made the final wiring harness connections today on his 92 pickup 3.4 swap project. Fired it up and it immediately ran rough and starting backfiring within 30 seconds. Only backfires while idling and stumbles at higher RMP's. Good oil pressure and the engine sounds good otherwise.
We did the following trouble shooting things.
1. Checked timing and its within spec.
2. Checked the cam sprocket marks at TDC and they both line up correctly. (we did a new timing belt)
3. Removed the aftermarket coil packs and reinstalled the original coil packs. Issue did not change.
We bought this motor from salvage yard and were not able to hear it run, therefore I can't confirm if this was an issue prior to the swap. I'm leaning towards a bad ECM? My theory is the ECM is sending incorrect firing signals to the plugs. Any other ideas? Thanks!

Please move this thread to the Tech Section. I put this in Chit Chat by accident.

 

[SteveH]

Does it set any codes?

 

[Telly]

No codes...I forgot to mention that.

 

[J1000]

Don't know much about these motors but I'd check the plug wires/coilpacks are on the right cylinders/in the correct order on the distributor/in the right connector on the harness.

 

[HDavis]

Is there a coolant temp sensor that sends data to ecu? My 3rzfe has one and when its unplugged it had these types of symptoms.

 

[On the RX]

Could the timing be off? After you exhaust the simple stuff, check to see if the timing belt has everything aligned.

 

[Telly]

So I'm feeling sorta silly because the fix was super easy. Like Jimmy suggested, I checked the plug wire configuration and yep...they were not correct. The funny think is, I installed the new wires exactly like the old ones, one for one. It did not dawn on me to verify the plug wires were in the correct configuration to begin with. Lesson learned is never assume any wiring is correct when you buy a salvage yard motor!

We took it for a spin around the block and the power gains over the 3.0 is like day and night. The swap was definitely worth the effort and my son is super stoked.

Picture of the finished product.