First of all, since your troubles began when you were welding on your truck, it is very possible that you damaged something inside the PCM. Did you disconnect the battery prior to welding? The wire that you shorted out going to the tachometer might have done something, but not likely. I would go through your ground wires again, even using battery size cable to ground the battery to the frame, battery to the engine, and engine frame. Make sure the ground wires for your computer are sound.
Tuner Pro requires that you have the proper BIN and .XDF file or it will give you erroneous information. There is a lot of information on this site for TunerPro help. If it is in closed-loop it should not be rich, fuel trims would be really negative but it should self adjust. The oxygen sensor could be bad, or the ground for the oxygen sensor could have a bad connection. Is this a one wire oxygen sensor or three/four wire?
Howell EFI is a good system using the older GM technology. I hope this helps, Jeff
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