00 F150 Ran lean discussion

I could use some thoughts on this one. This truck just simply defies logic. So here's the run down. 2000 F150 5.4 Auto 2 wheel drive. Truck came in with the dreaded coolant leak at the front of the intake. The guy that runs my shop now ( I've taken ill so i can't run it and I'm not there all the time ) talked with the customer and they decided to go with an after market intake. When it showed up it was dramatically different, I told him to call the company directly to discuss it and the company said it was simply a different design and they work well, i also told him to talk to the supplier about reimbursement if it had to come back off. Sure as heck the truck ran like crap. Ran lean, both banks. I told the tech we had to cobble up the old intake and put it back on to see how the truck ran then we had proof the new intake wouldn't work on this truck. Ran good again but lean on bank 1 oxy 1.Ordered up Ford intake and installed it, truck ran great, still lean hego 11. Other 3 hegos switching text book. I swapped out the front hegos, took the text book hego 21 and switched it with hego 11. Now hego 11 is switching text book in bank 2 but the once working hego 21, now in the bank 1 is not switching.It would maybe creep up on idle to .050 to .070. Has to be a vacumn leak right, NO. When I say I checked everything, I literally mean everything. I could go on for a while here but i will shorten this up. I had a new hego sitting on the bench for another customer and decided I would toss it in bank 1. The damn thing worked text book, all 4 hegos switching perfect. So how is it that I could swap out a perfect switching hego, stuff it in a different hole and have it not work, but the not working hego begin to work in another hole? How is that only a new hego in 1 hole could fix the issue?? It totally defies logic that a working hego would stop working and a non working hego would start working but a new hego would fix the issue.

Nobody yet I see. LoL. I'm going back out today and I think I'm going to check the resistance with the ohm meter to see what the difference between a new hego and the one on the truck.Maybe the old ones off the truck have a higher resistance. The customer has the truck back and he says the truck runs great now. This is just to weird.