back fires under load

[mech tech]

This is an econoline E-350 with a 7.5 it is fuel injected and was made with this system from 1988-92. My scanner is limiter to KOEO codes and KOER only codes. (None stored or present at this time) It back fire in park or neutral easing in to the pedal around 2500 to 3200 rpm. It is running lean for sure. There are no vacuum leaks. (Smoke Tested) Spark is strong and in time. The distributor is new. There is no slack in the timing chain. MAP/baro sensor works. Temp senders and knock sensors work. TPS also works. Engine vacuum is with in specs for 2000FT. Batt, voltage is good. Grounds are also good. Fuel pressure is with in specs; fuel flow is also in specks. It has this problem warm or cold, so the O2 sensors haven't even started to work when you first start it up and it still has the problem. It also will die on decell 1 out of 1000 times. T-plates and idle air are also clean and working.
So could some one tell me why this thing is running lean, what’s telling the computer to lean this thing up?
Help!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!