i can't even begin to count all the air flow sensors that i've replaced that have read good or hadn't coded. is the system reading lean or rich? if rich it may be forcing lean or if lean may be an intake leak. you speak of fuel pressure, what's the volume? what about a restricted air filter, has the customer been burning alternate fuel? is there a restriction in the exhaust? if all the basics are ok i would pull out the front 02's and give it a run. oh ya, welcome to the forum
never argue with a fool, they'll drag you down to their level and beat you with experience