So how are you tuning the car with no datalogger, watching the wideband and guestimating what cells to change?
When I replaced the engine in my teg a couple washers got swapped in the fuel rail and it was sucking the rail dry basically doing exactly what you describe. I threw a gage on the fuel filter and it was showing insane fuel pressure all the time. Basically I had pinched my fuel line off supplying the rail so everything before the pinch was high pressure, everything after would be stock until it couldn’t keep up and would drop off drastically.
Also as stated above a leak at the pump will make it suck.
I have that gage if you need it give me a ring and I’ll pull it off. My car hasn’t gotten driven much lately