Chevrolet: The 5.7 bogging saga, fuel pressure regulator, psi drop

Question
Jim... no Schrader valve so I had to Tee-in a FP gauge to the TBI supply line. Showing 13-10 psi during easy throttle operation but drops like a rock to 5 psi in WOT! Psi should remain constant in the 13 psi range, right? New fuel pump, pick-up tube and filters already installed. Local mech said probably not a collapsed return line since that would cause it to run rich, not lean. I think 5 psi at WOT is the problem now. How about you?

Answer
Skip,
     Bingo! 5 PSI is definately not enough to run those injectors at WOT. I would expect a 2-3 PSI drop, but not one that drastic. So... You need to find out if the fuel pressure regulator is built into the throttle body or external, as it would make a big difference both in price to fix it and time, plus if it is external, then it would be a good place to start. Other than that, I personally would probably pull the tank and look at the pickup and strainer and pump to see what they are doing... could be you have something in the tank like a piece of the old set up or a flake of metal or something floating around in there, or a bad tube that only sucks up when you are gassing on it. The strainer should pick all that up as they are made pretty rigid for screens, but if there is no squished lines I would have to go with something wrong on that end if it isn't the regulator. This has really been interesting and has stirred up my grey matter some... let me know how it all shakes out and if there is anything else I can do.... Good Luck!