I have the 6.4 bible downloaded on my computer but it is also available here.
The Powerstroke 6.4L Bible!
The reason I am adding this is because if you open the fuel system file there is a very good simplified schematic and good explanation of the system. This guy is right about the 3 +/- .5 psi., but as far as a tune causing a problem with this I just don't believe. The pressure is regulated by a mechanical poppet relief valve in the filter housing routing fuel back to the HFCM, has no communication with the computer. Also if you read on down is says the cracking pressure is 2psi. and actual pressures may be lower or higher.
I guess to get to the point, I just don't believe it is as critical as it is being portrayed and having a tune or any other mod. is going to have 0 effect on the operation of this portion of the fuel system. I'm not saying it "can't" fail but if it does it has nothing to do with a mod. J.M.O.
737tdi