My suburban is throwing the P0300 and sounds like it's running on about 4cyl. I have already replaced the fuel pump with a new Delco unit about 1.5 years ago. About 2 years ago I did the MPFI upgrade with a new Delco unit. I just replaced the fuel filter today. Pressure starts at about 52, then raises to about 60 under almost WOT load. From my research, I understand this is too low. I have pinched off the return line with the key on and pressure spikes to almost 90. Also from my research, I understand this indicates that the fuel pump is ok. At this point I *think* it's the regulator. Given that the regulator was new with the new spider it seems a little strange, but.... Does anyone have anything else I can try to check the regulator before I strip it down and replace it? Thanks!