p0300 obdii code

[handymann]

HELP? Throws p0300 code. Idles rough with noticeable miss. Not even driveable.

Ignition side: Replaced plugs,wires,cap,rotor,coil swapped in known good igniton module,swapped in known good crankshaft position sensor.

Exhaust side: swapped in known good EGR valve, dropped y-pipe and checked for exhaust restriction.

Fuel system: checked fuel pressure 48 psi. book says should be 60-66. okay maybe on to something. Replaced fuel filter. Checked fuel pressure bleed down. seemed to bleed down tooo fast . suspected regulator leak. 1997 express 2500 5.0l v8 by the way. injectors and regulator located under upper intake manifold. Sooo i installed the MFI injector upgrade while i was in there. no help. checked voltage drop at the pump it was only 1.2volts so i changed the one year old fuel pump with new delphi unit. no help.

vacume guage reads 17-18hg steady. checked for any vacume leaks best i could with propane and carb cleaner. hard to get to with van access. seemed okay. checked engine compression warm with 150 psi lowest and 165 psi highest. ANY IDEAS? on where to go next.