2000 deville still misfiring

My old pump wasnt building pressure so i put a new filter on and i switched wit a used one from an old car i had.key on engine off Pressure goes to 45 then instantly starts droppin? Once i turn key an do again same thing... Car is starting tho. fuel Pressure around 38 while idling. I pulled fuel rail up and looked for leakin injectors.. There was none yet pressure still bled down kinda quick. I also have my cat aloose to keep from messing it up any further. Only codes keep showin is po300 multiple misfires. I need help figuring this out please!:runaround:

Fuel isn't the only thing that can cause the P0300 code. Plugs, wires, coils are probably the main culprit here especially if any of the above are original to the car. I would start with plugs and wires, if that doesn't remedy the problem change the coils. Fuel pressure can cause the problem but a fuel pressure around 38 at idle is within tolerance.

motor is not original...i pulled it from a 2000 deville low mileage wreck a few yrs back. i went to yard today and got a coil to try it out