Ron6519

Here we go again. I think one issue is causing the other two. Maybe, maybe not. Fuel pump and filter are 2 years old on a 2001 GMC Savanna van, 5.0 liter engine, 104410 miles. I had compressed the nylon lines and then had to drop the tank again. Fixed the lines and put it back, getting 60 psi. Then in February 2020 the check engine light comes on with a Random misfire, P0300. Fuel pressure was 51psi( measured with 2 different gauges). I dropped the fuel tank again to see if the nylon lines closed up again and they were fine. While the tank was down I replaced the pump with the spare I had(AC Delco). Checked the fuel pressure and found it still at 51 psi.
Tune up done at about 100000 miles( plugs, wires, cap and rotor)around 2017-2018. No fuel pump codes. Van starts right up. No hesitation. Accelerates fine with no rough idling. Fuel pressure does not drop after engine is turned off and will hold at least overnight. When the accelerator is snapped wide open, the psi jumps to 61 psi., then drops back to 51psi. Spider module upgraded in 2016 with a new unit due to gas peeing out of the fuel regulator. There are no lean or rich codes .Fuel trims are within specifications.
Misfire code of P0300 morphed to P0304. Pulled the plug from 4 and checked spark. Spark was evident and the same for 4 and 2 plug. Cap and rotor fine.
After catalytic convertor oxygen sensor code based on low activity. Hovers at 450 mv., closed loop. O2 sensor a month old.
I'm about to check for electrical issues with the pump and oxygen sensor. I'll check the amperage draw of the fuel pump at start up and then look for compromised grounds. Will check the oxygen sensor for signal voltage, ground and ohm ratings.
Any other suggestions? Any issue with the one item causing all three issues?

Thanks
Ron

mountainmanjoe

Pressure is a little bit low. Should be 55 to 62 PSI.

Since you only get misfire on one cylinder then you can focus on that. Not sure if doable, maybe pull out the injector and observe it while cranking. You could try a compression test on the cylinder. I also know there are pressure transducers you can hook up to the fuel rail. You scope out the pressure reading to make sure injectors work properly.

Ron6519

This van has a spider injector under a plastic intake. There is no external fuel rail and no exposed injectors. If an injector was malfunctioning, wouldn't there be another code thrown for a lean or rich condition?
A compression test was done when I did the tune up 2 years ago and the compression was good on all cylinders. The van has 4000 additional miles since then.
A fact I neglected to put in the original post was that the PCM was replaced due to a white moist pasty substance inside the PCM and "communication" issues. This was a used, re-flashed unit based upon my VIN number. I'm thinking that the PCM has a programmed incorrect fuel pressure associated with it. I'll troubleshoot the electrical side to make sure of supply voltage and proper grounds and then contact the company that programmed the PCM.
Is the assumption of an improper fuel pressure in the PCM a possibility? How else would the fuel pump know how to regulate the fuel pressure?
**** Never mind, I think the ECM just turns the pump on and the regulator controls the pressure. I'll concentrate on the voltages and grounds for now.