99 silverado 5.3 p0300
Bought this truck a month or two ago, had a misfire on cylinder 8, swapped spark plug, coil and wire to other cylinder and the misfire stayed at cyl 8. I ran the engine with no valve cover on passenger side to check the rockers/valves and everything seemed fine. So I pulled the fuel injector for cyl 8 and the orings seemed fine i cleaned it up a bit and put it back, i think i swapped it with cylinder 6 but i forgot. After i did that the misfire went away and the truck ran fine. I put a couple of bottles of fuel injector cleaner in the tank. A week or two afterwards i would get an intermittent code for downstream O2 sensor passanger side.
Truck kept running fine until this week i started the truck and drove about 2 minutes and it started misfiring pretty bad i had to keep my foot on the gas a bit so it wouldnt stall. I got it home and checked codes, it was only one code, cyl 8 misfire. I pulled the passanger fuel rail off and cleaned them up a bit and put them back with new orings and plastic cap. I started the truck and waited for new codes as it was still doing the same thing. I got a misfire code for cylinder 7. I cleared the code and ran it until another code popped up. This time a p0300 mutiple misfire. I checked for a vacuum leak and there wasnt really anything leaking besides the throttle body leaking a very small about near the flap since its a drive by cable it was leaking from there. No leaks anywhere else.
I read fuel pressure and it showed 0psi but maybe it was just user error. I replaced the fuel pump/filter/fuel pressure regulator anyways as the cause wasnt working before. It now runs slightly better, but still stalls and hard starts/barely runs. Fuel pressure is up around 50psi now.
The only other thing I could think to check is the maf sensor or maybe a clogged catalytic converter or bad sensor.
What should I check next since there wasnt a vacuum leak and now the fuel delivery situation should be good. I only have a basic scanner so I can really do any monitoring, I have another truck just like it I can swap the maf sensor from to see if that does anything. I was going to try and unplug/ remove the upstream o2 sensors to see if the cats were causing a restriction but it seems odd that it would happen suddenly if it were that.
Thank you for any advice and please let me know what I could have done differently in my diagnosis also and what you would have done.