Help! P0300 Code

[Gen3 Cheyenne]

Hello there, wanted to see if I could tap into some knowledge out there.

Here is my problem - I just finished a 2002 5.3 motor swap into my 1973 C20. Has a brand new harness and PCM from PSI. Engine fires up but immediately runs pig rich. Idles like crap (although now after running a bit it will at least idle when before it just died immediately.

Some other details -
- Scanned PCM with EFI Live - Shows P0300 (Random Misfire)
- Pulled several plugs and they all show black soot heavy on the ceramic.
- The exhaust reeks of raw fuel.
- When revved, it seems to clear up, but then drops to idle and almost dies.
- I drove it around my block and it stumbles badly.
- I'm running the stock fuel tank - 3/8" (-6AN feed) and 1/4" return (-4AN) *I know it's small... but the stock tank only has this provision and I don't want to drop it if possible.
- I'm running a new walbro 255lph pump.

Anyone have any ideas for me to check? I've looked for the obvious stuff - vaccum leaks, etc.

I thought if it was isolated to one cylinder - I wouldn't see the fouling on all plugs. But they are all bad. So that kind of rules out a bad injector or coil I would imagine.

I thought maybe the -4AN return could be causing a high rail pressure but I've read about others using it w/o problem.

I don't have a gauge yet to check actual rail pressure -but it's pretty obvious it's very high based on the fuel load going in.

Help! Any thoughts?

Thanks

[jansen]

Have you verified with efi live that your coolant temp sensor is functioning correctly?

[Gen3 Cheyenne]

So I have to apologize here... Just figured out what my problem was.

I'm using a SLP Mass Air Sensor from an LS1 motor. Just read that the calibration settings are not the same as the 5.3 engine. My bad...

Unplugged the Mass Air Sensor and boom, problem solved. Runs perfect now..

Hopefully this can help others in the future.