P0300 after heavy rain
 

I searched briefly.

I'm at my wits end... 2003 Chevy Express 1500 w/ 5.3L engine.

For about 6 months ONLY in HEAVY rain, it would flash the check engine light... code reader said P0300 (random misfire). I only had maybe 30 minutes to drive, and the next day the van would run fine w/ no check engine light.
THEN, wife and I were taking a long trip. Heavy rain. Check engine flashing again, and I had to drive about 3 hours like that. Next days, and up to now (2 weeks), engine running rough and P0300 all the time. Can "drive" the van but it's dangerous.

Figured it was probably a ground that got soaked... Cleaned up all i could find. Problem remains.

Decided coils, plugs and wires were in order (about 80K miles since). Problem remains.

Knew I had put a couple gallons of questionable gas in it a couple weeks before this all started... figured fresh fuel filter... Problem remains

Son and I thought we heard hiss when dog house off... Intake manifold gaskets are often a problem... Had guy smoke test it, Leaking at #2, #4 and #6. Fixed the vacuum leak. Problem remains.

He said fuel pressure would drop from mid 50s to 30... Said "new fuel pump".... Interesting, I did one about 6 to 8 months ago (thru the floor). Purchased new one. Problem remains.

Fuel Pressure Regulator vacuum line is not full of gas (so I assume it's good, and not part of the problem).

I'm back to poor ground and too low a voltage to spin the fuel pump and maintain 55-62 PSI...

I cleaned the 2 on the back of the block just below the intake manifolds... one on the driver side has a horizontal bolt that grounds a wire coming down from the main loom. PCM ground, I think? The other has a vertical bolt (on the passenger side... I did this again since it was there and easy to do.

Any and all suggestions are welcome.

2003 Chevy Express 1500
277K miles

Thanks

John

update
 

Update: Took it to guy that replaced intake manifold gaskets. He was saying new fuel pump (chinese-ium) is still a problem. Put fuel gauge on it in front of me.. 28PSI. Supposed to be 55-62PSI.

Took it home... back probed fuel pump wires (black - and grey +)... my analog gauge read above 12 but below 15 volts. Can't find my digital one.. borrowed digital from neighbor. 13volts (didn't have decimals)... Anyway, that confirmed for me that the pump is getting enough voltage to run the pump at full speed.

Leaves ONLY the Fuel Pressure Regulator... Replaced that... problem remains.

Finally ordered "Genuine GM" fuel pump. Gonna have to wait until next Wednesday... Frown...

Got me even questioning if I put the fuel filter in the correct orientation at this point... I'll double check this evening when I button up the "dog house".

scratching my head.

I had a similar issue with my '22 Silverado after a big storm, code P0300 was popping up - ended up being a faulty crankshaft position sensor due to all the water getting into the ECM. Cleaned it out and it's been fine since, but yeah, definitely a thing to check after a heavy rain.

Heavy rain can really mess with the ECU's ability to read proper engine temp and fuel trim, it's not uncommon to see codes like P0300 pop up after that. I've dealt with a few 2022 Silverados that got this code after a particularly soggy rainstorm and it always ended up being a faulty IAT sensor or wiring issue.