04 S/C MC P0300 Code
 

So I get a PO300 code which is random cylinder misfire but it goes away after a couple days of driving than it comes back weeks later. Car runs perfect no bogging. I have brand new AL 104's gapped at .55 and made sure they werent cracked..Brand new OEM ac delco wires..I checked for any vac leaks and I couldnt find anything..The only thing ive noticed with the car is that when its idling the RPM's will jump up and down slightly...I ordered a new upstream O2 sensor to see if that helps....Car only has 47k on it...not sure what else it could be

I've had the same a few times. I have yet to figure out what caused it but have wondered if hitting the rev limiter can cause it. Before knowing that the car has a rev limit of 4000 in park or neutral, I had slowly brought the RPMS up to the 4000 range and got the rev limit to kick in, shortly after I scanned the codes and had the P0300. Didn't scan right away as I have had a MIL for Major Evap Leak for some time now.

If the plugs/wires/coils are good and you still have a random misfire, fueling can be the issue.

Maybe your fuel filter?

could be that, check and make sure you passage way from the egr passage way to the intake side. that was my problem before

Fuel filter is also new, maybe the FPR acting up?
I'll check on the EGR as well

FPRs are just so rare to go bad, but I guess you can check it.

Remove the vacuum line from it and make sure it's not wet / doesn't smell like gas.

PO300
PO403
PO405
PO446 ✴︎ ⬅︎ ⬅︎ ⬅︎

All these Codes were triggered by a faulty FUEL TANK PRESSURE SENSOR!

Wait for the Low Fuel Warning before attempting to DIY.
Start engine.
Pull FUEL PUMP Relay ... Let Engine stall to relieve pressure in system.
Remove GROUND Wire from Battery.
With car Raised and SAFETY STANDS securely in place.

Support Tank with Jack.

(I made a "Jig" to attach to the Jack. Sawed the square bottom off from Milk Crate. Drilled holes through Hockey Puck. Mounted Puck to center of Milk Crate with Heavy Duty Zip Ties. Affixed carpet to other side of Milk Crate bottom for padding.)

Lower tank enough to access Sensor atop Fuel Pump.
Unfasten Pigtail from Sensor.
Unclip and Remove Sensor ••• Straight-up.
DO NOT BREAK THE END OF SENSOR OFF INSIDE PUMP!!!

If you need a Charcoal Canister do it when you replace Sensor.

Removing Relay to releive Pressure throws a Code which may require a MIL Light reset.