Hey Y'all,
My 2003 5.3L Suburban 1500 has the dreaded P0300 random misfire code. I have read thread after thread and watched countless YouTube videos and feel like I can't find anything similar to my situation with a solution attached. Please bear with me, I feel like I've checked a lot of boxes from other threads with no luck.

For starters, this is a rebuilt motor. Warranty ran out 4 months ago (yay). Motor has about 40K miles and was done in 2022. 90% of the sensors and "whatnot" were replaced during rebuild. Injectors and coils were not done (I know what you're thinking. Keep reading ).

Long story short, the issue started with a simple check engine light. Was running fine all the way home from a 300 mile trip. I feel like the fuel economy was trash though. Got home and scanned it. P0300 with a P0445(?) EVAP code. Hoping it was a fluke since the truck was running fine; I reset the code. Couple days go by with nothing and then, boom, light is back on but this time I can slightly feel the misfire at idle. Same P0300 code and P0445 EVAP code. This time when I reset it, it came back on after driving ten miles. I've replaced all Evap canisters. One on top of the motor and the large and small by the gas tank eliminating the EVAP code. Reset codes...what do ya know...P0300 comes back.

Scanner live data for misfires says its cylinder 2 and 6 with 2 being the worst. At idle (rpms around 550) Cyl. 2 misfires about 30 times with 6 about half that. In gear, power brake, Cyl. 2 misfires up to 60 times with 6 about half that. To the untrained eye, everything else seems fine. Short and longer term fuel in bank 1 and 2 are all bouncing between one and zero with the worst being -3 on long term bank 2, if I remember correctly. A/F ratio is 14.6:1 which I've read is normal. THIS MOTOR IS RUNNING SMOOTH minus the misfire!

The part you've all been waiting for, troubleshooting:
-Hoping it was as simple as bad gas, at 1/4 tank I added injector cleaner and filled up with Premium fuel. Drove around for a day and not a single change.
-Checked MAF sensor on the air filter and cleaned it out.
-Checked and cleaned throttle body. Throttle body is new and wiring looks good.
-Sprayed starter fluid on vacuum lines and around motor while it was running and no changes happened with that.
-Replaced spark plugs and wires, just for fun. Old plugs were okay but not great so it felt like a good box to check.
-Swapped coil packs from good cylinder to bad with no change in misfire location.
-Removed fuel rail and it looks new still. Swapped injectors from bad cylinder to good with no change in misfire location.

This next part is where things get interesting:
Test drive connected to scanner reveals the misfire is only happening at low rpms or idle. Drove the truck up to 80mph with no misfires. Someone mentioned it could be the Crankshaft positioning sensor. The theory is that maybe its happening at idle because of voltage. Maybe it's a long shot but what did I have to lose.... So I took the sensor out and checked the wires on the connector and everything looked as it should. Tried to check the sensor with an ohm reader but got nothing (I think I'm doing it wrong so advice is welcome for that). It was 8pm so I made the decision to put it back in because it looked new, is an AC Delco and a new one was $90 at my local parts store. I did a Crankshaft positioning sensor relearn since I took it out and now ALL THE CYLINDERS ARE MISFIRING. With 2 and 6 still being the worst. I tried the relearn a few more times and had the same result.

A couple side notes: I did not do a compression test of the cylinder because I do not have the tool but will get one today and update. I will also start testing wires. A new crank sensor may also be on the list today depending on compression test. I've learned the firing order for this motor is 1-8-7-2-6-5-4-3 so I think the cylinder 6 misfire is residual from cylinder 2.

I'm hoping someone here could point me in a direction that I haven't already taken. It's beginning to feel like opening the motor is inevitable. Also, if anyone knows how to unlearn my relearn, please advise.

Thank you in advance!!