2007 Chevrolet Tahoe 5.3 Flex Fuel P0300

Working on a 2007 Chevy Tahoe with a 5.3L Flex Motor code P0300 but not 1000% sure whats causing the misfire. I can feel the miss no doubt about it misfire PID at idle shows miss on cyl. 1 and slightly on 8 tested for the following, Both upstream sensors mark rich..Had new plugs and wires changed before i got the truck. I hear ticking as well from engine. Truck had been sitting for at least 4 years(long time) and owner did not drain out old fuel only filled up half tank with 91 octane
-Spark and coils (swapped coil 1 and 3 miss still on cyl.1)
-Fuel pressure at @ 60psi running and @ 55 static with little bleed off
- Injector firing heard with stethoscope and connected fuel pressure gauge/ energized injector good and held pressure
-Conducted a Cylinder leakage test (cylinder 1) with less than 10% leakage
-Sprayed water on intake manifold with no vacuum leaks
-Created vacuum leak and 02 sensors responded switching lean
-Tested heater circuit for the upstream 02 sensors
-Connected vacuum gauge and was bouncing slowly 17- @ 18in hg ( Might be a mechanical problem causing misfire)
-Scanned for Fuel alcohol content @ 10% (normal i think)
-Spark plug gapped at .040"( right on the money)
-Cleaned MAF Sensor ( Scan tool showed 5 gs)
02 sensor are pegged rich and running OPEN LOOP, no STFT and LTFT command both at 0, Fuel cell at 37 (Fuel Learn disabled)
I unplugged the MAF sensor and weirdly got STFT and LTFT back -17 LTFT ( Rich condition) and Fuel learn was back active.
02 Sensors started to fluctuate rich/ lean slowly after MAF sensor was unplugged

If anyone has any input i would really appreciate it, It might just be a mechanical problem on cyl.1 and maybe 8, But what would cause both bank upstream sensors to be rich all the time but with MAF sensor disconnected they start to fluctuate again and trims return. Any input is appreciated and Thank Y'all.

Just for clarity, do you know the 8th digit of the VIN? I think VIN 7 (LMG) is the only option for this Tahoe, just want to be sure.

I think you're on the right track with a mechanical issue on #1. I'd disregard #8 for now, as that immediately behind #1 in the firing order, and is probably just picking up on the crank fluctuations caused by the #1 miss.

Do you have access to a compression gauge? Static readings on #1 and a known good cylinder would be a good baseline. What I'd be really interested in running compression readings on the same two cylinders. Those readings will paint a pretty good picture of what's going on with #1.

A good static reading but a low running reading would suggest something like a broken valve spring (allowing the valve to stay open) or a worn cam lobe/lifter (causing insufficient valve lift and duration). A low static reading would suggest cylinder leakage (which isn't likely, given your leakdown results) or an intake valve that isn't opening.

Thanks for the reply Tyler, 8th digit is "0"
I do have a compression gauge and should have done that as one of my tests, i will most likely do it do be for sure on the diagnosis. I was going to check the timing but i'm only getting miss on cyl. 1 and 8 so i probably wont since that would effect all cylinders, The old gas will need to be drained as well since its not helpig. I have never had a misfire with a mechanical problem till now so this is new to me, Thanks once again.

Ok so the end result was low to no compression on Cyl. 1 with the compression test that was performed. When the cylinder leak down test was done i did hear some air escaping from the TB (Bad intake valve)