mind boggling misfire/backfire everything has been checked HELP

[Cory Martel]

before we get to the fun stuff heres the story , i have a 2007.5 5.3 silverado ,500,000 miles ____ the past 3 years its ran fine with its little mods and with out adding or changing anything , i drive about 110,000 miles a year , oil changes every weekish , yea no ****** , i also tow heavy daily , like twice the gvwr and sometimes even more lol but everything has always been fine and then ****** started getting weird , repetitive misfires and backfires COMPLETELY random though and im unable to replicate it , well it hapopens more under load and sometimes during idle , happens consistently .
so did a few tests and couldnt find anything contributing to the misses .
NO CODES besides traction control , and abs ( abs light is for rear wheel speed sensor)
btw random misfires are on all 8 cylinders , not at the same time though
oddly the first time i changed the o2 sensors it ran great for 30 minutes and then started doing the same ****** again so ive changed them 3 times so far this month , same part number , denso i believe


im only running headers , straight pipe and cold air intake
afm is tuned out


took it to 3 reputable shops and they all gave up within days .
so heres what i did :

5.3 reman from autozone , lh6 same as old block
new plugs
new wires
new intake manifold and gasket
new oil pressure sensor
new iat sensor
new mass air flow sensor
2 new upstream o2 sensors
new crank shaft sesnor
new cam shaft sensor
did a crank relearn but was also worried about doing that considering it was still running like ****** and i didnt want the (relearn) to think thats how it "should" run
new ecm
ran through harness and replaced ecm grounds
new fuel pump
added chassis/bod/engine grounds ( extra grounds everywhere )
checked injectors , all firing
checked coils , all firing
added reducer to straight pipe to add back pressure just to clear up a theory
and guess what , nothing has changed

checked fscm , (fuel system control module ) all good and even bypassed the fscm just for ****** to see if it would make a difference and it didnt

went thru the chassis harness to check for shorts , nothing

damn near out of money now , out of options too , i feel defeated .
and my wife thinks i hate her because i stay tinkering with this god damn truck all day lmao

I NEEED HELP

[asilverblazer]

WOW...

 

So much has been done most shops (as you've found) aren't going to want to touch this mess (no offense). Without access to a good scan-tool and tuner that knows what they are looking at and what to adjust you're going to have to check thing manually.

 

Does the truck still start, run, drive? 

 

The original tuner that removed the AFM, may have adjusted other things so that the tuned PCM doesn't know that something isn't right (thus no codes). Does the new PCM detect any codes?

 

I would start with putting critical stuff back as close to factory as possible, particularly PCM (you have a tuned factory, and a replacement with who knows what programmed in it - my confidence in an aftermarket PCM being correctly programmed is zero), and install a set of AC-Delco up stream O2 sensors. You've already noticed that despite everything you've done the only thing to make any difference was O2 sensors... start there.

 

Presuming there are still no codes... a vacuum gauge, fuel pressure gauge, timing light, and compression check are in order. 

 

Interested why you chose to put a new engine in it, what indication (other than miss fire) was there that it was bad?