I'll start from the beginning..I purchased a 2000 silverado 5.3 from a guy it ran but had a lot of blowby...so I had a buddy of mine swap out the 5.3 for another it started up initially but ran horrible now all it does is turn over and backfire...changed cam and crank sensors and no change..getting fuel and spark white/blue...I am lost on this one anyone run into this problem?

 

Wouldnt hurt to double check all grounds, don't forget the one on the rear heads.

I've seen bad MAF sensors cause no starts too, sometimes unplugging them and trying to start it will work.

 

checked all the grounds..will try the maf it was SD tuned but it's still plugged in and yellow wire has been cut...a turbo setup with Trickflow stage 1 cam if that helps in diagnosing.

 

Unplugged maf didn't work.