What could it be.......

[Supercharged111]

Back when I still had my spider replacing the fuel filter netted a 1mpg increase.

 

[Wilecoyote]

Kinda lost track of what you did from the beginning... I think you replaced the fuel pump and recently the spider injectors? Seems like truck runs well now and with improved fuel mileage!? Except for the EGR code, right? Seems like a move in the right direction!

EGR on these are very simple and easy to check. Look into here: https://easyautodiagnostics.com/gm/4.3L-5.0L-5.7L/egr-valve-tests-1

Cleaned the egr and put it on backwards while trying to fix the miss fire.I finally had the truck towed to a shop to put a new mpfi spider in it. Driving it home the egr code came on. I checked it when I got home and it was on wrong. I found the gasket had slipped off when I cleaned it and put back w the sun going down weeks ago.Got a new gasket and put it back on right that night and erased the code.Also had the upstream 02 sensors replaced.Runs better than when I bought it. I think I read the computer needs about 100 miles to relearn?

 

[HotWheelsBurban]

Cleaned the egr and put it on backwards while trying to fix the miss fire.I finally had the truck towed to a shop to put a new mpfi spider in it. Driving it home the egr code came on. I checked it when I got home and it was on wrong. I found the gasket had slipped off when I cleaned it and put back w the sun going down weeks ago.Got a new gasket and put it back on right that night and erased the code.Also had the upstream 02 sensors replaced.Runs better than when I bought it. I think I read the computer needs about 100 miles to relearn?

Yes at least 50 miles. And has to be varying speeds too, not just " go blow the cobwebs out on the highway " like we did back in the day.....