Quote Originally Posted by jfman
Did you know an ls1 bolts right into your truck.... Not sure it'll fix you mpg issue howerver
the 5.3 is a gen3/4 motor, it's the same as an LS1, you can use LSx Heads, cams, etc.

but, as said before my guess would be the o2 sensor, because I have heard stories of the chemicals in the smoke messing them up. Also, you could have loosened a chunk of carbon that got stuck in your EGR, and you can try to clean it.