I've posted here before about the timing issues with my 98 3/4ton and i've received some great advice. but nothing seems to work. I have had the truck at a gm dealership and every tech in their shop could not seem to fix my timing woes. turns out that my truck is running 19 degrees advanced, they thought the distributor was off a tooth (it wasn't), they checked the timing chain (all good), they used the scan tool (no avail). I'm at my wits end here, truck has zero power and at best 8 mpg. trailer loaded with 5000lbs, foot the the floor in 4th gear - 55mph tops. i've read about a spout connector to reset the base timing, but i can't seem to find it on the firewall. I've watched a youtube video showing the techs adjusting the distributer, but i was under the impression that the distrubuter wasn't adjustable on a 5.7 vortec. am i looking at it wrong?

this truck was like this when i purchased it over a year ago. the guy i bought it from said a local mechanic had put in a rebuilt engine and it has never been right since i've owned it. it didn't used to bother me much because I was only using it as a farm truck. but recently my father passed away and this truck is being used to haul off his overwhelming collection of scrap metal, so being sluggish on power is very annoying for other highway drivers that happen to get behind me. i've replaced distributer, timing chain, plugs, wires, and crank and cam sensor. don't know where else to turn.

any advice would be greatly appreciated.