First, Im a newbie, not sure if im posting this in the right area of the forum.
Ive got a Chevy Suburban, 5.7 with 157K miles that im just starting to tune. But, ive got an issue at the moment that I cant seem to fix. It has a shudder/stutter that shows up when under load, high throttle settings. It goes away if I back off the throttle, or it downshifts. Ive run VCM scanner, attached is the latest data log file. Ive replaced numerous sensors and the usual tune up items in misguided/lazy attempts at troubleshooting. The data consistently shows two things that I dont understand. First, the timing drops erratically, sometimes from 30 degrees advance down to -10 degrees. Second, it goes into Knock Retard mode which is, I believe, why it is shuddering under load. I just dont know whats causing it to go into knock retard. I have not replaced the knock sensor(s) yet as I dont know if they are bad. Of course that hasnt stopped me previously.... Also attached are two screen shot, one of the timing drop and the other of the knock retard event.
99 Suburban Timing Anomoly.PNG99 Suburban Knock Retard Anomoly.PNGBurban scan after Brake booster.hpl