It's not the timing chain for a couple reasons... for one when they were brand new trucks I used to watch them do this and figured it was to give them a rumbly exhaust note but I also noticed they did it at cruise on the scanner. The other bigger reason you know it is not the timing chain is it is being triggered directly off the crank shaft not any thing driven by the timing chain. I bet if you could somehow stabilize it at cruise and idle near the higher numbers you would notice an MPG increase from that. Maybe you could put a regular distributor in with about 18* idle, 35* mechanical and about 10* vacuum advance then figure out a way to trigger the cam sensor and so bypass the computer control of timing but still keep it there squirting in fuel.
That would never work. The factory ECU would pitch a fit without some kind of modification to its firmware. I would just replace the ECU altogether at that point unless you need it for transmission control.
This thing is also compensating for emissions. Replace outright or flash it if that is even an option.
