Trying to help a buddy out. 01 4wd Silverado 5.3. On the interstate, ONLY above 70mph, ONLY when you are on a downgrade on the road, the light flashes the random misfire code. I've gone as far as swapping free parts i've had laying around like injectors, wires, new plugs, MAP sensor, clean MAF, turned the misfire code off w/ HP Tuners, and it still does this. Maybe its going into DFCO for some reason?

I've logged it a million times, the truck actually runs fine, and all sensors read out what they should. I even pulled timing out where it goes up when you aren't into the throttle when the light flashes(i.e the top of the table where it goes to upper 30s to 41 or so degrees upon decel), hoping it was bad gas, but he says its done it since he's owned it.

Any thoughts? I'm trying an extra PCM next

 

Misfire code is just the "summary" of what happened before,
flashing light is misfires -now- (real or imagined).

Look at the misfire current cylinder, 1-8, and see if there is
an "outstanding" one. A big imbalance in the rolling counts
indicates a cylinder specific problem. All of them being
large, is a global (fuel, most likely) one. Decel fueling is
not often gotten completely right. See if the narrowbands
are showing you pegged-lean but still delivering fuel (a
simple mixture problem in light decel, maybe not deep
enough to trigger DFCO).

I would think that DFCO would inhibit misfire detection
since no firing is expected. DFCO shows as zero advance
and zero injector pulse width.