My 2003 C15 6nz has same issues.
Changed frame mounted fuel filter head (Peterbilt said was sucking air)
Next they said it's a bad fuel line between fuel pump and tanks. I changed all fuel lines. Next they said it's a bad fuel pump, they changed it.
The list is endless. Still it pops and stumbles under load. 3X CAT drove with me, cutting out injectors with no resolve.
Here's my unique symptoms.
I'm 19 months into a platinum rebuild by CAT. New Tranny (18) behind it.
2 new diffs. Balanced driveshaft. New
Ujoints & hangar bearing. New load leveling valve.
Pops, stumbles and power goes down under load or pulling empty trailer.
Ease my foot off accelerator but still maintaining road speed on a flat surface and all is fine. Press down a touch to increase speed and pop...shake and shudder.
Here's the clincher...
Idle for a few hours and head out loaded and my engine runs smoothly and perfectly , for 2 to 5 minutes.
Last week I had a entire day of perfect smooth powerful engine running, for unknown reasons.
Tried using another fuel supplier for 2 tanks but no change.
Thought perhaps a cracked drop tube in fuel tank sucking air at top. Nope!!
I made up a brand new fuel line going from my new fuel pump directly into the fill hole on top fuel tank. Still no resolve. Pulled all 6 injectors to check for compromised O rings. All good. Did top set and closed her up. Still the same.
Shaking/popping/heavy low pitch bang under load or pressing down on accelerator slightly to maintain road speed on flat roadway it starts, back out of acceleration and all good.
On a highway when I'm exiting and slow going around a exit ramp, followed by hard acceleration to bring my rig up to highway / road speed again and for unknown reason the truck runs / pulls great again for....5-8 seconds, and thereafter it's "pop...pop...pop" as usual again.
I need help. If anyone has any ideas , even if I have to pay, I gladly will.
Again, 2003 CAT C-15, 6NZ
CAT warranty still applies but they are at a loss as to what's happening, as is Peterbilt and another local shop.
PLEASE PLEASE HELP.
Text me at 905 746 5505
I assure you that I will be grateful !!