Good morning, I'm Steve. Been around a while but just registered on here.
Truck is an Early 05' (Nov 04' build date) with new lift pump, lines, cp3 and injectors. Rail pressure and lift pressure have been stable and exactly where I'd expect them to be (lift 14psi stable, rail 7k at idle) then yesterday under light acceleration from a stop I got rail pressure fluctuating several thousand psi along with the bucking & surging to go with it. Pulled off shut it down to check rail sensor and FCA connections and they appear good. Fired up and ran fine for a bit then more rail fluctuation. No codes, bat and alt both test fine, fuel is clean and treated, no hard start issues and grounds are all solid. At this point I'm waiting on a new rail pressure sensor and relief valve from DPP but I'd like to get some perspective on the issue from some like minded folks. Thanks

 

What about that solenoid on the CP3? Mine did that bucking bronco act a few years back and it was the crankshaft sensor. I replaced both cam and crank with Cummins genuine sensors. But your case might be different. Have the injectors been replaced?