so just recently my rp gauge is reading about 4200 at an idle and only 6000 cruising at 65 mph if i floor it it will jump to 20,000+. my rail is capped and the truck runs good.i am getting a little bit of a fuel knock while cruising too.i unplugged all the connections and cleaned and dielectric greased them with no change,also there are no dtc's.any ideas why its so low while cruising.thanks

 

I would check your PRV (Pressure Relief Valve) first to see if its leaking.

John

 

His rail is capped so it can't be the PRV.
CP3 acting up maybe?

 

FCA not working right,maybe sticking, small cross over tube leak or the injectors flowing a bit too much back.

John

 

i'd say the injectors are starting to let some flow back. does it seem to take a little longer to start

 

no it seems to start normally. what are some symptoms of a fca going bad. could it be the rail sensor? if so how would you check it.thanks