2003 cummins 170k miles. I'm pretty sure my rail pressure is lower than it should be all the time (8-10kpsi at 60mph). On top of that if I'm driving and let off the gas when I get back on the gas it stutters and the rail pressure drops to almost nothing (1-2kpsi). I'm guessing a cp3 issue? Any help greatly appreciated and also is there a way to diagnose before just guessing and throwing money at it? Thanks.

 

when was your last filter change? and have you checked the pressure after the lift pump?

 

It could be several things. Check to see if there are any codes before doing anything.

 

First check lift pump pressure, sounds like a supply issue. But an easy check to do is unplug the FCA on the cp3. This will pretty much dead head the cp3 and give you some different and most likely higher rail pressure numbers. But check codes first before you touch anything. This will give you a baseline because you may create codes by unplugging stuff.

 

My rp at 65-70 mph runs 7-10 and when i roll on the pedal it jumps up to 16-19 depending on conditions.so i don't think your pressure is out much, you need to check your lift pump pressure, seems like it is a supply issue. My lift pump pressure runs 8-9 psi steady and will drop to 7 or so under heavy acceleration i have a gauge mounted on my steering wheel.