common rail loping idle, not what you would think...
 

alright guys, im new here so take it easy on me. i have an 03 common rail, 236k miles. truck has a rythmic loping idle. it will idle normal and then drop down to about 450 and then back to normal and so forth, all of this happens as quick as you can snap your fingers, every once in a while it dies. drive down the road fine, sometimes will die when you come to a stop but fires right back up and keeps loping away.

all data looks good on scan tool, no codes of course. injectors have been disabled individually with scanner. had a buddies truck and tried his FCA, fuel rail pressure sensor and what im calling the pressure relief valve on the rail. same results. loping. also tried his APPS to no avail. thought i found the problem when i found the crank sensor had been half chewed off, installed new one with same results. ive tried wiggling and twisting harness, does no good. truck has a new in tank pump, and the filter is spotless. im about to pull out what little hair i have left. has anyone out there seen this kind of problem? or fixed something like this? and i have tried reading through old posts looking for similar problems, but havent seen anthing like this

check your fan clutch wires, all grounds and battery connections.

Sounds similar to the pro lemme I had with my 03. It was the fuel pump regulator. Ran fine some of the time and then it would do the same thing as yours other times. After the regulator was replaced not a single time has the problem returned

I've heard of bad cam and crank sensors causing this problem in gassers but not diesels. if nothing else anyone suggests works possibly try the fca I know it can mess with your rail pressure a lot when they go bad but never heard of one causing the symptoms your having but its still an affordable part before you go to looking at big stuff.