MY buddy owns a stock 04 cummins 6-speed manual and its running rough. When he starts it in the mornings, it'll idle about at 450 or 500 rpms until it jumps to high idle at 1000. Then driving, it seems like it doesn't have all the power it used to, kinda lugs. when he slows down for intersections it will drop down to 450 to 500 rpms like its going to die, it has died a couple of times, so I'll step on it, rev it up and the rpms will then bounce between 500 and 900 and after while it'll even out back to 750. I changed my fuel filter about 4 days ago. I haven't check my fuel pressure yet. And I don't receive any codes on the dash either. Any suggestions? Bad injector, lift pump?
I just dont know I'm not a CR guy yet

 

It could be a lift pump headed south. No fuel pressure gauge it is hard to tell. Has this trucks LP been upgraded to the in tank unit? Also, read this tread. Intermittent short may cause issues.

https://www.dieseltruckresource.com/...ad.php?t=81936

 

no mods what so ever 100% stock 70,000miles

 

I would think the LP would cause that to be kinda crapy across the board and maybe even worse at higher RPMs.What about a crank trigger problem?

 

That is what above suggested reading is about. Has to do with shorted 5 volts on the camshaft position sensor. The possible short is on the High/Low pressure A/C switch on the high pressure line.

 

Changed FCA and no help what so ever....on to the next thing