The truck recently started having this problem and it has become worse and worse.

It takes some extra cranking to get it started some of the time, and is unable to go any faster than 50mph. It will jump up to full power for a couple seconds, then lose power and decelerate again. This happens every couple seconds, or even every second. So it's basically in breakdown mode whenever I'm on the highway and will not reach 3000rpms ever.

Throttle position sensor reading does maintain accuracy when the problems are going on.
No engine codes.
A mechanic hooked up his scanner and read the rail pressure. He said that it's losing rail pressure when you shut it off and that would be an indication of some component in the injection system leaking.
I replaced the return pressure relief valve with a Bosch unit and there was no change.
There was a temporary improvement when I added about 10 gallons of diesel to the tank, but the problem came back pretty quickly. I also added some diesel additive which made no difference.

What are my next diagnostic steps?
Has anyone else had this problem and solved it?

 

Have you had the fuel pump looked at?

 

It makes fuel come out of the fuel bowl, so I would say the lift pump is fine.

I can't check the high pressure side because my scanner only does very basic live data. I think I need a DRBIII or some kind of high end Snap-On scanner to figure this out.

 

Question becomes, is the lift pump supplying enough fuel for the engine under load.......

 

it is supplying far more fuel than the engine needs. My engine doesn't use a gallon per minute

 

Is that from testing? Or merely the pump spec? How old is the pump??

 

I have not pressure tested it. But the flow that comes out of the fuel bowl is very fast, which is why I estimated a gallon per minute. The fuel bowl fills up from empty in about 3 seconds. Also the symptoms do not match the previous 24v I worked on which did have a failed lift pump and was fully repaired after the lift pump was replaced.

Are you saying the first step in the diagnostic process is testing the low pressure side while driving?

 

I would think that is something you want to know..... It sure sounds like fuel starvation... unless it's really smoky? (which would imply excess fuel.)

 

It's not smoky. But really the problem cuts on and off so fast it can't be a steady state failure

 

Need to do some data logging and see what things look like when the problem presents itself. Once you FIND it, it'll probably be an easy fix.