Truck Repair: Cummins N-14 Low Power, dodge cummins, cummins engine

Question
"This engine will (eventually) get a full load up to 78 mph. But it won't climb or accelerate at all. It is actually a bit dangerous in a merging situation on highway. It simply falls flat on it's face. It has a new wiring harness, new TPS (APPS?) and numerous fuel filters. It has been checked by a private mechanic several times. (Owner is too cheap to take it to Cummins dealership)

Owner is ready to give up on it and wants to sell it. Truck is in remarkable shape( because it is so slow!) and is a pleasure to drive ON THE OPEN HIGHWAY once it reaches 60 mph on flat ground. It has no boost gauge but the post turbo pyrometer will reach 900 degrees F max. I drive this lemon and was thinking about buying it, if I can fix the acceleration. I don't want a hot rod, I just want it to drive like a regular Cummins engine in a used truck. I'm funny about merging onto a highway at about 50 mph. I merge at 30 mph if it's downhill and I'm lucky.

 I think it's a fuel issue, and here's why. When it is warm and I shut it off, it will crank for 5 or 6 seconds before it will fire. If I push the skinny pedal just a bit, it will fire right off.
It idles fine, but it will only rev to 2100 rpm. (Governor?) Is the fuel line too small or what? I swear my '01 Dodge Cummins will make this thing cry like a girl. "  

Answer
Either something is derating it or there is a fuel restiction.Charge air cooler might be busted.I would get those things checked first.Derate could not log a fault if the sensor is reading in its specifications but not letting it fuel because of the input to the ECM,ie showing too much boost pressure will cut the fuel back to compensate.the easiest way is to check them with the engine not running & key on to see if any read unnormally.