2 weak cylinders?

It does sound like a possible injector issue. Have you tried shooting the exhaust manifold at each port with a temp gun when its acting up?

 

Have the injector harnesses ever been changed? I have seen many that would not set any faults, but give intermittent issues. We have 7 NXS's in our fleet and every one got new harnesses. Some as early as 500K others when over 800K, but they all got changed for similar issues. Some were bad enough to set injector or actuator faults, but most did not.

You cannot not find the issue with an ohm meter unless the wires are completely broken (at that point it would set a fault anyway). The only sure fire I found to pin point a bad harness is load testing each wire. I unplug both harnesses totally (disconnect at the injectors, intake actuators, jake solenoids and unplug them from the main harness). The I use a jumper to power each pin and use load pro test leads to look for a voltage drop at the other end of that wire. If you don't have test leads that load the circuit, you can use a light bulb. The 4" round incandescent brake lights work well and are cheap. Just wire in the light and measure voltage drop with the light on.

Also while you're in there, check the overhead adjustment. It could be injectors, but it's not as common as harness issues in those engines.

 

Please allow me to throw this out there in response to you guys talking about your truck running better when the tank is full. It's not out of the question to have a pinhole in the standpipe in your tank. It could be sucking a little air when the fuel level gets below the hole.

Just some food for thought.

 

Mine has the pick up from the bottom of the tank. My Kenworth had the tube your referring to and I bypass that.

 

Is the injector with the trim code in question one of the injectors that ET is telling you is questionable?

 

Even if it pulls off the bottom the vacuum required to pull the fuel increases at lower fuel levels. Could be sucking air in a line on the suction side.