Cummins ISX air in fuel return line

Hello, 06 ISX has a noise and vibration under a hard pull with some smoke. It also only gets 30 pounds of boost when it acts up like this. It will normally go up to 38. I hooked a clear 5/8 line to the fuel return and ran it into a bucket. At idle (800 rpm) the line is half air and half fuel - foamy mix. I pinched the line and the foam clears up but there is still air bubbles in the fuel. I could also feel a regular pulse in the line. I unhooked the supply line between the primary filter and the engine and ran it out of a bucket. Same amount of air in the return line. Also fuel returns quickly to the tank after engine shutdown. When I unhooked both lines I didn't get one drop of fuel.

So, if it's an injector is there any way to isolate which one? Could it be sucking air at the primer pump or secondary filter base. I pressurized the system and did not find any leaks.
Thank You

 

Thanks I tried that and still have the same amount of air in fuel.

 

not yet. I also noticed the fuel was getting dark. So I changed the fuel pump because I was told the seals can go bad and let oil into the fuel. So far most other people I talk to don't seem worried about the air in the return line. I would hate to change all the injectors and that not be the problem. I am running a few tanks of fuel through to see if that clears up the oil. Or is it getting dark from exhaust??? Very frustrating.

 

There are no fault codes. I get some dark smoke when I first grab a gear, does not appear excessive. And then a little dark smoke when its pulling hard. No coolant or oil loss. No air leak in Turbo control line. New Turbo and controller. New egr cooler.1 year old egr valve. New egr temp sensor. New exhaust manifold pressure sensor. Manifold pressure and egr pressure tubes are clear. I paid Cummins $600 they could not find the source of the vibration.

 

I took a fuel sample today and the fuel was much clearer. So it was the fuel pump leaking oil into the fuel. I'm going to run a couple bottles of Lucas through to clean things up.