Black fuel filter(pics included)

If you'd have done the fuel analysis, you'd have gotten the results already. No guesswork involved, and takes seconds.

Still betting on combustion chamber gasses. Seen it many times. Oil cannot get into your fuel through injector o rings or cups, and while it's possible the fuel pump could, never seen it in 27 years of Detroit work.

 

Looks like oil in your fuel. Check you fuel tanks to see if they are discolored.

 

No. It is almost always minute amounts of gas getting into an injector from one of the injector tips not sealing off completely.

Unlike a loose injector or bad injector o rings, where the amount of gasses affect driveability, the small amount getting into the fuel through a tip is rarely noticed until the fuel darkens.

Don't just throw 2k into a set of injectors though. Do the fuel analysis first to verify the reason it's dark.

 

I never heard that it can't happen to a 12.7. It can happen to any diesel engine through the return fuel line to the tanks. It looks like oil to me, its possible you got bad fuel somewhere. I recommend you get your fuel analysis done, before you proceed any further.

 

I had the black fuel and pieces of fuel lines. Replaced the lines from tank to GT return valve. They were probably 15 yrs.old.