I have a 2004 GMC sierra 6.6 diesel LLY motor in it and the problem I’m having with it right now is when I hit the gas paddle the truck doesn’t wanna move then I put it in neutral hit the peddle again then it revs up then I put in back it drive then it works fine for a bit then it does it again does anybody know what it could be? It used to do that before then I would change the fuel filter and then it wouldn’t do it anymore more but now it’s doing it all the time and I changed the filter last week

 

When its in gear can you feel the tranny engage and can you hit rev limit? It may be time to look at fuel lines and possibly failing injection pump....(assuming it has one) I have not worked on one before....how about air flow?

 

Fuel pressure test.