I have a 2003 duramax lb7. I got fuel about a month ago and it had water in it. It started missing really bad and sounded like it was knocking. I changed the fuel filter and a week later it started doing it again So I changed the filter again put injector cleaner in it and water dry for diesel. It has been running fine until the other night I started doing it again. I came home drained took the plug out where you drain the water out. Its running fine now but what could be causing this?

 

you buying fuel at the same place everytime? If so maybe try a different shop?

 

I have bought it from 2 different stations since then.

 

checl the oil and see if it smells like diesel, does the idle fluctuate at all?
the knock noise your describing, its it like a loud tap that responds to the throttle?

 

It idles fine. It was cutting out and start knocking when I would drive down the high way at 55 or 60 mph.