I wanted to write up something fairly simple in hopes that others do not encounter the frustrations that I experienced in getting back on the road. Quick summary below main writeup.
Recently I broke down on the side of an interstate due to air getting into the fuel. At first it seemed like a loose fuel filter but after checking tightness, it was not. The push button priming valve on top of the fuel filter assembly was leaking fuel when depressed, so it was an obvious o-ring failure that was causing air to infiltrate through the priming button.
As I was evaluating options, I called a GM dealership just one mile north. I gave them my VIN and explained the situation and that I was fairly confident that I knew it was a bad o-ring. They stated that they did not have the o-ring set in stock and that my only option was to buy the fuel filter assembly (part 12642623 for my 2008.5 LMM duramax). I told them that I would take it since it was still cheaper than a tow and repair. I picked up the part and came back to the truck on the shoulder of the interstate. I had a basic socket set and Torx screwdriver set with me but did not have specialized tools to successfully navigate that very tight area. I made the decision to keep the existing fuel filter and fuel filter assembly mounted to the engine block and to field replace the pump o-rings by taking the new filter assembly apart and swapping the o-rings. It was fairly simple to do and most importantly, it worked. I got off of the interstate and took the truck home.
Since I spent a decent amount of money on a fuel filter assembly direct from a dealership (still cost less than a tow & OEM non-dealership part), I was determined to change the entire part out in the safety of my own driveway. Due to the tight quarters, it was somewhat difficult to remove but I got it done. I then field stripped the new o-rings out of the old assembly and put them back in the new assembly. I put everything back together and installed the assembly onto the engine block, however I had damaged the original rubber fuel lines that connect the fuel filter assembly to the fuel rail. They were damaged during removal when I tried to remove them from the barbed fittings on the fuel filter assembly (they were already cracked and fairly brittle). Well this put me in a pinch and I ubered to the closest GM dealership to my house a few miles away after unsuccessfully trying to find the part numbers of the hoses.
We spent 20 minutes looking at schematics but nothing was clear on which hoses in the diagram connected to the fuel filter assembly or not (there were 4 hoses in the diagram). The fuel filter assembly was pictured in the diagram, but there were no connecting lines to show what hoses connect to the assembly. Without a LMM duramax in front of us, we had to call in the senior mechanic and we still guessed. Since I needed the parts that day, we took a stab in the dark and hoped that would be it. Luckily it was the correct hoses.
NOTE: these fuel hoses have tight bends, so generic fuel lines could work in a pinch, but they are not a permanent solution (I tried but they were kinking too much that I knew I would have a restriction if I used them).
The reason for the writeup - I spent at least 4 hours trying to figure out the correct part numbers for these hoses and I wanted to try to share the info so that I can save someone else time. If you ever replace your fuel filter assembly or need to replace these fuel lines, be prepared for tight quarters and make sure you have a good needle nose with a long handle. TLDR - The fuel hoses that connect directly to the Fuel filter assembly are not clearly defined in the 2008.5 LMM duramax diagram (TC03-769). I have included the part numbers below so you can be sure you are getting the right parts for your LMM.
Fuel Filter Assembly Part # 12642623
Fuel Feed hose 1 (from fuel tank line to fuel filter assembly) - Part #12625281 (listed as 350 in TC03-769 diagram)
Fuel Feed Hose 2 (from fuel filter assembly to fuel rail that leads to high pressure fuel pump) - Part #12625284 (listed as 351 in TC03-769 diagram)
I would also recommend if you have the original o-rings on your fuel filter at this point, buy a spare set to keep on hand or proactively replace them before they fail. I could not find OEM o-ring kit, so you may have to purchase aftermarket. Otherwise prepare to pay about $200 for a new fuel filter assembly. You can field replace them with a Torx screwdriver in about 20 minutes so you dont have to do what I had to do. There is a youtube video that breaks down how to do a complete o-ring replacement (this is not me, but I thought I would share) -