08 LMM 0% fuel filter / power derate

2008 GMC 3500. 4x4 Sierra. Had the truck a month now every time I hook up to the horse trailer the engine light comes on power derates and it goes to 0% fuel filter life. Has new AC fuel filter and replaced both hoses on filter housing hoses by tranny and on top all look good and not linked. not running any tunes stock exhaust for now. It has a air dog 200 with new filters as well but I have it bypassed and still the same problem. It will also go to 0% filter life at 80MPH when passing with the truck empty but it won't derate unless the trailer is on. After a cool down it will clear and go back to running and clear itself. Stealer ships in area are clueless. Local diesel shops want me to leave it with them. I can fix it just gotta learn these junky systems. I'm really hating this truck. Can't wait till I get my 96 dodge 12 valve fixed from being in a wreck. Any help would be greatly appricated. :nail:

Ordering the shim kit for the relief valve and an adaptor so I can monitor fuel supply pressure at the filter housing. Or should I just get the race plug ? I'm looking to make this truck last a long time. I'm planning on doing a full EGR and DPF deleats open up the exhaust and go with EFI Live for tow hauling and better fuel mileage. Or go option B and put a Cummins in it !!!

Any chance you have the wrong fuel cap on it?

Nope. Still has the factory fuel cap with the teather cord on it.

Ok. So far I've shimmed the rail relief valve .060 inches ( 3 shims ).
Hooked the air dog 2 back up.
Still doing the loss of power due to fuel filter life at 0%.
Is there a strainer screen before the pump mounted on the back of the CP3 ?

No it's in the tank on the bottom of the sender.

Read the link Rob posted. Lots of good info he provided.

Looks like I'm pulling down the tank after I get home with the horses. A 2 hour trip has turned into 7 hours so far. I'm really missing my dodge

Ok. I pulled the bed pulled out the fuel sending unit from tank. Had no trash in it and all hoses and fittings are tight. Put all that back crap back on then pulled the filter head and replaced all the O-rings in it and put it back on. Test drove without the horse trailer and at WOT around 80 MPH it goes to 0% fuel filter life left and then derates power. If you shut down the truck for 5 minutes it will run with full power and after a bit the engine light will go off as well. Appricate all the help I can get.

Sounds like it's dropping rail psi. No dtcs stored? I have seen cp3s cause the same, and have seen injectors returning too much fuel cause the same symptoms.

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You Need to Perform a Bottle Test Before and After You shim the rail, to make sure the plug (or Valve) Seated correctly.

Pretty much the only things after that would be CP3 or Injectors which a shop needs to diagnose. Normally it wont show up on balance rates, so a return rate test will have to be performed. and in your case it might be an Enhanced return rate test