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LBZ Duramax, low rail pressure code/limp mode

lbhsbz

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Friend of mine and her mom and daughter (from Texas) is in Havasu right now with a 2007 LBZ that's throwing a low rail pressure code and going into limp mode on hills towing their travel trailer (28ft or so).

Truck has an Edge tuner, FASS filter/lift pump setup, and EGR delete that I know of.

They took it to the dealer but the dealer won't touch it without putting it back to stock first. Robinsons is telling them they're a week or more out before they can look at it. Anyone else in town that might be able to help out?

Leaving the trailer in Havasu in driving the truck down into Riverside/LA is another option....any recommended shops that I could relay to them? I'm not a diesel guy by any means...just trying to help a friend out.

Edit: just spoke with her again and the dealer was able to command 20K psi of rail pressure and got it...so that indicates the high pressure pump is probably OK as is the pressure control valve and probably injectors aren’t leaking bad enough to cause this...that leaves the in tank screen/strainer. I changed the filters on the FASS setup over the weekend. She mentioned the aux. tank is full of crud so that supports the clogged strainer theory.
 
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rivrrts429

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I had this same issue and turns out I didn’t tighten the main fuel filter housing enough.

It was the damndest thing because it would go into limp mode towing the trailer but I could drive the piss out of it empty and not have a problem. Nothing obvious when the dealer scanned it.

Young tech said he wanted to reset the filter housing and it worked like a charm.

Be sure they have a tight seal on the filter housing.
 

Gary Maschner

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Crud in the tank could be causing a restriction. Checking pressure output from the fass would be a good idea. It's a low pressure pump so you'll see about 8-10psi if all is well. If there's a restriction on the suction side it could also cause failure to the fass pump. There could still be other issues on the high pressure side too. Even though commanded and desired rail psi match when testing it could be a different story when towing and everything is hot. Excessive fuel could be returning to the tank from a weak relief valve, injection pump or injectors to name a few. Like mentioned above the fuel filter housing can cause this also, but with a fass installed you'd likely see fuel leaking from it. I work at Strictly Diesel in Phoenix and would love to help out but we're a few weeks out as well. If I can help in any way let me know.
Thanks
Gary Maschner
 

Racey

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Crud in the tank could be causing a restriction. Checking pressure output from the fass would be a good idea. It's a low pressure pump so you'll see about 8-10psi if all is well. If there's a restriction on the suction side it could also cause failure to the fass pump. There could still be other issues on the high pressure side too. Even though commanded and desired rail psi match when testing it could be a different story when towing and everything is hot. Excessive fuel could be returning to the tank from a weak relief valve, injection pump or injectors to name a few. Like mentioned above the fuel filter housing can cause this also, but with a fass installed you'd likely see fuel leaking from it. I work at Strictly Diesel in Phoenix and would love to help out but we're a few weeks out as well. If I can help in any way let me know.
Thanks
Gary Maschner
Another thing is make sure the fass pump is actually running and not dead causing a fuel restriction to the engine
 

lbhsbz

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Another thing is make sure the fass pump is actually running and not dead causing a fuel restriction to the engine
FASS pump is definitely running.

Thanks.
 

lbhsbz

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View attachment 1011207
He's local and one of the best, give him a call
Thanks...will relay the info.

To be clear, I don't really give a shit what's wrong with it....and since I'm 250 miles away I don't want to fix it. I was just looking for someone competent who might be able to check it out today or tomorrow so they can carry on with their travels.
 

monkeyswrench

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Just for other's knowledge, maybe not the case with this one, but I have had a handful (3 or 4) loose rail pressure on the scanner under load from bad primer bulbs. The injection pump would pull enough ahead of the FASS pumps, and suck air.
 

hawgty55

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Any idea of what the codes are? I have a couple codes im getting on my 08 cummins i just picked up. Hard for service techs to diagnose with qll the parameters different than stock. But mine is throwing low fuel rail pressure and it throws a relief valve code but no limp mode. Mine has to do with the pressure relief valve spring getting tired over time. Truck is tuned and deleted and has edge monitor.
20210608_144857.jpg
 

Gary Maschner

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Any idea of what the codes are? I have a couple codes im getting on my 08 cummins i just picked up. Hard for service techs to diagnose with qll the parameters different than stock. But mine is throwing low fuel rail pressure and it throws a relief valve code but no limp mode. Mine has to do with the pressure relief valve spring getting tired over time. Truck is tuned and deleted and has edge monitor. View attachment 1011418
Are you diagnosing and repairing yourself or taking to a shop for repair. The first thing I would check is fuel psi from the lift pump. If that checks normal at no less than 6psi there's a few possibilities. DM me if I can help further.
Gary
 

Dettom

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I had the same problem with my 2008 Duramax. Usually, when I was 1200 miles from home and 500 miles from a dealer. I dicked around with it for three years. Dealers looked at it, and even Daryl Robinsons crew messed with it. I never did figure it out. Daryl told me I could spend a lot of money tracking it down, or just go buy another truck. Next day, went out and bought a new F250. Problem solved. I just got tired of never knowing if I was going to make it to the next destination. It ruins vacations. Expensive fix, but worth it to me, when I’m in Bumfuck South Dakota.
I hear sometimes flushing the radiator fixes it too. Good luck.
 

lbhsbz

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Did they get it sorted out?
They got a dealer to change the in-tank pickup/strainer, and it’s been good, but there’s a new leak...if it sits for more than 30 minutes, they have to manually prime it to start it.
 

hawgty55

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Are you diagnosing and repairing yourself or taking to a shop for repair. The first thing I would check is fuel psi from the lift pump. If that checks normal at no less than 6psi there's a few possibilities. DM me if I can help further.
Gary
Thanks gary. Yes diagnosing myself codes all lead to the tired spring in the fule pressure relief valve?
 

LargeOrangeFont

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I had this same issue and turns out I didn’t tighten the main fuel filter housing enough.

It was the damndest thing because it would go into limp mode towing the trailer but I could drive the piss out of it empty and not have a problem. Nothing obvious when the dealer scanned it.

Young tech said he wanted to reset the filter housing and it worked like a charm.

Be sure they have a tight seal on the filter housing.
My friend had the EXACT same issue.
 

LargeOrangeFont

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They got a dealer to change the in-tank pickup/strainer, and it’s been good, but there’s a new leak...if it sits for more than 30 minutes, they have to manually prime it to start it.
Fuel filter housing deal Is leaking. Get the billet replacement one.
 

DaveH

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i had the same problem with my LMM. talked to lots of people including a friend of mine who was the head diesel tech at a dealer ship.

i have proven engine temp will cause this. most people only have this issue in the summer, towing....AKA extreme duty on the engine.

the trouble is the engine temp gauge is highly inaccurate. first thing I would do is pull the cooling stack out it (intercooler and radiator) and give them a very thorough cleaning. what appears to be a clean radiator to the eye isn't. all sorts of small debris get in there and reduce cooling efficiency, especially the AC condenser which is in the front and greatly reduces airflow to the radiator which is in the back.

using a lower gear and keeping engine RPM around 2k on grades keeps the water pump at a higher RPM circulating more coolant and helps greatly.
 
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