Low fuel pressure code

[aridgedell]

We have a long trip planned next week and I feel a little silly all of a sudden being concerned about the low fuel pressure code I've gotten a few times. When I look at the log data for the code it only ever comes on at idle and seems like after it has sat for a while. I haven't done any other tests other than making sure the pump runs. The fuel filter and water separator have been recently changed. I didn't realize the fuel pumps were an issue till I started researching this issue. Should I be concerned now or only if the code starts popping up while under higher load?

[ewo1]

Originally Posted by aridgedell

We have a long trip planned next week and I feel a little silly all of a sudden being concerned about the low fuel pressure code I've gotten a few times. When I look at the log data for the code it only ever comes on at idle and seems like after it has sat for a while. I haven't done any other tests other than making sure the pump runs. The fuel filter and water separator have been recently changed. I didn't realize the fuel pumps were an issue till I started researching this issue. Should I be concerned now or only if the code starts popping up while under higher load?

What engine do you have and what codes ( numbers please) is it throwing?
It would help in trying to give you an honest answer.


Without this info, I would say go for it and take care of problems down the road but….
I have found that on occasion, on a breakdown, you might actually come out of it with some good memories….make new friends…..do things that were not planned!

With more info maybe someone here might have the correct answer.

[aridgedell]

Originally Posted by ewo1

What engine do you have and what codes ( numbers please) is it throwing?

It would help in trying to give you an honest answer.





Without this info, I would say go for it and take care of problems down the road but….

I have found that on occasion, on a breakdown, you might actually come out of it with some good memories….make new friends…..do things that were not planned!



With more info maybe someone here might have the correct answer.

2003 5.9L 24v ISB. Code 0779

[Booyah45828]

Typically a 779 is usually a lift pump fault. If you have that code, the ecm has been updated at some point and a fuel pressure sensor installed.

The original design lift pump is inadequate and they're pretty unreliable.

Cummins and the aftermarket have done several updates and workarounds, but the moral of the story is loss of fuel pressure will lead to a broken injection pump, typically sooner then later. If you have a few thousand laying around to replace the injection pump when it finally goes out, then keep running it as is.

If it was mine, I'd install a manual gauge to read fuel pressure in the cab. Anything below 5 psi and you best start looking for faults. And then when you discover the weak lift pump is letting the pressure drop below 5 under load, I'd replace it with the updated airtex pump.

How long ago was the filter and water separator changed?

[PorchDog]

I am assuming that has a VP44 injection pump, and like Booyah said-- the thinking is that low boost pump pressure contributes to the VP44 failures. I would at least replace that lift/boost pump. I am pretty sure it is electrical. On a Dodge pickup-- it bolts to the side of the engine block on the drivers side. Around $100 maybe?

[aridgedell]

Originally Posted by Booyah45828

Typically a 779 is usually a lift pump fault. If you have that code, the ecm has been updated at some point and a fuel pressure sensor installed.

The original design lift pump is inadequate and they're pretty unreliable.

Cummins and the aftermarket have done several updates and workarounds, but the moral of the story is loss of fuel pressure will lead to a broken injection pump, typically sooner then later. If you have a few thousand laying around to replace the injection pump when it finally goes out, then keep running it as is.

If it was mine, I'd install a manual gauge to read fuel pressure in the cab. Anything below 5 psi and you best start looking for faults. And then when you discover the weak lift pump is letting the pressure drop below 5 under load, I'd replace it with the updated airtex pump.

How long ago was the filter and water separator changed?

Filter and seperator were changed about 500 to 1000 miles ago. I just put a guage on the outlet side of the filter and without it running it was about 14 psi. at high idle it stayed over 10 psi. I feel comfortable that the lift pump isn't totally shot at least. I bench tested switch and it seems to be working properly. I looked at the wires and connections for the lift pump and pressure switch and I don't see anything wrong there.
That does seem like a good idea to put a guage in the cab to monitor fuel pressure. Atleast that way if the light comes on I can verify if it really is loosing pressure then go form there.

[Booyah45828]

Originally Posted by aridgedell

Filter and seperator were changed about 500 to 1000 miles ago. I just put a guage on the outlet side of the filter and without it running it was about 14 psi. at high idle it stayed over 10 psi. I feel comfortable that the lift pump isn't totally shot at least. I bench tested switch and it seems to be working properly. I looked at the wires and connections for the lift pump and pressure switch and I don't see anything wrong there.
That does seem like a good idea to put a guage in the cab to monitor fuel pressure. Atleast that way if the light comes on I can verify if it really is loosing pressure then go form there.

Definitely get a gauge in it.

With the engine off, the fuel requirement is nothing, so even a weak pump will show pressure with the engine off. So long as it can pump more then the bleed orifice. Truthfully, even at an idle(high idle included) the fuel requirement is minimal. You'll read online a lot of people recommending 14+ psi, and that much isn't necessary. They came up with that number because that's the point the pressure relief valve opens in the banjo bolt on the side of the pump. I think I remember cummins stating the 5 psi(or was it 6?) number in a bulletin they published.

Regardless, you'll often times be in spec with the engine off or at idle, where as driving under load will put you below spec. So run your test while driving, particularly under full throttle, and see what it does.

[aridgedell]

Thanks for the advice! I picked up a delphi replacement lift pump to keep as a spare just in case. I also got a guage temporarily installed and will get it moved into the guage cluster at a later date.

[Booyah45828]

hfp923 or hfp953? I hope 953, as that's the airtex style upgrade, which is a better performer then the carter style.

[aridgedell]

Originally Posted by Booyah45828

hfp923 or hfp953? I hope 953, as that's the airtex style upgrade, which is a better performer then the carter style.

I did get the 923, I couldn't find the airtex style locally in stock. From the pictures it looks like I would have to do some modifying to the existing hoses to make it work. My outlet line is a hard line banjo style. I feel I could swap the direct replacement in the parking lot if I had to, I'm not sure if I could do the upgrade one without a few more tools. If I don't wind up needing it I think I'll return it to get the upgraded pump like you say. Do you know what you have to do for the upgraded one to work? I see some people recommend mounting the lift pump away from the engine too.

[Booyah45828]

A few modifications, nothing really complicated or requiring extensive work to do. The upgrade comes with hardware to retrofit to the current system IIRC.

You can mount the pump rearwards toward the tank. IIRC bluebird made a harness to do such a thing. You could also mount an air dog pump system, raptor pump system, etc. like the dodge boys do. I've never installed one of those systems on a bus though. After you install a monitor system (pressure switch and light, or a gauge) to show your fuel pressure, you'll see the carter pump is often inadequate. The airtex pump isn't as good as those other systems, but it's not as expensive either, and it's better then the carter units and works with a stock system with minimal modifications.

[aridgedell]

Well we started our trip today, made it 20 miles and the lift pump crapped out. Thank goodness I did bring the spare pump with us and we got it changed out in the walmart parking lot. Everything has been working good since then.