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Hi everyone, I have a 2001 LB7 that I recently put a motor in. The reason I changed the motor is because the one in the truck the headgaskets let go and I got my hands on a motor with only 67,000 kilometers on it (I'm from Canada). The truck ran perfectly before and everything from the old motor accept for the cp3 was swapped over to the new motor. This is because the motor I got was bare aside from the cp3, injector, injector lines and the high pressure lines (all sensors on the lines and cp3 as well). The motor was sitting for quite a long time (around 2005) but it was sealed up and nothing would have gotten into it.

I ran new fuel lines from the tank to the motor along with a new fuel filter.

Once I had everything hooked up I primed the fuel system using the hand pump ontop of the filter then cranked then bled via the screw ontop of the filter and so on until the the truck fired. It fired up and I shut it off to look for any type of leak and then returned to fire it again. I primed the pump and cranked it and it fired up fine. It ran for only a short period of time and then slowly died. I then pumped it again and it fired it up but it wanted to die. I could keep it running by continuesly pumping the hand pump.

I then did some research and found that the filter head is common for sucking air and was hard to diagnose unless a lift pump made it leak. I had a lift pump from a duramax cube van handy so I wired it and temporarily plumbed it in under the hood just after the ficm and just before the filter head. I then fired the truck up but it was still hard starting and almost seamed like it had gotten worse. It inevitably stalled again and I have not been able to get it running or even to fire since. With the lift pump on it I see no leaks. I do hear fuel returning to the tank through the return( I am assuming thats normal).

Needless to say the truck will not attempt to fire anymore with or without the lift pump plumbed in. Again the only thing that I did not change from the original motor that was running fine aside from head gaskets is the cp3, injectors, injector lines, high pressure fuel lines and sensors on them. The reason I did not switch these things over is because the motor cp3 and injectors only had 67,000 kilometers on them. With a scanner I am only getting 1.4 mpa rail pressure.

Any help is appreciated.

Thank you.
 

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Did you replace the rubber fuel lines at the tank? They could possibly be sucking shut and stopping fuel flow. A lift pump will not cure that problem. Haven’t had it happen to me, but have read of numerous others with that problem.


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Did you replace the rubber fuel lines at the tank? They could possibly be sucking shut and stopping fuel flow. A lift pump will not cure that problem. Haven’t had it happen to me, but have read of numerous others with that problem.


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Yes I replaced them. Also I just tried bypassing the filter head and sucking directly from a jug of fuel. Fuel rail pressure went from 1.4 mpa to 1.9 mpa but no change aside from that.
 

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So after running the lift pump directly to the back of the injection pump bypassing all low pressure lines and also sucking fuel directly from a jug I still only got 1.9 mpa rail pressure while cranking. Is there anything besides a failed cp3 that could cause such low rail pressure. In my opinion it sounds like a failed cp3 is the probable cause as nothing that I have researched could cause it to have such low rail pressure. If anyone thinks there is something else that could cause such low rail pressure when bypassing all low pressure fuel lines and directly pumping fuel to the back of the injection pump with a lift pump fed by a jug of fuel please let me know. I would rather not go through the job of replacing the cp3 if someone thinks it could be something else.

Thank you.
 

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Fuel rail pressure relief valve?

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Fuel rail pressure relief valve?

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Took the valve apart and also took the one out of the old motor. Spring is different and the needle is different as well ( see pics). Swapped the pieces from the old motor over. Still have same problem. No rail pressure.
 

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As you have discovered, there is a minimum rail pressure which must be established before the FICM wakes up and begins to fire the injectors.

Another possibility for low rail pressure, that is not too hard to check, is high injector return rates due to failure of the ball/seat in the injector’s internal return system.

A check of total return rate at the line before it enters the tank will tell you if that is your problem, although you won’t know which bank or injector.

Search “return rate test” here or (I think) “Duramax Return Rate Test” on YouTube.

If you search here, enter my user name since I believe I posted that YouTube video at some point.
 

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Discussion Starter #9
As you have discovered, there is a minimum rail pressure which must be established before the FICM wakes up and begins to fire the injectors.

Another possibility for low rail pressure, that is not too hard to check, is high injector return rates due to failure of the ball/seat in the injector’s internal return system.


A check of total return rate at the line before it enters the tank will tell you if that is your problem, although you won’t know which bank or injector.

Search “return rate test” here or (I think) “Duramax Return Rate Test” on YouTube.

If you search here, enter my user name since I believe I posted that YouTube video at some point.
One issue I have thinking injectors is that I am showing next to no rail pressure. For example key on no crank I show 1.4mpa. Then if I crank the engine it shows 1.4mpa.. maybe flutters to 1.6 mpa. The only way I have ever seen 1.9 mpa is with a lift pump installed. I am assuming 1.4 mpa is the base setting for the sensor and cannot read lower as it is at that with key on not cranking. Also would all the injectors not have to completely fail all at once to get next to no rail pressure?

I see a return line comes off the cp3 as well. Is there a way fuel could be flowing in the cp3 and right out the return on it for some reason? When I hooked a lift pump up to the back of the injection pump directly from a jug of fuel it pumped it in and almost immediately returned to the fuel tank. It would pump straight through the system and out the return with the engine not even cranking.

Thank you.
 

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Anyone?

I’m testing the limits of my LB7 knowledge and encourage old timers to weigh in...please.

To be clear, the engine you now have in the truck has its original fuel system (CP3, lines, injectors, etc) and that engine sat since 2005.

To answer your question about failed return circuits in the injectors, the CP3 provides a high pressure, low volume supply of fuel and even one sketchy ball/seat in one injector would act like a hydraulic short circuit, and rail pressure would not rise to a level necessary for starting. A more likely scenario would be all the injectors may be somewhat comprised, with the same effect on rail pressure.

67,000 km is about 47,000 miles which would put your replacement engine well below the 70k +/- time to failure of typical LB7 injectors, but all the time your replacement engine was on the shelf is worrisome :frown2:. As you know, diesel fuel has a limited storage life and, just speculating here, but I wonder what evil old fuel could have been doing to the system over all those years?

You mention you can hear fuel returning to the tank.

The return test I suggested only amounts to a quite small amount (? Don’t recall the number but mm’s) during a 15second crank and it seems unlikely to me there would be enough flow to hear. Where do you hear it?

Seems to me a good test of the CP3 would be to find a gauge of appropriate pressure range and connect it to the CP3 output. Measuring pressure there would seem to eliminate downstream problems. The fact that when you put fuel directly to the CP3 and it immediately came out the return side may be what the police call “a clue”.

I’ll stop rambling now, and wish you the best.

Here’s an LB7 fuel system diagram and an internal view of an injector, showing the return circuit, that may be helpful:
 

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Discussion Starter #11
I’m testing the limits of my LB7 knowledge and encourage old timers to weigh in...please.

To be clear, the engine you now have in the truck has its original fuel system (CP3, lines, injectors, etc) and that engine sat since 2005.

To answer your question about failed return circuits in the injectors, the CP3 provides a high pressure, low volume supply of fuel and even one sketchy ball/seat in one injector would act like a hydraulic short circuit, and rail pressure would not rise to a level necessary for starting. A more likely scenario would be all the injectors may be somewhat comprised, with the same effect on rail pressure.

67,000 km is about 47,000 miles which would put your replacement engine well below the 70k +/- time to failure of typical LB7 injectors, but all the time your replacement engine was on the shelf is worrisome :frown2:. As you know, diesel fuel has a limited storage life and, just speculating here, but I wonder what evil old fuel could have been doing to the system over all those years?

You mention you can hear fuel returning to the tank.

The return test I suggested only amounts to a quite small amount (? Don’t recall the number but mm’s) during a 15second crank and it seems unlikely to me there would be enough flow to hear. Where do you hear it?

Seems to me a good test of the CP3 would be to find a gauge of appropriate pressure range and connect it to the CP3 output. Measuring pressure there would seem to eliminate downstream problems. The fact that when you put fuel directly to the CP3 and it immediately came out the return side may be what the police call “a clue”.

I’ll stop rambling now, and wish you the best.

Here’s an LB7 fuel system diagram and an internal view of an injector, showing the return circuit, that may be helpful:
Thank you for the information. I appreciate it. I will find the appropriate gauge and test the pressure coming from the CP3. I understand where you are coming from saying possibly all injectors are faulty but how is it not more likely that one thing failed immediately then 8 things simultaneously. Is there a check valve or relief valve that could be faulty in the CP3? I understand it may end up being injectors but I have a spare cp3 and it would be way easier to change then the injectors so I would like to rule everything out before jumping to injectors. Thank you for your help.
 

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Just to close off this post with a conclusion and fix. It was the injection pump that crapped out. I imagine just from sitting for so long. Runs excellent now with new injection pump.
 

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Thanks for the update.
 

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Just to close off this post with a conclusion and fix. It was the injection pump that crapped out. I imagine just from sitting for so long. Runs excellent now with new injection pump.
Glad that you got it figured out. Lots of great info on this forum.
 
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