LB7: LB7 Advise before I burn this thing!

custom8726

Active member
Feb 25, 2008
2,784
0
36
Upstate N.Y
I Would appreciate some advice on were to go next if possible.
Long story short, (Kind of) :eek: I bought a 03 Kodiak (Duramax/allison) from a State auction about a year ago, bought it site on seen with no history given.
It had dead batteries and the large ground cable that goes from the block to the frame was broken off. I replaced those items, hit the key and she fired right up, motor sounded perfectly fine, good oil pressure, no smoke, etc etc. I then decided I was going convert it to a dump truck and keep it for my business. so for the next 6 months or so I started it here and there pulling it in and out of the shop to pull the old utility bed off and install a dump body, paint the frame, etc. It then sat for another 6 months or so simply because I was to busy to finish it up and I didn't really need the truck at the time. So just recently I decided to finish this project up, It started right up, drove it in the shop, worked on it all day, fired it back up to move it outside for the night, next day it will not fire. It acted like it was not getting fuel and no matter how much I pumped the primer it never got super hard and wouldn't fire. I had a fuel pump sitting on the shelf off a 6.0, I figured what the hell, Ill throw it inline quick since being it was now a dump body the fuel line was easily accessible. Well it fired right up but ran ruff do to there being way to much rail pressure. I pulled it back in the shop and tried to track down a leak somewhere or a area it was sucking air but could not find any issues. I then changed the fuel filter, no change, I then installed a new filter head and replaced all the rubber fuel lines as some of them were dry rotted, still no start unless I used the pump. I overzealously assumed the worst and figured its a LB7 it must have a injector sticking or something injector related not allowing it to start. So Today I finished installing all new Injectors and it is doing the exact same thing, just cranks, won't pop or anything and acts like its not getting enough fuel to fire. I had another FICM sitting in the shop so I exchanged that and still nothing. So my question now is, what am I missing? I realize now I should have logged it originally and that will be my first step tomorrow when I get back to the shop with my laptop and EFI. What exactly should I log? what is the rail pressure suppose to be when its cranking over? Could the CP3 have simply died over night? Thanks in advance for taking the time to read this long winded post and I appreciate any and all advice to help me figure this thing out
:hug:
 

IOWA LLY

Yes, its really me
Feb 23, 2007
2,275
4
0
I have seen regulators all the sudden just up and quit. That would explain why it will run with a lift pump but not without.

Definitely start by logging rail pressure while cranking. Should be in the 35MPA range depending on temperature.

Also, you should still be able to get the hand pump to get hard. Might be best to throw a new filter on there to be safe. But word of advice, buy the newer one for an LLY-LMM truck. There are around $130 and will fit the older trucks as long as you re-route the fuel lines. GM wants a ridiculous amount for the lb7 specific filter head.



Sent from my SM-N920V using Tapatalk
 

c20elephant

C20ELEPHANT
Apr 25, 2013
2,065
0
0
Phoenix, Arizona
I Would appreciate some advice on were to go next if possible.
Long story short, (Kind of) :eek: I bought a 03 Kodiak (Duramax/allison) from a State auction about a year ago, bought it site on seen with no history given.

It had dead batteries and the large ground cable that goes from the block to the frame was broken off. I replaced those items, hit the key and she fired right up, motor sounded perfectly fine, good oil pressure, no smoke, etc etc.

I then decided I was going convert it to a dump truck and keep it for my business. so for the next 6 months or so I started it here and there pulling it in and out of the shop to pull the old utility bed off and install a dump body, paint the frame, etc. It then sat for another 6 months or so simply because I was to busy to finish it up and I didn't really need the truck at the time.

So just recently I decided to finish this project up, It started right up, drove it in the shop, worked on it all day, fired it back up to move it outside for the night, next day it will not fire. It acted like it was not getting fuel and no matter how much I pumped the primer it never got super hard and wouldn't fire. I had a fuel pump sitting on the shelf off a 6.0, I figured what the hell, Ill throw it inline quick since being it was now a dump body the fuel line was easily accessible.

Well it fired right up but ran ruff do to there being way to much rail pressure. I pulled it back in the shop and tried to track down a leak somewhere or a area it was sucking air but could not find any issues. I then changed the fuel filter, no change, I then installed a new filter head and replaced all the rubber fuel lines as some of them were dry rotted, still no start unless I used the pump.

I overzealously assumed the worst and figured its a LB7 it must have a injector sticking or something injector related not allowing it to start. So Today I finished installing all new Injectors and it is doing the exact same thing, just cranks, won't pop or anything and acts like its not getting enough fuel to fire.


I had another FICM sitting in the shop so I exchanged that and still nothing. So my question now is, what am I missing? I realize now I should have logged it originally and that will be my first step tomorrow when I get back to the shop with my laptop and EFI.


What exactly should I log? what is the rail pressure suppose to be when its cranking over? Could the CP3 have simply died over night? Thanks in advance for taking the time to read this long winded post and I appreciate any and all advice to help me figure this thing out
:hug:

Much easier to understand...:thumb:
 

custom8726

Active member
Feb 25, 2008
2,784
0
36
Upstate N.Y
I have seen regulators all the sudden just up and quit. That would explain why it will run with a lift pump but not without.

Definitely start by logging rail pressure while cranking. Should be in the 35MPA range depending on temperature.

Also, you should still be able to get the hand pump to get hard. Might be best to throw a new filter on there to be safe. But word of advice, buy the newer one for an LLY-LMM truck. There are around $130 and will fit the older trucks as long as you re-route the fuel lines. GM wants a ridiculous amount for the lb7 specific filter head.

Thank you, I will look into the regulater :thumb:

Sent from my SM-N920V using Tapatalk

Much easier to understand...:thumb:

man,no kidding,took me 5mins to read.
I'd be going after the regulator,

Sorry for the run on sentences guys ^^ I was rushing to get a post in quick
before I had to run out. Thanks for the advice thus far I appreciate it.
 

custom8726

Active member
Feb 25, 2008
2,784
0
36
Upstate N.Y
Update.

Changed out filter head (Again) no change

New fuel filter (Again) no change

Thought maybe the FPRV could be stuck open from running such a high psi Lift pump, Dug that out and switched it out with a spare I had, No Change!

Log showed 13830 rail pressure while attempting to crank.

Pulled filter head and blocked off the out put side with a bolt and clamp inline to verify filter head was good, pump got rock solid and held.

Re-installed fuel line and the pump (Filter head) built up but would not get rock solid.

Changed Regulator, No Change.

Decided to yank the CP3 out and exchange it for a known good one I recently pulled from my race truck..

Problem solved :) only took 24hrs of wrenching, exchanging nearly everything fuel related in the entire system and a good amount of swearing lol..

I did gain some (LB7) knowledge and also realize I need to learn some more about data logging these things, that would have saved some time for sure on diagnosing it. That and if 3 of the damn glow plugs didn't snap off when I was trying to check for injector issues would have saved a significant amount of time.
:banging:

Live & Learn :D
 

custom8726

Active member
Feb 25, 2008
2,784
0
36
Upstate N.Y
H-O-L-Y-S-H-I-T!! Lol! Glad you got it running. Like you said, you learned a heck of a lot tracking down the problem lol!

Yup ^ I'm being optimistic at this point, lots of should of could of would of's but at the end of the day whats done is done ;)

I was just about to suggest swapping pumps too. :D. Good thing you got a boneyard.

The funny thing is I have been giving away LB7 parts to anyone I know that needed them for the last couple years thinking I would never need them and they were just taking up space. Then I bought this kodiak and picked up a super clean Reg cab (04) a few months back. Luckily I still had a decent amount lb7 parts kicking around and have gathered a bunch more pulling the motor and most of the stock parts on the regular cab recently :thumb:

The one thing that sucks more then all the time I waisted tracking down a bad CP3 is that I pulled out a perfectly good set of injectors for no reason:( I had a feeling when I pulled them out and noticed all the injector lines were marked (Previously removed) and the injectors looked brand new with no carbon, even the copper washers were still shiny and looked new. I think it was having issues thats why the state sent it to auction with only 90k miles when they A. couldn't figure it out or B. decided they didn't want to dump anymore money into it:confused: I also am really surprised the CP3 went bad with under a 100k on it Hence why I waited until the end to change it. After it was out I knew right away when I spun the gear and there was hardly any/no resistance on the pump. Anyways lesson learned, Just figured I would document it and hopefully this thread may help some one else in the future :beer