• Welcome to The Truck Stop! We see you haven't REGISTERED yet.

    Your truck knowledge is missing!
    • Registration is FREE , all we need is your birthday and email. (We don't share ANY data with ANYONE)
    • We have tons of knowledge here for your diesel truck!
    • Post your own topics and reply to existing threads to help others out!
    • NO ADS! The site is fully functional and ad free!
    CLICK HERE TO REGISTER!

    Problems registering? Click here to contact us!

    Already registered, but need a PASSWORD RESET? CLICK HERE TO RESET YOUR PASSWORD!

2002 LB7 won't start. Low fuel rail pressure

Nick 6.5

Active Member
Messages
197
Reaction score
84
Location
California
Hey guys. I'm looking for any help or tips. I got an 02 lb7 that I have in my 96 crew cab. This was a running duramax/Allison combo. I physically drove the truck so I know it was. Im nearing the end of my swap/build (about 4 years later) and I cannot get it to fire. It will fire with just the smallest squirt of ether but will not stay running. The live data shows my fuel rail pressure is at 350 without cranking. It only gets up to about 600-700 and kicks over like it wants to fire then drops down to 350ish. From what I read it could be fpr or cp3. Any tips?? Thanks in advance!!
 

Nick 6.5

Active Member
Messages
197
Reaction score
84
Location
California
I forgot to add: when it does fire on ether the fuel rail pressure will spike. The highest it got was 1900 while "running" then drops down to about 300 when it dies. Is the fpr controlled by the Ecu? I'm wondering if my wiring has the fpr staying open if it is Ecu controlled. Or maybe just faulty? Again thanks in advance!!
 

THEFERMANATOR

FRANKENBURBAN
Staff member
Lead Moderator
Messages
9,811
Reaction score
3,826
Location
ZEPHYRHILLS FL
If your fpr was stuck open, that gives you max pressure. It sends power to it to close off, restrict fuel to the cp3, and that is how it lowers fuel pressure. You need at leadt 1500 psi for the injectors to open. Any less and there isn't enough differential pressure for them to open. 1st off make sure you have a GOOD supply of air free fuel to the cp3. 2nd is throw your starting fluid away, NEVER take it to your engine again, AND PRAY TO GOD you haven't SCREWED your engine up. YES, just a wiff of it can DESTROY your LB7. I would pull your glow plugs and make certain you haven't blown a tip off of a glow plug as even a cold glow plug can break when starting fluid ignites, and then you can say BYE BYE to your engine. 3rd, if you have good fuel to it, but can't build rail pressure for it to start, you need to do a fuel return test to see if you have an injector sticking, bad cp3, or some other issue.
 

Nick 6.5

Active Member
Messages
197
Reaction score
84
Location
California
If your fpr was stuck open, that gives you max pressure. It sends power to it to close off, restrict fuel to the cp3, and that is how it lowers fuel pressure. You need at leadt 1500 psi for the injectors to open. Any less and there isn't enough differential pressure for them to open. 1st off make sure you have a GOOD supply of air free fuel to the cp3. 2nd is throw your starting fluid away, NEVER take it to your engine again, AND PRAY TO GOD you haven't SCREWED your engine up. YES, just a wiff of it can DESTROY your LB7. I would pull your glow plugs and make certain you haven't blown a tip off of a glow plug as even a cold glow plug can break when starting fluid ignites, and then you can say BYE BYE to your engine. 3rd, if you have good fuel to it, but can't build rail pressure for it to start, you need to do a fuel return test to see if you have an injector sticking, bad cp3, or some other issue.
I knew staring fluid was bad but didn't think it was that lethal. I've read on other forums of people using it in very minor amounts just to get it to kick. But thank you. I will check my glow plugs. I do have a air dog 150 lift pump and a fresh factory fuel filter. The most pressure I had in the fuel rail was 3400 after I unplugged the fpr. Is there any possibility that this could be a wiring issue or Ecu problem from when I had it converted to stand alone? Or definitely a mechanical issue?
 

THEFERMANATOR

FRANKENBURBAN
Staff member
Lead Moderator
Messages
9,811
Reaction score
3,826
Location
ZEPHYRHILLS FL
If there was a wiring issue, the fpr would go full pressure as it wouldn't get power. With the fpr unplugged, does it start? Only let it idle, and not for but a few seconds, but it would rule out the electrical side as no power to the fca gives max pressure and gives the cp3 all the fuel it can take. Otherwise your next step is to isolate some fuel lines, and do a return test to see where you're losing your fuel.
 

Nick 6.5

Active Member
Messages
197
Reaction score
84
Location
California
If there was a wiring issue, the fpr would go full pressure as it wouldn't get power. With the fpr unplugged, does it start? Only let it idle, and not for but a few seconds, but it would rule out the electrical side as no power to the fca gives max pressure and gives the cp3 all the fuel it can take. Otherwise your next step is to isolate some fuel lines, and do a return test to see where you're losing your fuel.
Copy that. No with the fpr unplugged it will not start. It only gets up to 700 psi under its own cranking power. I will look into getting a fuel rate return test done. I have no idea how to run that test. One last question, is it common for fpr to get stuck in the almost closed position? Not allowing fuel the cp3? I'm only asking cause I'm trying to do the most I can do before I have to spend money for a mechanic to run tests.
 

THEFERMANATOR

FRANKENBURBAN
Staff member
Lead Moderator
Messages
9,811
Reaction score
3,826
Location
ZEPHYRHILLS FL
Not normally will a fpr stick closed. It has a pretty stiff spring in it to make it go open. You can het some pieces of hose and do a crude return rate test yourself to see what is going on. Turn your lift pump off, unplug your fpr, disconnect the 2 plugs at the ficm, take the 5/16" fuel line that comes out of the top of the cp3 and atatch a section of 5/16" hose to it, plug off the section of hose ypu removed that goes back to the return line, place the hose you put onto the cp3 return into a jar, next disconnect the fuel return line at the rear of the engine, slide a piece of hose over the return line, and put that piece of hose into a different jar. Turn the engine over until you have fuel coming out of one of the hoses. Empty the jars, then crank the engine over for 15 seconds. Now measure how much fuel is in the jars. The cp3 shouldn't return more than 20 milliliters, and the injectors shouldn't return more than 30 out the return. If you get more than 30 out of the return, the next step is to remove the fuel line from the fuel pressure popoff valve(should be on the drivers side, pointing inward, with a 5/16" fuel line atatched to it), with the fuel line off, crank the engine over for 15 seconds. Ideally you shouldn't see any fuel come out of the popoff, but it can return up to 20 ml according to GM specs(if it does though, you're pretty much guaranteed the popoff is bad, and will open under pressure).
 

Nick 6.5

Active Member
Messages
197
Reaction score
84
Location
California
Not normally will a fpr stick closed. It has a pretty stiff spring in it to make it go open. You can het some pieces of hose and do a crude return rate test yourself to see what is going on. Turn your lift pump off, unplug your fpr, disconnect the 2 plugs at the ficm, take the 5/16" fuel line that comes out of the top of the cp3 and atatch a section of 5/16" hose to it, plug off the section of hose ypu removed that goes back to the return line, place the hose you put onto the cp3 return into a jar, next disconnect the fuel return line at the rear of the engine, slide a piece of hose over the return line, and put that piece of hose into a different jar. Turn the engine over until you have fuel coming out of one of the hoses. Empty the jars, then crank the engine over for 15 seconds. Now measure how much fuel is in the jars. The cp3 shouldn't return more than 20 milliliters, and the injectors shouldn't return more than 30 out the return. If you get more than 30 out of the return, the next step is to remove the fuel line from the fuel pressure popoff valve(should be on the drivers side, pointing inward, with a 5/16" fuel line atatched to it), with the fuel line off, crank the engine over for 15 seconds. Ideally you shouldn't see any fuel come out of the popoff, but it can return up to 20 ml according to GM specs(if it does though, you're pretty much guaranteed the popoff is bad, and will open under pressure).
Ok copy that!! I'll run that hopfully this week. Thanks again I appreciate it!!
 

Nick 6.5

Active Member
Messages
197
Reaction score
84
Location
California
I got it fired! Faulty fuel pressure release valve. All the glow plugs checked out obviously. Thanks for your help ferm I really appreciate it!!
 

MetalMaineak

New Member
Messages
4
Reaction score
6
Location
Limestone Maine
Hello! I've got a similar prob on my 02 LB7. The LP side is air free, Disconnected FPR no change, Return rate checks out, installed elec lift pump, worked for awile then died when i let off the throttle once, will not restart. Cracked #1 inj nut and turned over w/hood closed (for obviose reasons) , but only get small amount of fuel, lots of air, and not a lot of pressure. Live Data has FRP @ 278 not cranking and about a 50# rise while cranking. FYI FPR was @3800# idling before lift pmp and 5600-6200 after. and sounded better then it died after revving it to 2500rpm and let off. Please help, prayin' it is not the CP3
 

THEFERMANATOR

FRANKENBURBAN
Staff member
Lead Moderator
Messages
9,811
Reaction score
3,826
Location
ZEPHYRHILLS FL
What do you mean by checked return rates? You have to pull the valve covers and install the beakers to check return rates. I would pull the return line from the CP3 and see how much fuel the pump is returning.
 

Rockabillyrat

Well-Known Member
Messages
132
Reaction score
272
Location
Ohio
They sell injector line cap offs. Usually I do one bank at a time. Cap off all 4 and crank it. Do that for both sides and see if it makes a difference in rail pressure. If not then it's the pump.
 

MetalMaineak

New Member
Messages
4
Reaction score
6
Location
Limestone Maine
Theferminator: No I didn't do the beaker thing, however I did what a guy did on a YT vid. I cut the return to the tank near the installed lift pump, then put a piece of hose on the eng. side of the line and placed it in a paint meas. cup , blocked the other side, used the lift pump make sure syst. was primed. Emptied cup and unplugged lift pmp, then turned engine over for 15 secs. the cup had 30ml in it. The guy said that you are allowed up to 100ml using this method any more is considered excessive.

Rockabillyrat: Thanks for the tip! Makes sense. I should be able to make a set by welding up old ones, right? Being the cheap SOB I am LOL!!
 

THEFERMANATOR

FRANKENBURBAN
Staff member
Lead Moderator
Messages
9,811
Reaction score
3,826
Location
ZEPHYRHILLS FL
15 seconds of cranking would be allowed 50 ml max for the entire system(30 ml for 8 injectors and 20ml for the cp3 return), but that is worst case scenario. Whenever I've seen a failed CP3, they always poured fuel out of the return line of the CP3 while cranking.
 
Top