HELP! low rail pressure, new cp3 no start

J

joshpete

New member
Jul 8, 2012
8
0
0
i bought a 2001 duramax. 260000 miles from a guy. the only way itll start is if you give it a shot of either an then it fires right up and runs great. anyways i know the problem is because when crankin it over the rail pressure is only at 600-700 psi an i need 1500. i didnt know if it was a problem with lines goin to the cp3 so what i did was hooked a line directly to the cp3 and filled it with fuel while holdin it straight up an down. still no start i then took an air compressor and pushed air into the line while crankin it over. this made it start.

i then ordered a new fuel filter housing an replaced all rubber lines and it started for like 10 times without either. (i did have to crank it over a little exta long it seemed like tho) anyways, like after the tenth time of starting it it went back to not starting unless i use either.

i then decided maybe its the cp3. somethin is prolly wrong with it an maybe its having trouble sucking fuel. i replaced it with a new one and finally got it all back together and primed the lines and it fired up. i shut it off and it started back up again. it did seem to crank a lil longer than normal tho. i then drove it around and bout the tenth time i went to start it, it wouldnt start! the fuel rail pressure wont get past 700 psi. i need help!! im using an Edge to monitor the fuel pressure
 
H

Hot COCOAL

May the farce be with you
Jun 9, 2012
4,433
0
0
Ok ok . So im on a frprv(fuel rail pressure relief valve) kick
Only cuz i just helped a guy that owns an lly with a similar problem, just before the 4th
his truck had been experiencing rail pressure codes tho, but it became increasingly more difficult to start, to the point where it would only start "after" it sat for a bit, like hours.
i explained to this guy how the fprv worked, skeptical, this guy let me work on his truck in my garage, i put a shim and ported rail fitting on his truck, he was running an edge programmer, i thought an efi live program would be better, gave him robs name, forum address.
Greg's truck started right up, he has my number, hasn't called back to say anything is wrong, so ill take that as a job well done, he should be coming home tonight so I'll try to get feedback as to weather or not we fixed his problem
his only gauges were on his edge so i never knew what rail pressure he had
All that back story to say, it wont hurt to, and is inexpensive to install the frprv shim
idk if it will even help at all, some may yell and say its a dumb idea, but ya never know, weird chit happens.
 
Subman

Subman

Old Geezer
Jun 27, 2008
3,233
10
38
80
Madras, OR, Pahrump NV
Hot COCOAL said:
X100 ether on a dmax or any modern diesel for that matter is a huge no no
Click to expand...

Totally agree, you can get a run away and scatter your engine in a heartbeat. Try unplugging the Fuel Pressure regulator on the back of the CP it should max out the rail pressure when you start it and make a funny quacking sound. If it does then you know your cp is good. I can't see the fuel pressure relief valve causing your problem, that usually only happens when you are commanding high rail pressure. You could clamp off the rubber fuel return line coming from the relief and see if it starts easier just to eliminate that as well. I have never had any issues doing this, infact raced White Out that way one day when my pressure relief valve was leaking and I couldn't hold rail pressure at WOT.

Could be injectors a balance rate test is a good thing to start with. Any white smoke at idle when you get it running? If so then could be injectors or (hate to say it) bent rods. Bent rods cause lower compression and can make for a hard start. Just a few ideas.
 
Stingpuller

Stingpuller

The Pusher Man
Jan 11, 2007
2,019
35
48
57
central Ohio
No start

You need injectors. When the fuel is cool it will make enough pressure to start. When the fuel gets hot it thins out and will not make pressure. Ramping up the pressure at idle will not tell you everything because there is little fuel demand and the pump is turning fast enough (idle rpm) to make pressure. Without doing a return test (very hard on lb7) it's hard to tell for sure.
 
LWATSON

LWATSON

future trans limpers
Jul 30, 2008
2,587
1
36
54
Scotland Neck NC
Hot COCOAL said:
Ok ok . So im on a frprv(fuel rail pressure relief valve) kick
Only cuz i just helped a guy that owns an lly with a similar problem, just before the 4th
his truck had been experiencing rail pressure codes tho, but it became increasingly more difficult to start, to the point where it would only start "after" it sat for a bit, like hours.
i explained to this guy how the fprv worked, skeptical, this guy let me work on his truck in my garage, i put a shim and ported rail fitting on his truck, he was running an edge programmer, i thought an efi live program would be better, gave him robs name, forum address.
Greg's truck started right up, he has my number, hasn't called back to say anything is wrong, so ill take that as a job well done, he should be coming home tonight so I'll try to get feedback as to weather or not we fixed his problem
his only gauges were on his edge so i never knew what rail pressure he had
All that back story to say, it wont hurt to, and is inexpensive to install the frprv shim
idk if it will even help at all, some may yell and say its a dumb idea, but ya never know, weird chit happens.
Click to expand...
CoCo Puff you need to get off the fprv kick! LLY's 05 and down did seem to need them more but the 06+ LLY, LBZ, and LMM trucks dont have the problem as often. I have over 180k miles on my truck and 170k of them are over 500 HP. I still havn't installed my shim kit, it is in my glove box and I will not risk damaging my fuel rail until I know I need the shim kit. Good advise to new members would be to do a bottle test to see if they need the shim kit instead of just throwing one in.
 
H

Hot COCOAL

May the farce be with you
Jun 9, 2012
4,433
0
0
My valve popped around 128k
i had only been running a tune for 4k miles
at the time i had no idea my tune could cause the frprv to open
one time i commanded full fueling @ w.o.t in my efi race tune and it wasnt pretty, the unexpected loss in power put me in a dangerous position
since then i have been sorta an advocate to anyone running a tune, especially if a tune has been leaning on the frprv spring for a while.
Sorry, but i dont know how, or that you could damage your fuel rail by installing/running the shim.
Making a pass on some one and popping your frprv can have bad consequences too tho.
I apologise for passing along potentially bad advise, it is just an opinion after all. I like to try and take preventive measures, not wait for things to happen.
I build custom homes, when i have to wear a safety harness on a roof, i like to make sure that i have less slack in my rope than height off the ground, i have actually seen a guy fall off a roof and hit the ground while tied in. He looked pretty dumb.
 
J

joshpete

New member
Jul 8, 2012
8
0
0
ya ima stop using the either.. anyways the edge shows that the balance rates are all good.

i unhooked fuel pressure regulator an then squirted a lil wd40 in to get it started (not sure if thats a good idea either?) it had bout 26000 psi. if i shut it off i could watch the rail pressure drop and if i tried starting it before it got to like below 10000 psi it would start up with no help otherwise it would need help.

i unhooked the fuel return line at that quick coupler on the drivers side and if i pump up that hand primer on the fuel filter it makes fuel slowly come out that line. is this normal? or is there a valve thats suppose to hold the pressurized fuel in?

dont think theres any fuel in oil. once in a great while i may see a lil white smoke coming from the exhaust if i let it set and idle for awhile but its barely any at all. if i run the truck hard or for awhile i smell like burning oil on the exhaust but i dont know where its comin from. could be from the tranny or motor i spect or possibly even diesel fuel?

it looks like theres two different fuel return lines on these motors? one goes to that quick disconnect but the other goes like back behind the motor but i have no idea what its for.

if it is an injector is their anyway to tell which one it is? im gettin fed up with this truck. sorry bout all these questions guys.
 
redws6rocket

redws6rocket

Member
Apr 22, 2007
406
0
16
Odessa, MO
The best way is to use the tubes and lines that come in the Kent Moore kit, one screws in to one head and the other line in the other head. So you have one hose screwed into to each where the fuel returns out. You will crank the motor over for 15 sec. and they should be the exact, the one that is higher is your problem bank. This is a hard test to set up, you pretty much have to take the engine harness loose to get to the fuel lines. You have got an injector returning to much fuel bud, lb7s are known for it.
 
J

joshpete

New member
Jul 8, 2012
8
0
0
im starting to figure out how to test the return flow now.

do you guys know where i could buy a kent moore kit?

also would it be possible to cap off like the fuel rail on the passenger side of the truck and see if it builds pressure? then i would know which side its on?
 
J

joshpete

New member
Jul 8, 2012
8
0
0
well i narraowed it down to injectors. because i took that banjo bolt off that screw into the fprv and then i cranked the motor over. all the return fuel comin was from the line coming from the injectors. none came out of the fprv.

so now i needa figure out which injectors are bad. i would like to just replacce bad ones. ill prolly try do some tests when i get time here.

i got this link here from a guy that ill prolly use
http://www.gmtruckhq.com/diesel-tech/lb7-fuel-system-diagnosis-high-pressure-side-3761.html
 
J

joshpete

New member
Jul 8, 2012
8
0
0
well i did a quick test today. i unhooked the return line at that quick disconnect and hooked a line up to it and cranked motor over till line was full then stuck it in a container and cranked motor over for 15 seconds. i measured the fluid in the container and there was a little over 70 CCs. i did it a couple times to make sure.

with that said, it looks like its definetly injectors. im thinkin bout not even tryin to figure out which ones are bad and just replacing them all. Is this a good idea? i would really like to put some like 50hp injectors in it.. would like some input on this. is it worth the money for the bigger ones? im gonna be using the truck for pullin once in a while but mostly on long trips and just driving it. heres a couple links to some injectors i found. one is just stock and the others are bigger but do cost more.

http://www.ebay.com/itm/200788924502?ssPageName=STRK:MEWAX:IT&_trksid=p3984.m1423.l2649

http://www.ebay.com/itm/Chevy-Gmc-D...Parts_Accessories&hash=item1c283761a6&vxp=mtr
 
D

dher

New member
Sep 13, 2015
8
0
0
I know this is an old post but im having the same exact problem with my lb7 did new injectors end up fixing your truck
 
You must log in or register to reply here.
Top Bottom