my 01 did the same thing and it was the cp3

 

Did it do the same exact thing as whats happening to me? would it still run fine once you got it started?? just tryin to make sure before i replace a major part i dont need to

 

I drove the truck to work one day and then tried to drive it out for lunch. As soon as I started the truck, it immediately cut off and would not start at all. After a lot of work, I found out the truck does have a minimum crank speed however I do not remember what it is - whether it is 600 or 800 rpm with a minimum 5000 psi of fuel pressure to fire the injectors. The tech 2 said I had 600 psi. There is a fuel return line coming out of the front of the passenger side head and the rear of the drivers side head. I took those off and no fuel came out. I checked and I had fuel at the filter and to the cp3. I replaced the cp3 and the truck started. It is still going strong (the truck is all stock).

 

alright ima try get time tomorrow and check and see if i have fuel at those lines. i know i have fuel up to the cp3. and there is a valve that looks like a air conditioning type fitting on a fuel line that ive used to get air out of the line. theres no air in the line im pretty sure and plenty of fuel upto the cp3

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a read a couple places about doin a return fuel rate test. they said if i you unhook the reurn line that is on the drivers side by the head and hook a rubber line up to it an then start the motor so that the line is full of fuel then shut it off and unhook a relay then crank the motor over for 15 seconds and collect the fuel that comes out and it should be like 40cc? does this sound accurate?

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well i took that return line off where theres a quick connect fitting and then cranked the motor over and watched it. there was a little fuel comin out while cranking it over, but not a whole lot and my fuel rail pressure stayed at 600 psi. but what really dont seem right is that when i pumped up my hand primer on the filter, it made fuel come out of the return line that i had unhooked. does this seem right. im guessing this is what causes my hand primer to become soft after a few minutes of letting it sit. what would cause this??

 

anyone have any ideas why the fuel would run out the return line when i pump the hand pump?

 

bad cp3???

check out this link:
How to check fuel return rates - Chevy and GMC Duramax Diesel Forum

it gives the return rate info for the injectors

 

i really think its a bad cp3 but i just dont know enough about these diesels to be for sure.

If i hook like a 2 foot piece of clear rubber hose upto the line goin into the cp3 and hold it straight up in the air and fill it with diesel so that there is no air in it, i then crank the motor over and watch the fuel level. shouldnt i be able to see it goin down when i crank it over?? usually it dont go down very much at all. sometimes if i crank it over long enough or somethin its like it grabs and you can see it start goin down then it fires up.

Isnt the cp3 suppose to suck fuel? it dont look like its sucking much at all when i do this until it starts.

 

what you describe sure sound like the CP3 is sucking air, the weird thing is everything done to eliminate any air getting in and now with you connecting direct to the pump and still having problems

see if you can supply a few psi to the clear line and stat it. if the truck starts every time then it may be a CP3 problem.

it also makes me think the gear pump on the back of the CP3 is not doing its job

 

the place where i hooked the clear line into is where the rubber hose that comes from the fuel filter goes to. its like right in the center of the motor. i doubt it could still be sucking air somewhere but i dont know.

is there any way i could just replace like that gear pump on the back of the cp3?

also, i dont think the hand primer is suppose to stay firm after the truck sits and its been pumped up. my dad has an 08 dmax with no problems and the hand primer is the same as mine. it stays loose till you pump it up then gets firm, but if you let the truck sit, even without starting it, it becomes loose. just thought id throw that out there

 

well i filled that clear line up with fuel that i got goin to the cp3, i then pressurized it with my air compessor with probably 30-40 psi. i probably didnt need that mush but its what i used. while i had it under pressure my brother would try start it and it seem to start everytime when it was under pressure.. thinkin maybe bad cp3??

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well i figured out what the biggest problem was thats been casuing this i think. theres a short rubber piece of hose that hooks into the cp3. it was completly wore out. looked like it could been collapsing or letting air in. anyways, i replaced it and the truck will now start without ether, WHICH IS AWESOME! but it seems to crank a little longer than it should, any suggestions?
Was thinkin maybe a lift pump would help it??