LB7: Need HELP

[Chevy1925]

Ok i pulled a vacum test on the return side of the fuel system. I blocked of the fuel line coming from the filter to the cp3 and pulled vacuum out of the return line going to the tank. I pulled 25psi and it stayed the for 15 minutes. I also primed the fuel filter while the line to the cp3 was blocked off held prime for 15 minutes. As soon as i put everything back to normal. Once u prime the truck it starts. You can prime the system and with in 15 seconds it loses prime. I know if the fpr is bad with will lope but could the fpr be stuck open just a hair that its leaking prime? Or os there a check valve in the cp3 that is bad. Also tested rail pressure with efi live. When primed as soon as it hit 2500psi it starts and it does that right away. Shut off and rail pressure drops to 1200 psi and stays there. Crank the motor to start with out re-priming and the rail pressure drops to 200psi. All injectors have been replaced and balance rates are all 0.** except one is -1.25. My guess would be something in the cp3 but could it just be the fpr? Or am i leaving something out of the equation? Any ideas or suggestion i would appreciate it. Thanks for the help

so rail drops to 200 and wont come back up? or is it very slow to rise? you need to check the return rate at the cp3. from what your describing, frp sounds fine

 

[05chevy]

so rail drops to 200 and wont come back up? or is it very slow to rise? you need to check the return rate at the cp3. from what your describing, frp sounds fine

Once you prime the system it will start. Shut it off and rail pressure stays at 1200. Start to crank it over drops to 200. Re prime system and it will start. Is there a test port on the cp3 i can test?

 

[Chevy1925]

Once you prime the system it will start. Shut it off and rail pressure stays at 1200. Start to crank it over drops to 200. Re prime system and it will start. Is there a test port on the cp3 i can test?

ok well what is rail pressure doing as you crank, staying at 200? no sign of building pressure as you crank?

pop the return line off the back of the cp3 and put another hose on the cp3 hard line to a beaker. henry has some procedures somewhere that state what is acceptable and isnt

 

[05chevy]

Staying at 200 while cranking. Not building any pressure

 

[Chevy1925]

got ya

here is some helpful info
http://www.duramaxdiesels.com/forum/showthread.php?t=26249&highlight=return+rate+spec

 

[05chevy]

I tested the return on the cp3 and injectors and they come out to factory spec. I tested the rail pressure and it is with in gm specs. Replaced all fuel lines and rebuilt the filter head with new filter. Tested the ficm for leaks and it is good. Still having the same problem prime the system starts runs fine. Turn off wont start till i re-prime system. Anyone have any other ideas or suggeations? Id hate to just put a lift pump on it to solve the prime problem and it be something worse down the road. But every thing test out to spec. Thanks for any help

 

[Sledhead]

Did you ever win this battle?