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Discussion Starter #1
On the way to the lake today and I started hearing a tapping noise. Let off the throttle the noise quit. Made the noise again when I pressed the throttle. Started slowing down to get off the road it quit before I completely stopped. I have a lift pump and when I loosen a filter, fuel does come out. BUT the lift pump is quite until I loosen the filter and fuel pours out. Tighten it and it quiets down. Kinda Sounds like the pump lost prime. It’s always loud when it is primed.

What can I check myself to confirm it’s the CP4?
No check engine light.
I do not have a programmer so I don’t know my fuel pressure.
Truck is a 2012 Duramax.
If it is the CP4, is the updated CP4 okay to buy or do I need a CP3?

Thanks for the help
 

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Nearly impossible to diagnose without checking for codes first. I’m guessing you have codes, but the CEL just didn’t illuminate. Pull off the rubber hose that goes to the injection pump from the factory filter and check to make sure there’s fuel coming out when you turn on the lift pump. IMO, it doesn’t sound like a cp4 issue tho.


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Discussion Starter #3
Thanks for the reply. I’ll check this morning. My truck is at my dad’s house. I used a basic diagnostic tool and it didn’t show any codes. I’m going to order a more sophisticated one today.
If I have fuel going to the pump, what’s my next step?
 

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Discussion Starter #4
i have fuel at the factory fuel filter.
it acted like it was going to start once but no additional luck.
As soon as I turn the key off my lift pump loses prime. I reprime and make sure I have fuel at factory fuel filter. What should I try next?
 

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If your losing prime as soon as you shut the key off, and there are no fuel leaks, then it sounds to me like you have a leak on the suction side of the lift pump.

Which narrows down your area to look since it's from the lift pump to the tank. Because from the pump to the motor is pressure so you would tend to see the leak

Is yours the newer style with the draw straw or the older style that hooks to the factory lines? Or do you have a sump?
 

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Discussion Starter #6
I bought the truck with the system so I don’t know.
Since I have fuel at the factory filter, should it start?
 

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If you have fuel flow at the factory filter outlet (to the high pressure pump) it should start, or at least try to.

It's a little bit or a pain to get at, but still relatively quick to check. But see if you cant pull the regulator out of the pump and see if there is any metal on it. It is held on to the pump with two torx screws.

If you have metal on there, that's bad news for the pump.

Now not to get too far ahead, but if there is indeed a leak on the suction side of the lift pump, that very well could have taken out the cp4 due to aeration.
 

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How are you determining that you’re losing prime at the lift pump?

If you have a good scanner, watch your desired rail pressure vs actual rail pressure, and see if the injection pump is building any fuel pressure. If not, check the regulator.


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Discussion Starter #9
I can tell I’m losing prime by the sound it makes. I crack a filter and it reprimes.
I don’t have a scanner.
I’ll take the regulator off.
 

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Whelp, it looks like you have your answer there.
 

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Keep in mind you're not just replacing the pump, you're in for lines, injectors etc too. Whole fuel system typically.
 

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Discussion Starter #16
Yeah... I’m shopping now. It will be done right. Thanks guys for the help.
 

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Keep in mind you're not just replacing the pump, you're in for lines, injectors etc too. Whole fuel system typically.
All of the high side fuel system stuff, yes.
 

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Discussion Starter #18
I’ve searched and mostly find rants and advertising videos related to the CP4. Any suggestions on specific YouTube videos and write ups? I’m most concerned about getting all the debris out of my fuel tank and lines. Thanks.
 

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Discussion Starter #20
Truck is deleted and tuned. Bought it that way. I’ve kept it on economy the whole time I’ve owned it from 140k to 190k miles. Bought a kit from injectorsdirect yesterday. Now I’m trying to find a good how to write up to make sure I do it 100% correct. Any videos or threads would greatly be appreciated. I’m starting Friday morning and working on it all weekend. Luckily my truck is at my dads. He owns 2 Peterbilts and has A LOT more tools than I do.
 
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