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Discussion Starter · #1 ·
Hey everyone, I have a 2007 Lbz creeping up on 190k miles.
truck has been tuned through Ppei for the last 20k miles. I recently started having low fuel rail pressure. So far I have changed the fuel filter, and installed a Ppe race plug. In the 150 tune im seeing 26k desired rail pressure and WOT and my actual fuel rail pressure drops down to 6k. Yes 6k psi. I’m lost as to where to start. Low pressure side of things first? high pressure? Any input is appreciated. Thanks
 

· Former GM Tech and parts
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Could be a couple things really. Does it do fine under normal driving, only falls off under WOT? I've encountered this on a couple different trucks. It could be that under high demand the cp3 just cant keep up because it is failing. Or it could be the injectors are returning too much fuel under high demand making the pressure fall off.
If its a cp3 starting to fail, a lift pump could help prolong the life and get you by for a while. But, I would hate to be the guy to tell you put a 700$ lift pump on and then still have the same issue after because the injectors are returning too much fuel.

And the injectors could test good on a return rate test- but that's a test done at idle so it won't be under heavy demand like you are when you are having the issue. But, this at least gives you some direction to go to start with
 

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Discussion Starter · #3 ·
It’s fine driving normally. I’ll be cruising at 60 and go to pass someone, truck will downshift to 5th. It holds 26k psi for a little while till the Rpm gets high. Truck won’t shift to 6th and the rail pressure drops down to 6k. I know I need a lift pump at some point, so I’m not too worried about spending the money on it even if it doesn't help my case right now. Would bringing to someone with a Tech 2 and having them command max rail pressure give me an idea if the cp3 was failing? Also another idea might be a dumb one. Could winter blend fuel have anything to do with this? I never had issues over the summer with it. I haven’t changed anything since then besides fuel obviously and the fuel rail plug.
 

· Former GM Tech and parts
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Commanding it to max pressure will tell you if it will get there, but the injectors wont be using all the fuel while you're dong it so it may not tell you definitively. But, if it won't do it then, that will tell you it is junk so it won't hurt.

Winter blend shouldn't make a difference on if the truck will reach and keep desired rail, so it has to be a truck issue and not the fuel itself.
 

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First, read this thread, from the Beginning!


Then this may be your issue but without the information mentioned in that first thread we really don't know, do we?

 

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When the CP3 starts going bad you start to notice it under loads. I would take it somewhere and have them put it on there computer hopefully its not the CP3. I wouldnt just go out and buy a CP3 and hope it fixes it either though. It can as simple as sucking air from somewhere, check simple things first I always say.
 

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Could be a fuel line collapsing too. My dad has a 07 lmm with 190000 on it and the fuel line coming from tank to quick connect was collapsing. He has a temp fix on it now with small pair of vise grips crimped just enough to keep it open. Hasn’t had code since. Pulling load or not.


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This is a code that a lot of people get confused by.

1. I never installing a race plug until you data log what is happening, did you happen to do that? The reason why is if someone installs it incorrectly it will leak and cause exactly what you are seeing.

2. Do you have a lift pump?

3. I would do a bottle test to see if you're leaking past the race plug, typically this would test to see if the FPRV is leaking.

4. If bottle test is clean, then your CP3 is starting to fail, you might be able to limp it longer with a lift pump if you don't already have one, but it's something you'll need to plan on replacing soon.

The fuel demand of a larger tune combined with heavily loaded is what makes these things shine. A guy I met and became good friends with had thrown parts at his truck to the tune of $10k trying to chase this issue when it was an improperly torqued race plug. Dealers and shops had him adding lift pumps, replacing injectors, replacing CP3, etc... Someone got him in touch with me, we properly diagnosed it and he was up and running quickly.
 
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