Need help with Fuel Rail Pressure at WOT on LB7 with Max Effort Tune (+230 HP)

[cludwin]

Hi There,

My name is Courtney and I'm new to the forum but I've got a puzzler that I've been trying to diagnose / fix and I was hoping someone with more experience could help me out.
Hopefully this may also help someone else out in the future

Thanks in advance!



Truck:
2003 Chevy 2500HD w/ LB7
Allison
270k

Mods:
EFI Live Max Effort tune
New Bosch Injectors installed this year
New Fleece CP3k installed this year ( it’s a modded LBZ CP3 pump w/ reg remapped by tune )
AirDog 165 4g lift pump cracked up to about 15 PSI at idle
PPE fuel rail race valve / plug
PPE boost increase valve
AirHorn
S&B Intake
CAT 2 micron fuel filter ( ~4 months old )

Problem:
On stock or tow tune the tuck will hold rail pressure at wide open throttle no problem…
When commanded 23k the actual is right there at 23k

But on large tune (+230 HP Max Effort) at WOT the rail pressure drops to between 14k and 16k depending on RPM.

Diagnostics / troubleshooting:
I’ve recently replaced the fuel filters at the lift pump and filter head.
I’ve verified low side fuel pressure @ idle to be 15PSI w/ gauge at port near alternator.
I’ve dropped the tank to check for debris / obstructions and cleaned it ( nothing there it was clean to begin with )
I don’t have any leaks
The filter head is new and primer button is always firm so I don’t suspect any leaks
Visual inspection of all the hoses and I don’t see any kinks / obstructions.
Injector balance rates are all in spec
Checked engine oil and it is clean (no diesel in oil)
Tested return rate on injectors + CP3 w/ 15 second crank test (https://www.youtube.com/watch?v=t9Da8rcL1jA) and it came back at 70ml ( <100ml is normal )



Interesting findings…
When preforming the return rate test I noticed that when I turned the key to the accessory mode which engages the lift pump I’d get a slow stream of fuel coming out of the return line to the tank. My guess is that the PPE fuel rail race valve is the reason. The stock fuel rail pressure relief valve has a spring tensioner which keeps the valve closed under normal operating conditions but pops open when rail pressure spikes to avoid damage. The PPE valve is actually just a slug with a hole machined in it and therefore always bleeds off a little fuel to prevent damage. Is this correct, it seems logical but I haven’t worked on enough LB7’s w/ race plugs to be certain?

One theory I have is that with the injector duration at the highest tune combined with the constant bleed from the PPE valve that the Fleece CP3k can’t keep up. I could test this by reverting back to the original factory FPRV.

A second theory I have is perhaps to increase rail pressure I could have a custom tune made up which commands more then 23k PSI, my understanding is that the LBZ CP3’s handle between 26k to 29k so it could be that a tune could just ask the pump for more.

Another theory I have is that while return rate is within spec I have just enough return from one of the injectors or CP3 that it just won’t hold rail pressure on the big tunes.

I hope it’s not an injector or a CP3 as I’ve just replaced both within the last year and neither my back or my wallet are looking forward to doing either any time soon

For fun and a little extra info I’ve attached a screen shot of a trace I took while getting on the highway where I put a foot on it and drained the rail pressure.

[cludwin]

Also another possibility is that maybe with the lift pump filter + the cat 2 micron I just have too much filtration going on which is restricting flow to the CP3.

One final thought is that the truck starts, idles and drives fine. It doesn't smoke / haze while idling. It does roll coal when I put a foot in it. The only downside is that I think there is a lot more power left in it if I could get the rail pressure sorted out.

[Mike D]

Also another possibility is that maybe with the lift pump filter + the cat 2 micron I just have too much filtration going on which is restricting flow to the CP3

Thanks for posting. We will get some tech help to you. I would remove the Cat Filter for now and just run the Factory or Raccor. That would be a simple test. The Cat filter is very restrictive and not really needed if your have the AirDog.


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[cludwin]

Mike, Thanks for the quick reply I'll give it a shot and let you know how it goes!

[cludwin]

OK brand new OEM Fuel filter installed and I've still got roughly the same results.

It looks like the CAT wasn't a restriction I'm leaning toward the PPE race valve being the culprit
http://dmaxstore.com/products/?route...roduct_id=1631

I'm going to keep working on it and update here when I find a fix.

Any suggestions are welcome!

Thanks,
Courtney

[DMAXDR]

The PPE race valve is solid and has no bleed hole. The return fuel is most likely from the injection pump returning fuel with the Airdog on. Did you run the supply hose from the air dog all the way yo the engine fuel supply quick connect fitting? This will rule out and compromised factory fuel feed lines. The core of the flex sections in the factor fuel lines are a plastic tube that may be kinked internal with on outward signs.

[cludwin]

I wanted to provide a quick update here. So since the ppe LB7 race valve is really just a plug and there is no hole then I think that the key is in the return test.
I've been able to get a trickle of fuel that comes back to the tank in a return test when I engage my lift pump without cranking the engine.

My guess is that I should get nothing at all back because my lift pump is putting max 10-15 PSI on the system. I wonder if that is a symptom of a bad CP3 or bad Fuel Pressure Reg. I have a hard time believing that it's due to an injector because it's happening under lift pump pressure and not even at rail pressure.

Is it safe to assume that I shouldn't get anything past my CP3 at 10 PSI ( pressure from lift pump only )?

[cludwin]

More updates I'm leaning toward a bad CP3 or regulator...
This video shows that with lift pump on at 10.6 psi I'm getting fuel returned past the CP3 pump.

VID #1) https://photos.app.goo.gl/GjOulD3w3DMAvuTH3

Another test I did was a simple bottle return test where I collected from the CP3 return as well as from the injectors (at the same time tee'd in 2x) and only a few drops of fuel came from the injectors but 75-100ml came from the CP3 after 15 seconds of cranking (this time no lift pump).

VID #2) https://photos.app.goo.gl/3Y4uvU9adlW18UHD2

The strange thing is that at idle using EFILive I can command full rail pressure of 26k PSI and the pump will meet that pressure, it just does not hold rail pressure at WOT.

VID #3) https://photos.app.goo.gl/9IW0sfUs1b6Na0Zh2

My If you look at vid #2 closely you can see tiny air bubbles going through the clear line my guess is that could be an indication of bad pump / regulator.

[DMAXDR]

More updates I'm leaning toward a bad CP3 or regulator...
This video shows that with lift pump on at 10.6 psi I'm getting fuel returned past the CP3 pump.

VID #1) https://photos.app.goo.gl/GjOulD3w3DMAvuTH3

Another test I did was a simple bottle return test where I collected from the CP3 return as well as from the injectors (at the same time tee'd in 2x) and only a few drops of fuel came from the injectors but 75-100ml came from the CP3 after 15 seconds of cranking (this time no lift pump).

VID #2) https://photos.app.goo.gl/3Y4uvU9adlW18UHD2

The strange thing is that at idle using EFILive I can command full rail pressure of 26k PSI and the pump will meet that pressure, it just does not hold rail pressure at WOT.

VID #3) https://photos.app.goo.gl/9IW0sfUs1b6Na0Zh2

My If you look at vid #2 closely you can see tiny air bubbles going through the clear line my guess is that could be an indication of bad pump / regulator.

I have been away for a bit but, there will always be return from the injection pump. The air bubbles are a concern. I was unable to see where your clear hose was from. So what I would do is determine where the air is conning from that is not good. Simply air compresses and fuel doesn't as well as air can cause all kinds of rail pressure issues.

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[DMAXDR]

I have been away for a bit but, there will always be return from the injection pump. The air bubbles are a concern. I was unable to see where your clear hose was from. So what I would do is determine where the air is conning from that is not good. Simply air compresses and fuel doesn't as well as air can cause all kinds of rail pressure issues.

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A while back a was wondering if you had the lift pump plumbed all the way to the engine ? Also a quick test is to see what the fees from the lift pump outlet looks like. Should be a steady flow no air or trickling

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[cludwin]

Quick update It's been along time and I've just been driving the truck as is on a stock tune but I did this week just replace the fuel pressure regulator which was a bear ( those 3 bolts took me the better part of a day ). The synopsis is that the fuel pressure regulator didn't do the trick I still get the rail pressure to drop at WOT on larger tunes.

DMAXDR - The clear line was coming right off the return from the CP3.

I'm going to re-examine all the soft lines going to the CP3 and make sure none are cracked / falling apart. The interesting bit is that the lift pump holds 10-11PSI of positive pressure going to the CP3 which leads me to believe that I shouldn't be getting any air in the system (e.g. filter head or hose or something) and that if my lines were bad I'd actually see a leak of fuel coming out.

Good news is that based upon the fact that my balance rates are good and there is only a few drops of fuel that come back on the injector return I feel pretty safe in saying it's not a bad injector.

[Roger O’Dell]

Although I have an lml with lbz ecm and injectors. I really like to read troubleshooting process, it may or may not apply to my situation. There is always something to learn.

[DMAXDR]

What is the calculated fuel rate at idle park operating temp a/c off? The on times for your injectors? This will help me base line your fuel system.

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