Low rail pressure codes

[02greysixer]

Alright I have a buddy thats having some issues. His truck is a 2006 with the 6 speed allison. I thought that meant it was an lbz but according to his vin its an lly. I ran a check with my v2 and I could not connect to it as an lly, but when I changed the "connect to" to lbz it connected and let me do the log. Point is its a late lly or early lbz.
Mods are as follows: Bully dog set on stage 2 (he never told me what kind of bully dog it was) A power play MP-8 (a box that sits under the hood and plugs in line with the fuel rail pressure sensor and the MAP) has stock exhaust, intake, and no lift pump. All mods have been removed since problems started.
Problems are as follows:
Truck had bad diesel knock and just shut off one day. Was able to restart and get it home. The owner found one fuel injector was bad (don't know how he did). he changed it and took the truck for a drive, then blew all the freeze plugs out of the cp3. Changed the cp3 twice with brand new ones and the truck ran good, but now anytime it gets driven when it hits 2k rpms it throws a low rail pressure code and goes to limp mode. The desired and actual rail pressure were the same at idle. when revved up in D with the brakes on the desired goes up to almost 21K psi but the actual never breaks 10k. It did not throw a code while i was doing this. I can't figure out how to post the log. Any ideas?? Could the rail pressure sensor be bad causing a false reading tricking the computer into thinking it has low rail pressure when it really doesnt?

 

[Chevy1925]

Alright I have a buddy thats having some issues. His truck is a 2006 with the 6 speed allison. I thought that meant it was an lbz but according to his vin its an lly. I ran a check with my v2 and I could not connect to it as an lly, but when I changed the "connect to" to lbz it connected and let me do the log. Point is its a late lly or early lbz.
Mods are as follows: Bully dog set on stage 2 (he never told me what kind of bully dog it was) A power play MP-8 (a box that sits under the hood and plugs in line with the fuel rail pressure sensor and the MAP) has stock exhaust, intake, and no lift pump. All mods have been removed since problems started.
Problems are as follows:
Truck had bad diesel knock and just shut off one day. Was able to restart and get it home. The owner found one fuel injector was bad (don't know how he did). he changed it and took the truck for a drive, then blew all the freeze plugs out of the cp3. Changed the cp3 twice with brand new ones and the truck ran good, but now anytime it gets driven when it hits 2k rpms it throws a low rail pressure code and goes to limp mode. The desired and actual rail pressure were the same at idle. when revved up in D with the brakes on the desired goes up to almost 21K psi but the actual never breaks 10k. It did not throw a code while i was doing this. I can't figure out how to post the log. Any ideas?? Could the rail pressure sensor be bad causing a false reading tricking the computer into thinking it has low rail pressure when it really doesnt?

do what we call a "bottle test". there is a thread here where you will stick a gallon jug under the hood and take the hose off the top of the fuel pressure regulator and put it in the gallon jug. Then go run the truck at WOT a few times and see how much fuel is in the bottle. there should be damn near non. if there is a good amount the FPR needs to be shimmed or changed out with a new one and shimmed that new one.

 

[02greysixer]

Oh I forgot to put that in, I did tell him to do the bottle test and see what he got. We didn't have time today. And his fprv is shimmed as well, he did that after it threw the code the first time. Little to no change.


Sint frum tha orijinul smert fone

 

[Chevy1925]

Oh I forgot to put that in, I did tell him to do the bottle test and see what he got. We didn't have time today. And his fprv is shimmed as well, he did that after it threw the code the first time. Little to no change.


Sint frum tha orijinul smert fone

still a good thing to check. if it was not shimmed enough it can still leak and it doesnt take much to kill rail pressure, even if that fpr is a little loose too. those CP3s can give tons of pressure but they highly lack in volume

 

[02greysixer]

True, I will tell him to check. What causes it to need shimmed though? Is there something that keeps some trucks from building pressure?


Sint frum tha orijinul smert fone

 

[Chevy1925]

True, I will tell him to check. What causes it to need shimmed though? Is there something that keeps some trucks from building pressure?


Sint frum tha orijinul smert fone

yeah, unlike our trucks, the LLY and up have weak springs in the FPR so it makes it easier to pop them when under pressure. the LB7 has a very stiff one but once then pop, they keep popping from what i have read. Rare to find though. You basically put a couple washers of a certain thickness in between the spring and housing to stiffen up the spring. some guys weld them shut or run a PPE plug but ive been hearing they leak after a while.

 

[02greysixer]

Interesting, thanks for the info. I will tell him to give that a shot, any other thoughts?


Sint frum tha orijinul smert fone

 

[Chevy1925]

If you dont see any fuel or a very minimal amount, go into the DVT tool on the scanner side of EFI Live and command rail pressure to 160mpa. log the rail pressure as you do this (both actual and desired). they should match eachother at idle. it will sound like the truck is gunna loose a rod but it will be fine. if they dont match, your lookin at either a weak CP3 with a high return rate, bad FPRV, or high returns on a few injectors IF balance rates are good.

 

[02greysixer]

Balance rates on injectors were all within +/- 1.5. And I did try to crank up the rail pressure with the DVT part of the tool. There was no change in the idle at all. That's what made me wonder if the sensor has something going on. We will try to shim the crap out of the fprv first though.


Sint frum tha orijinul smert fone

 

[Chevy1925]

Balance rates on injectors were all within +/- 1.5. And I did try to crank up the rail pressure with the DVT part of the tool. There was no change in the idle at all. That's what made me wonder if the sensor has something going on. We will try to shim the crap out of the fprv first though.


Sint frum tha orijinul smert fone

Just do a bottle test, takes much less time and headache

 

[02greysixer]

Will do :thumb:


Sint frum tha orijinul smert fone

 

[Bobo]

Put it back to stock and pull the mp8 off to diagnose it. Once it's fixed, put the tune in it and drive it. If its ok, put the mp8 back on and test it again. That will make it easier to diagnose and repair. Email me if you need some help!

 

[McRat]

Always bottle test before shimming the PRV. I wrote a brief tutorial on how to do it, it's pretty easy to shim, but a MOFO to get the bitch out of the rear driver's side fuel rail.

LBZ+ don't always need shimming. IIRC, Wendy (540rwhp, 12.5 ET) isn't shimmed.

But what really worries me is the CP3 explosion. I wonder if there is debris downstream now? CP3 kapows are normally caused by lubrication failure. ie - water in fuel, or debris.

Anyhow, do the bottle test to get that out of the way. It's fast and easy.

 

[02greysixer]

It is back to 100% stock till we figure it out. And I will read up on the bottle test then go from there. The pump blowing up scares me too because couldn't that damage injectors and give them high return rates, even though the balance rates are normal? Although if that were the case wouldn't the rail pressure be low at idle as well?


Sint frum tha orijinul smert fone

 

[02greysixer]

TTT I was bored today so I decided to mess with this truck some more. The guy checked the return rates for injectors this weekend and the were normal (according to him, he didn't give specifics). I told him to do the bottle test to the cp3 this weekend and let me know. I did some more logging and messing with it today. The desired rail pressure was all over the place, even at idle. It bounced wildly from 8k all the way to 30k and the actual never broke 10k. I unplugged the FPR and there was no change, shouldn't that have sent the rail psi to max? I know he needs to do the bottle test to know for sure but since there was no change in unplugging the FPR doesn't that point to a stuck/bad FPR or pump? I will post the results of bottle test when he does it, I just didn't feel like doing it today :embarrassed:


Sint frum tha orijinul smert fone

 

[DIESELJEFF]

did you ever figure out the problem? im having the same issues and cant get it figured out.

 

[ripmf666]

did you ever figure out the problem? im having the same issues and cant get it figured out.

You need to look at the rest of your fuel system that had the veggie in it. Your already getting help in your thread not like no one is posting help.