Duramax Diesels Forum Truck of the Week
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Old 11-08-2018, 11:41 PM
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Default Lb7 rail pressure issues

I picked up an 02 lb7. Truck has a slight lope at idle. Mostly when cold then comes and goes. When accelerating im about 10k below the desired fuel rail pressure but the trucks not throwing a code. I have an airdog 165gph but its not installed yet. Wonding if the fpr can cause this and why it is not throwing a code for low rail pressure. I did notice the 2 lines coming off the filter head to the ficm are cracked. Ill be replacing them as well as a new factory filter and rebuilding the filter head. Not sure if the fpr is bad or i have a weak pump or a mixture. Any help is appreciated. Im new to the lb7s but not duramaxs.
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Old 11-09-2018, 08:50 AM
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Yes to all of the above. You may also have injectors or CP3 with high return rates that drag the pressure down
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Old 11-10-2018, 03:55 AM
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Injectors are not that old but i will look into it after swapping the rubber hoses, filter and fpr. Then will add ligt pump after that regardless. Was just curious why im not getting a cel for rail. Also wasn't sure about the regulator. I know they will cause a lope but wasnt sure about the rest.
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Old 11-10-2018, 07:40 AM
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Early lb7 won’t throw a low rail code. It might throw a large fuel leak code or something like that. But I’ve seen them like 16k down on rail and it just keeps going with no light.

Try an fpr but I bet it ends up needing a pump, I’ve seen this scenario a lot on 01-02 trucks. Especially with freshish injectors. Most trucks the owners never know due to no light, it just lacks power.
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Old 11-10-2018, 08:27 PM
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01 would throw rail pressure codes for no reason, GM changed it for late 01/02 so they don't throw codes easily. Mine was dropping to 12K when desired was over 25K, and still wasn't coding. If the cp3 has 150-200K miles, I wouldn't waste time replacing the regulator, just do the cp3.
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Old 11-12-2018, 12:45 AM
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Thanks fellas. Worth upgrading to a lbz cp3 or just another reman lb7 one from Lincoln diesel?
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Old 11-12-2018, 06:33 AM
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Quote:
Originally Posted by Rancelot211 View Post
Thanks fellas. Worth upgrading to a lbz cp3 or just another reman lb7 one from Lincoln diesel?
Id stay lb7 for ease of tuning. Especially if its going to be a relatively stock truxk
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Old 11-12-2018, 08:51 AM
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The LBZ regulator on the pump will allow it to make a few thousand for psi but requires tuning to run properly
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Old 11-12-2018, 08:59 AM
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Quote:
Originally Posted by 2004LB7 View Post
The LBZ regulator on the pump will allow it to make a few thousand for psi but requires tuning to run properly
it will only allow a little more flow IF the cp3 is a little on the weak side which can gain you a little more rail if you are loosing it in comparison to the desired. the stock lb7/lly reg will run up beyond 26k rail pressure if commanded.

that may or may not be what you were stating 04 but just wanted to clarify. back in the day, it was said only the LBZ reg would net you 26k rail pressure till a couple tuners stepped in showing it can be done on a LLY/LB7 reg in the tuning.
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Old 11-12-2018, 09:05 AM
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Learn something new every day. Didn't know that
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Old 11-12-2018, 10:08 AM
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Quote:
Originally Posted by 2004LB7 View Post
Learn something new every day. Didn't know that
yup! been running 26k in my truck on LLY stock regs for the last 3 years and on stock LB7 regs for the last 5 years before that.
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Mike L is my role model. I want to be like him when I grow up.
James

02 LL7 2500HD CC/LB dmax/alli "The Limo"

BIG THANKS TO: Mike L, Steve, Trent, Guy, Dan, Ruben, Josh H, Scott, Brian and everyone else who has put a hand in to help me.

best time to date: 12.39

motor died/bent rods = 3/11/11
back from the dead 7:04pm 4/9/2012

Limo is back in the garage, now driving a built chevy tracker
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Old 11-12-2018, 10:55 AM
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Cool; never knew either! I did replace a failed lb7 regulator with a new style and found the rescaling info is out there (google search!) free on the net from an old post of Nick's if you are trying to DIY. Worked well.
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Old 11-12-2018, 11:05 AM
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Yup, that info is out there for sure. some trucks are pickier than others though (as well as owners). ive had some trucks that took forever to keep any lope away (cold idle, warm idle, and coming to an idle as you coast to stop). specially dual pumps, you gotta really play with direct and time adjustments to keep things happy. but again, im picky. if its not like stock, it bugs the hell out of me. hell i had to play with the direct and time adjustments on stock regs with dual pumps cause they were over shooting.
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Mike L is my role model. I want to be like him when I grow up.
James

02 LL7 2500HD CC/LB dmax/alli "The Limo"

BIG THANKS TO: Mike L, Steve, Trent, Guy, Dan, Ruben, Josh H, Scott, Brian and everyone else who has put a hand in to help me.

best time to date: 12.39

motor died/bent rods = 3/11/11
back from the dead 7:04pm 4/9/2012

Limo is back in the garage, now driving a built chevy tracker
Tailgate Crew Member #4
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Old 11-12-2018, 12:57 PM
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I did an lbz cp3 because lbz/lmm cp3's were $100 cheaper than lb7/lly cp3's. A quick bend to the banjo fitting on the return line, and it all works fine. I did have to spend quite a bit of time rescaling the current table because just copying the lbz table over did not work. It got it running, but drove like crap with pressure under/over shooting all over the place. Took me about 2 hours to get a few drives in until I got the table where I was happy with it on both of my tunes.
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Old 11-12-2018, 01:47 PM
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Quote:
Originally Posted by THEFERMANATOR View Post
I did an lbz cp3 because lbz/lmm cp3's were $100 cheaper than lb7/lly cp3's. A quick bend to the banjo fitting on the return line, and it all works fine. I did have to spend quite a bit of time rescaling the current table because just copying the lbz table over did not work. It got it running, but drove like crap with pressure under/over shooting all over the place. Took me about 2 hours to get a few drives in until I got the table where I was happy with it on both of my tunes.
That's interesting, because I've always just scaled the LBZ table to fit the LB7 axis and had good results. It isn't a copy-paste thing, sure, but 5 minutes in Excel with some formulas and it seems to work fine. But that has always been a LBZ regulator on a LB7 CP3, never a full CP3 swap.

Would you mind sharing your table?

My truck has a LBZ CP3 now, but it's a 12mm stroker, so I just made that table up from scratch, for fairly obvious reasons.
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