LB7 Duramax Idle Lope and Surge Issue Fix! Limiting the supply pressure to the CP3

If your LB7 is loping/surging at idle, especially when in drive, this is one possible solution to your issue.
As stated in the video, there are several things that could be the real culprit. But after plenty of forum research and talking with my local shop, we identified the supply pressure from my lift pump being to high was my issue. By cutting the return spring down slightly, I was able to relieve a little of the supply pressure to the CP3, which was enough to allow the truck to idle smooth and perform correctly.

[@jellyfrosh9102]

Finally a channel that doesn't have a 5 minute long intro talking about bullshit that has nothing to do with the title. Thank you!

[@FixedVaneDuramax]

Jellyfrosh Thank you! Im doing my best to only post content that has actual value. That means I don’t make as many videos as some of the big names but what I put up is there for a reason.

[@stevewaclo167]

Always amazes me how tiny, tiny things, like one turn of a spring or the little check balls and their seats in the injector return circuitry can have a profound effect on operation of a 7,000# truck. Luv a happy ending! Well done, sir!

[@nou8257]

It's always the little crap that all builds up to big issues together.

[@TruckMaster]

Great video thanks for the tip

[@FixedVaneDuramax]

Thank you sir! Thanks for stopping by the channel!

[@hardhead7056]

Basically. This is a good video. Basically. Thanks for posting it. Basically. Have a good day. 🤣

[@gavinw5530]

I have the same lb7 (183k miles) with the same Fass pump with same loping issue, but mine isn't super consistent on when it occurs. Really cool to see you work through it. Anyways, I don't have the pressure gages to check if that is my issue. Do you think this would be the most probably cause? Haven't had the pickup that long to know otherwise, nor have I driven it when it was loping, but from the severity of it, I don't think I would want to be haha. Anyways, good work, can't wait to check out your other videos with this pickup!

[@jacobparks9952]

Could you do a video of you lift pump setup to the ficm and cp3

[@JS-er2te]

Thanks for the tips!

[@cody6.6max]

Just installed new Cp3 and my truck actual pressure is going up to 11-17k and goes back down after a min, driving it’s fine . Thought might have been FPR but unplugged it and made it max out so it’s not that . Than unplugged the fass and it went back to normal at idle so I need to do this I reckon correct sir ????

[@bannana6290]

dude wear gloves my dad is a master and diesel specialist and got skin cancer at 45 and so did half the guys at gm some type

[@HtownSpartan]

Hello I only have a jumpy idle just when in cold start after running for 30-40 min it's drives smooth and normal what could it be I have changed out FPR and fuel rail sensor as well everything else is stock truck has 170k original miles

[@theawezomedude]

Do you have a video of the lope trying to diagnose my 04

[@1nHaLeZ]

Nice video ! I have a 02 Sierra , I have no haze at idle but sometimes when I take off it’s kind of boggy and shoots blue smoke then if I get in it it clears out and runs fine with no smoke , just did the fuel filter would you suggest injectors or regulator ?

[@1nHaLeZ]

Also it doesn’t lope at idle or in general

[@FixedVaneDuramax]

@@1nHaLeZ If its not idling rough your regulator is probably fine. If you can try and check your balance rates with a scan tool or an edge cts to see if they are in spec. If you can try running an injector cleaner through as well before you dig into replacing the injectors. There are a few different youtube videos showing how to unhook the supply and return lines and running the cleaner through the the motor.

[@evanch123456]

Yesterday a p1094 code showed up on my truck. Trucks runs fine no issues...

[@FixedVaneDuramax]

evan hale I just did some quick forum searching. You have an LLY truck? That code generically is a fuel rail low pressure at idle code. Which could be either a weak fuel rail pressure relief valve or the FPR itself. If its the FPRV you can get a PPE “Race Valve” that essentially blocks that off, helping the cp3 maintain pressure. Similar to how you plug off the return port on the LB7 fuel distribution block. For now I would clear that code and see if it comes back. If the truck seems to be running good it may be a fluke but watch it.

[@evanch123456]

It's an lb7. I cleared it we will see if it returns! Thanks man!

[@cholochedd6752]

I have an 02 lb7 that lopes at idle I have an air dog 150 I replaced the cp3 which also had new fpr but still lopes can it be the pressure relief valve it has close to 300k miles but does not have absolutely no blow by and dealer and other shops all said injectors read goods very thing else is stock and I have no gauges but do have an obd2 that reads live data

[@Tyler_LB7]

Did u fix the issue? Im having the same head scratching problem

[@rca_yj3928]

any fix? i got the same issue on mine.

[@cholochedd6752]

It was the pressure relieve valve. Or of you have a lift pump check the check valve to make sure it’s still ok on the lift pump.

[@Tyler_LB7]

I have a lift pump on my lb7. Could the check ball be the problem? Same deal just over pressurizing

[@rca_yj3928]

thank you ill try that out. i have the industrial injection cp3 100hp and the air dog lift pump. when i disable the lift pump it stops.

[@Parker565]

Now riddle me this, twin dueler LBZ cp3's one in the valley, one on the pulley system, Fpr delete with my Wherli S300 kit yet it surges at idle while it's cold and my frp jumps around at idle and lopes when it's cold. Ppe duel fueler controller, and ppei tuned for the Lbz pumps. Only dose it at a cold idle though. I also have a Fass 150, anything else I could be missing? Love the content man, been working on a build series of my truck but haven't posted any of it yet! Nice to see some one with good information! PS I'm also running 100% over Sac injectors, not that that matters much but I'm just curious because my truck never used to surge with just the Fass lift pump, Fpr in place, and trans safe tunes.

[@FixedVaneDuramax]

Sounds like an awesome build! I'll be excited to see those videos when you get time to put them together. I'll be honest in that situation that may go over my head, may be worth taking a data log and sending down to Kory to see if he has any thoughts on it. If I had to take a stab in the dark I would think the pumps are providing more fuel than necessary at idle or the injectors are staying open too long (pulse width) but like I say, I can't say with any confidence I'm shooting near the mark on this one. More and more I am realizing that I need to buy a V2 and start digging into tuning myself just to learn the ins and outs of how these trucks function. Would be a fun challenge.

[@kratez707]

I have this exact same problem. I've snipped about half a circle of the spring and got down to about 10psi and it doesn't seem to bounce. Did you happen to throw a P089 code along with it? Also started after the FASS150 install.

[@riveraf5053]

I installed a new fass lift pump and I got a p0089 code I replaced the fuel pressure regulator and after awhile it gave me the same code, you think if I cut the spring it might help

[@cody6.6max]

I have same problem at idle with p0089 and when I unplug my lift pump It stops

[@white24valve]

What your tach...if it is bouncing/moving up and down...fpr giving you fits......and or...Duramax does not like over 9 to 10 psi of fuel psi

[@FixedVaneDuramax]

Agreed. Cutting my spring down helped to control the surging at idle, but if your fpr is going out it will not completely solve the issue. I actually have an fpr ordered at this point, while this procedure did help a lot, my truck has once again started to have a lopey issue and watching the rail pressure it is bouncing around more than I would expect when the engine is under light load (idling in drive) and cold so the FPR is getting swapped when I get some time.

[@251suzuki]

@@FixedVaneDuramax did the fpr fix your issue?

[@riveraf5053]

I installed a new fass lift pump and I got a p0089 code I replaced the fuel pressure regulator and after awhile it gave me the same code, you think if I cut the spring it might help

[@FixedVaneDuramax]

On an lb7? There is a decent chance that is your issue yes, especially if it started happening after you installed the Fass pump, and changing the FPR didn't help. You can either cut the spring down a bit, or call Fass and get a lighter spring from them for cheap.

[@FixedVaneDuramax]

One more note here...there is a port on the oem fuel filter head for 1/4 npt gauge I believe, it would maybe be worth the $10 to get a cheap pressure gauge and rig that up there to check your supply pressure, and be able to cut the spring to hit the 8-9 psi range if you go that route. Don't want to lead you down a rabbit trail but just a thought after doing some forum searching.

[@riveraf5053]

@@FixedVaneDuramax yup on a lb7 awesome thank you for the advice!!

[@cliffwaltman5903]

Since you have the supply line directly to the cp3 what did you do with the ficm?

[@FixedVaneDuramax]

Plug the inlet and outlet.

[@cliffwaltman5903]

Won’t that over heat the ficm. Since no fuel cooling it?

[@jarredtmiller]

Just ordered a fass 165 lift pump for my 2004 lb7 with 243,000 miles injectors have been done just curious if I should change my fpr before installing the lift pump or was wondering if I will have any issues with installing it on a stock lb7 thanks for any input

[@FixedVaneDuramax]

If the FPR isn't giving any issues now the fass install shouldn't change that by itself. If it was mine I would install the lift pump and just pay attention to how it idles. If it starts to get lopey, I would try the spring clip trick or reach out to Fass and have them send you a lighter spring.

[@jasonscruggs986]

When you say surge do you mean like a slight shake at idle after truck warms up My truck is doing that and come to think of it didn't start doing it until I put a fass 165 on truck. I replaced fuel pressure regulator about 8 months ago with new Bosh one. One thing I noticed is on the edge cts2 monitor desire frp is 4.4 and actual frp is going 4.4 down to 4.2 back and forth.

[@FixedVaneDuramax]

Jason Scruggs Yeah thats kind of what I mean, when the truck is cold you can hear and feel the motor slightly loping. When it was in drive, and at idle, it was literally attempting to surge through the brakes until the truck was up to operating temp. Like I say in the video it wasnt really noticeable until the weather turned much colder, but that could just have been since the truck takes longer to warm up. You could very well being seeing this a little bit too. I really wouldn’t have found issue with it if it hadnt been trying to lope/surge through the brakes when in gear but it made it very uncomfortable to drive.

[@jasonscruggs986]

@@FixedVaneDuramax cool I will put a gauge on my truck next week and see what the lift pump is at with truck off then truck on. I bet mine is too high too I have a lly

[@FixedVaneDuramax]

Jason Scruggs You’ll have to keep me posted, Im not sure if they made any adjustments to the 165 gph model or if its exactly the same. I have a lbz cp3 and fpr so I thought this shouldnt have been an issue but apparently its more the ficm that has a problem with it vs the fpr itself.

[@MrAgatto2]

i have a question..... my older lb7 was looping a little bit... so i figured the fpr.... well i put a new bosch one in and when i started the truck its looping terribly bad now.... i think i may have messed the oring... but i am not sure .. any idea what direction to go... ?

[@FixedVaneDuramax]

The O-ring is the most probable cause, it is really easy to roll or crimp when installing the FPR itself. If you have a monitor or a shop that can check your injector balance rates and pulse that would also be good. I haven't seen in person but have seen videos of a truck with a stuck injector loping hard. But if it is much worse purely after an FPR install that would point to that FPR itself still. Not sure how helpful this is but that would be my thought.