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2015.5 GMC Sierra Denali 3500 DRW
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Discussion Starter #1
I have been reviewing the site and haven’t been able to find my particular issue so looking for any insight before I really dig into troubleshooting.
I have a 2015.5 Sierra 3500 DRW, with a FASS 150, CTS2, factory filter delete with a pressure transmitter to CTS2 and run Optilube since 20k miles when I purchased Feb of 19. Deleted at 35k and now at 42k.
Was pulling my 24’ trailer unloaded from NC to Florida. Stopped to stretch and when I got back on the highway, truck fell on its face and got the Fuel Filter change warning, but would still cruise. Began watching the CTS and supply pressure stayed between 9-10 as normal, but actual FRP was lagging demanded when accelerating, but fine cruising. Within an hour down the road, actual versus demand difference continued to get worse and finally just wouldn’t go but would still idle. Was able to get a tow on to my farm.
Sounds like first steps are to pull driver rail regulator and then CP4 regulator and check for shavings. If that checks good then begin checking injector return rates? Did a balance check on the CTS when it finally lost all power and those are good.
Maybe I got lucky and it failed but didn’t grenade?
 

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Former GM Tech and parts
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I think when they fail they stop running altogether. No check engine light or codes? Could be you got some bad fuel and there's a slug of crap in the filter- I would change the filters and see if that makes a difference as well as check for restrictions in the fuel lines
 

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2015.5 GMC Sierra Denali 3500 DRW
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Discussion Starter #3
No codes at least on CTS, just reduced power message on cluster. Will check with EFI tomorrow and see if it shows anything. I have an aux tank and filled up and ran same fuel all the way to where it began acting up with no issues, but will change the filters to see if that helps. My transmitter is in the filter delete bowl at the factory filter location so restriction would have to be from factory head to CP4 inlet. I checked at the vacuum fitting and have good pressure/flow with engine off and FASS running.
 

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2015.5 GMC Sierra Denali 3500 DRW
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Discussion Starter #4
Did a code review with Autocal and no codes, same as CTS. Picked up new filters for the FASS tonight and will get them changed in the morning. Beginning to wonder if one of the soft hoses between the stock filter location and the CP4 has failed and collapsing. Hope to get into it more tomorrow.
 

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I did the stock fuel filter delete bowl thing for a while too. The more I learned about the CP4, I thought better of it and put a Racor filter back in place. With the FASS filters ahead of it, I get 30-40k miles out of a Racor without issues now. No more miles than you put on your truck, you could probably just change it every year or two. I hope that's not your issue:) Good luck!
 

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I don't think it's a cp4 failure either. I think it's a clogged/dirty fuel filter.
 

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2015.5 GMC Sierra Denali 3500 DRW
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Discussion Starter #7
Pulled filters this morning, no water or other evidence of contamination. Just nice clean fuel. Purged the system of air at the FASS, stock filter head, and vacuum port. Verified good pressure (10.5 psi at stock filter location) and good flow from the vacuum test port. Attempted a start and now won’t build over 1700 psi, but does try to idle at that low of pressure. Demand goes up over 11k trying to compensate. Just won’t do it. Now to get a shop bay cleared to try and get it inside and dig a little deeper.
 

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2015.5 GMC Sierra Denali 3500 DRW
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Discussion Starter #8
Well CP4 let loose. Was able to still drive it in the shop this morning. Cleared up and started holding 4K on the rails so I could get it inside. Now to spend a few dollars. As in my earlier posts this truck has had a lift pump and Optilube religiously should have just done the CP4 to
6C4F4041-D957-4FAE-B2AB-BEAA8D52988B.jpeg
CP3 when I deleted it.
 

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Former GM Tech and parts
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Dammit, that sucks. Now on to replace the complete fuel system....
 

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2015.5 GMC Sierra Denali 3500 DRW
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Discussion Starter #10
Yep, replacing rails and injectors, planning to do a full flush on all of the hard lines and replace the rubber on the motor, pull my sump and wipe the tank, flush body lines and go back with a CP3, LB7 up pipe and new down pipe. Hopefully should be good for a long time after that.
 

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That's crumby news!

Which Optilube product were you using just out of curiosity?
 

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I don't think it's a cp4 failure either. I think it's a clogged/dirty fuel filter.
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I learned my lesson in time, after reading all the misery of folks with the CP4. What about YOU? Will you have learned YOUR lesson in time (meaning "bought the bullet and gone to a CP3..?)
 

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Discussion Starter #13
Initially was running the Red bottle that takes 6oz’s per ~30 gallons. After that swapped to the summer + cetane which doses 6oz to 60 gallons. Had a 3oz container for my factory tank and 6oz to dose my 60 gallon aux tank. I will continue to use it. I think that’s the only reason why the truck would still idle and it didn’t just stop. At this point, just glad it did it close to my farm versus middle of Atlanta!
 

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Gee, I guess for those speculating a lift pump will potentially save the life of a CP4 this thread has not left me much confidence.

I personally don't run a lift pump. Just good old Racor fuel filters only.

I have been religiously running Optilube as well. I have tried XPD, Summer, Boost, Winter and their XL (purple bottle). That's their lubricity formula which is what I now only run all year with every fill up. I'm in Canada and will run the Winter formula for anti-gel properties in the colder months though.
 

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I learned my lesson in time, after reading all the misery of folks with the CP4. What about YOU? Will you have learned YOUR lesson in time (meaning "bought the bullet and gone to a CP3..?)
Wow...what an asshole. OP...sorry about your issues...best of luck getting it worked out.

Sent from my SM-A102U using Tapatalk
 

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Discussion Starter #16
Got all of my new parts installed this morning and truck runs well. Went with a modified CP3 from Lincoln Diesel and so far so good. Builds good rail pressure to 30k and starts as soon as you hit the key once I got the new rails and injectors primed.

The fuel system was full of shrapnel and the truck would still idle (barely). If your CP4 goes, be sure and drop the tank and change all of your high pressure parts. Was amazed at how much metal was in fuel tank, rails etc.
 

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suggest ANYONE with CP4 get the bypass kit as it can happen anytime after warranty runs out!
 
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