LMM rail pressure issues

[Lowboost]

Like many before I am chasing the p0087 gremlin. Here are the details. 07.5 LMM with 212k. Emissions intact. FASS Dmax lift pump. Brand new GM filter head and AC Delco TP3018 filter. Balance rates are not more than -1 or +1 on any cylinder. Bottle test on injector returns are empty. Rail plug installed and helped some. Here is the issue: empty the truck never stumbles. Put a trailer on with 4K lbs, it will limp every 30 miles. Had it on a scanner today and it holds 145-ish Mpa until 5 or 6 gear then it slowly drops to below 130mpa at full throttle. Can anyone help me with where to look next?

 

[Ne-max]

Test return on the cp3

 

[Lowboost]

Test return on the cp3

Is this something I can do or need my diesel shop to do? I see tons of these p0087 threads and I'm trying to avoid redundant questions and this isn't an answer I've commonly seen. All help is greatly appreciated.

 

[Ne-max]

It's similar to the injector return test. Just unhook return line from cp3.

 

[2004LB7]

I think it is 35 mL or about 1.1 oz for the cp3 with 15 sec of cranking

 

[SSchmi5519]

It needs to be holding 180MPA. Anything less than that is sign of a failing CP3...especially at a stock power level.

 

[Lowboost]

It needs to be holding 180MPA. Anything less than that is sign of a failing CP3...especially at a stock power level.

Ouch. Is it common for cp3's to check out at this mileage? I'm the second owner of this truck and knew the first owner. Always got a new fuel filter every 12k miles since new, I consider that reasonable maintenance.

 

[Lowboost]

It's similar to the injector return test. Just unhook return line from cp3.

How difficult is it to get to the cp3 return line?

 

[SSchmi5519]

Yeah at 215k you're in that territory. Keep in mind the CP3 will always return the fuel that doesn't go through the regulator(FPR), especially with a lift pump....you'll want to disconnect that probably to get any sort of valuable information.

 

[Burn Down]

Lmm’s have fuel line issues. Check the fuel lines at the left rear of the engine where they quick couple to the hardlines. I had a truck with p0087 issues with a lift pump and it was that line kinking under load and popping rail codes. Feel the line and see if it’s been kinked.

 

[Lowboost]

Lmm’s have fuel line issues. Check the fuel lines at the left rear of the engine where they quick couple to the hardlines. I had a truck with p0087 issues with a lift pump and it was that line kinking under load and popping rail codes. Feel the line and see if it’s been kinked.

Thanks, will check tomorrow. Curious if you know how this happens. I do see something that's been leaking on the frame right in there.

 

[Burn Down]

The flexible tubing they used is sh*t basically.

 

[NC-smokinlmm]

Yep, dimes to dollars it's the fuel line collapsing...

 

[Lowboost]

Update. After replacing all the soft lines on driver side, the low rail pressure issue is gone. While it was under the knife I went ahead and put a 3" down pipe, 5" magnaflow exhaust and DSP5 tunes and switch from PPEI. Wow is all I can say. Tune 5 sure feels like it could break something. I have not gone full throttle for more than 3 seconds. Is tune 5 from PPEI big enough to break a piston?

 

[NC-smokinlmm]

Lol. It's a gamble on any tune. If your max effort stock trans have fun and pucker your butt!!!

 

[Lowboost]

Lol. It's a gamble on any tune. If your max effort stock trans have fun and pucker your butt!!!

Not sure if it's max effort or not it's just PPEI regular #5 tune. Supposed to be 150 I think. It seems to run harder than my 15 LML that was max effort. It certainly seems to light the turbo a lot faster.