Chevy/GMC Duramax 04.5-05 LLY Discussion of Chevy and GMC Trucks with LLY Duramax Turbo Diesel Engines

Fuel Pressure Issues

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Old 11-01-2016, 12:21 AM
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Default Fuel Pressure Issues

Iím having fuel pressure issues that I canít seem to diagnose. Please bear with the long background but hopefully it gives a better idea of what I can try next.

Original issue was a loss of power when towing/under heavy loads. P0087 code would come up, put the truck into limp mode and prevent anything above 2000 rpm. To get by I would simply clear the codes with EFI Live and the truck would work as normal until the next heavy load.

I replaced the fuel filter multiple times and then replaced the fuel pressure regulator with no change. I have checked balance rates and they appear to be within spec. I shimmed the fuel pressure relief valve and did a bottle test to verify I wasnít loosing pressure through it. The next thing observed was the inability to increase fuel rail pressure above 80 MPa. I would increase pressure in 20 MPa increments while performing the bottle test but could not get anything above 80 MPa.

At this point I put the truck in the shop to replace head gaskets and while doing so I replaced the CP3 with a remanufactured one that came with an other new fuel pressure regulator. This is when I did the head studs, downpipe, egr delete, up pipes and exhaust manifolds. As you can imagine, it was incredibly frustrating to have the same issue once everything was back together!

While monitoring fuel pressure (actual vs desired) while driving it maintains pressure right up to 80 MPa and then drops off after that. I replaced some kinked fuel lines up near the brake booster, again with no difference. I have an o-ring rebuild kit on route for the fuel filter head thinking I may have air entering the system but the problem has now changed.

The truck now idles rough and sounds like a power stroke. Monitoring with EFI Live now shows P0087, P0088, P0089 and the actual fuel pressure is way higher than desired. Idling and very light driving shows a desired pressure of approx. 40 MPa with an actual pressure of 120 MPa or more! As soon as I try to accelerate (light or heavy) it bogs right down and the actual fuel pressure drops to about 20 MPa. After replacing some more rubber fuel lines (cross over by CP3) I no longer have the bogging down issue, but constantly have high pressure.

At idle, desired pressure is 40 MPa, actual is 90 MPa after warming up and pressure will rise relative to rpm. Unplugging the regulator while idling has no change on actual pressure but if I unplug the pressure sensor on the passenger side fuel rail while idling it will instantly jump to 180 MPa!!

Iím lost. Reading numerous forms on different sites all have partial similarities but nothing seems to match my situation. Any suggestions would be very much appreciated!
 
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Old 11-11-2016, 01:05 PM
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do you have a lift pump? and your new cp3, which engine series is it for, lly, lbz/lmm? and what series FPR did you have before and which one is in it now.
 
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Old 11-21-2016, 02:00 PM
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I don't have a lift pump. I'm trying to find the issue causing the problem before adding one.
I bought the cp3 for my engine series, LLY. It came with a Bosch FPR on it. I'm not sure of the original FPR that I replaced.
 

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