n03 LB7, P1093 code, diagostics have not found anything yet

texascadillac42

texascadillac42

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Have an 03 C4500 with an LB7 here at the shop. Truck has no power above 2000 rpms, which I assume is a limp mode, and will throw a P1093 code. Here is what has already been done, and monitored using a Snap On Solus Pro....

- Replace fuel filter with new Wix unit
- Check for vacuum at inlet to CP3, close to 20 inHG
- Commanded rail pressure to 160 at idle, actual is at 159-160
- Used clear line between filter head and CP3 to check for air leaks
- Balance test showed one injector around -3.xx, the rest were all closely grouped between -1 and +1.

Truck starts very quickly, regardless of how long it has been sitting. Idles smoothly, and does not seem to have any kind of miss at any RPM.

If you clear all codes, and drive the truck gently, the desired and actual rail pressure stay the same up to around 120, after 120, the actual rail pressure falls off sharply, down to 20 or so. When the RP drops to 20 or so, FPR % never goes to 0. If there is a leaking injector, or any weak component in the fuel system, shouldn't it? (0 means full pressure as I understand it)

A GM dealer replaced all 8 injectors a year ago, and a previous mechanic had installed a Bosch reman CP3 4 weeks ago.

I am pretty familiar with Cummins common rail systems, diagnostics, etc... but the Dmax is new to me. Thanks for any help.
 
First off check when the fuel filter was replaced last. If you do not know then put a new one on and recheck.
 
replace with gm fuel filter, wix have been known (in my cases) to make matters worst, even when they are new, might explain the 20inHG vacuum, that would indicate a restriction in my book, should be around 3-5inHG, i see it's a c4500, not sure but they may have 2 fuel filters, cant remember, if there is it would be on the frame rail, it may not have tho, hope this helps!
 
sorry specs say that on that particular vehicle (c4500) that the vacuum could be as high as 12inHG under load (highway) but 5in idling!
 
Didn't measure restriction, only measured how hard the CP3 transfer pump was pulling (checking to make sure the system was creating vacuum to draw the fuel in) Thanks for the specs though, they will be good to know for next time.

Turned out to be an improperly routed rubber fuel line at the FICM cooler, line was bent 90 degrees and basically pinched off. With the line correctly routed, the truck runs fine and has no codes.

Thanks for the ideas.
 
when you checked the vacuum and you said you got 20inHG, the bent/pinched line was your restriction, so that makes the vacuum pump on the back of the cp3 pump harder and causing the higher inHG numbers, so you kinda did check restriction, junk fuel filter (wix) can cause the exact same issue, glad you got it fixed!
 
Checking vacuum by pulling the rubber line off of the fuel filter head, and sticking a vacuum gauge directly into the rubber line is how I tested is the transfer pump was working properly. I wanted the pump to be working as hard as it could when we tested it.
 
okay, so your taking the fuel filter back to the tank out of the picture, i've done that a similar way but that would work too, good info for me as well!
 
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