I am looking for help in diagnosing a major problem with my 2006 GMC 2500 HD Duramax
fuel mileage. I have 155,000 miles on the truck and have been very very happy and problem free until about 8 weeks ago. I have had many other GM 2500's but this is the first diesel. Most miles on my truck are long distance highway miles and I have used since it was new, an Edge controller set to economy(number 1) for better mileage. I had been getting as good as 22 MPG up until 8 weeks ago. Now I can hardly get above 13 when I nurse it.

8 Weeks ago I was on a long trip and the Low fuel rail pressure light came on and the vehicle went into safe mode. I reset the code once and continued for another 10 miles when it came on again. I went immediately to the GM dealership in Rock Springs, Wyo where the tech told me I had bad fuel. The tank was drained, cleaned and refilled. He indicated all appeared well and there was proper flow through the fuel system. I finished the drive back to Illinois without any more issues except that the fuel economy was not good and it seamed like a slight loss of power.

On the next drive to Bozeman, MT I barely got 11 miles a gallon. I noticed the boost reading on my Edge unit was registering higher than in the past. At highway speeds what used to be 5- 6-7 was now 9-10-11-12 and starting from a stop or passing the boost was into the upper 20's. When in Bozeman I decided to unplug the Edge unit and see if things improved. Somehow the stock program did not reload correctly so I had to be towed into the Gm dealership there where they reprogrammed my truck computer to stock. I explained my mileage problem but they could find no issues. I did not reinstall the edge unit at that time wanting to see if anything had changed. While in Bozeman they did changed my fuel filter. The truck appeared to run fine except I still had the terrible fuel mileage.

I drove to Boulder with no improvement in mileage and ended up going to the Boulder GM dealership where they did another diagnostic and found nothing wrong. They saw the empty Edge holder and blamed all the problems on that. They do not do much diesel work so I put little stock in there abilities. By now I had the same problem with the Edge unit in and out of the vehicle. At this point I reinstalled the edge unit and made sure it was updated to the most current program.

The vehicle still seems to run well but the mileage is still not where it should be. The boost registering on my Edge gauge is still a couple points higher than what it would run at before. Yesterday I discovered while at highway speed and going down a very long downhill I shifted into neutral and the RPM's WENT UP to about 2k and the Boost stayed at 2 PSI. I tested again and they same thing. At startup or once I pull up to a stop all appears where it should be. It seems to me the truck coasting should idle down to 670 rpmish and the boost should be 0. This is the only clue to what may be going on. I went back to the Boulder dealership and explained my observations. The tech was pissy and completely useless in trying to solve the problem.

I am desperate to solve this as I can not afford the brutal fuel costs due to the long distance driving I do for work. If anybody has any clues about where to begin looking for the problem I would be very grateful.

Thanks

 

Can you check your fuel rail pressure with the edge.
Darryl

 

x2 u can use the edge to do some monitoring, i know u can check and look at the rail pressure. i think you can look at vane position as well (turbo).

first thing to observe would be the rail pressure while driving and under WOT

 

Thanks for the help. Yes my programmer can tell me fuel rail pressure as well as a whole bunch of other indicators. Please let me know if there are some other indicators(turbo vane%, CFR, manifold pressure?) I should be looking at. I will go do a test drive again and get a fuel rail pressure reading then let you know the outcome. Any idea what the fuel rail pressure should be at when driving at highway speed of 68-75?

Thanks

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Here goes... During the test drive at 65mph the rail pressure was about 9.5 on level ground and 10.5 on a very slight incline. The boost was averaging 5-6 which is higher than I have needed in the past before the problem. At idle sitting still rail pressure was about 4.5 and boost does go back to 0. I tried again putting the truck in neutral while at 65 mph and the boost would not drop below 2 as well as the engine rpm went up to 2000. Once I got home I did check the vane position at idle and it seems that it is at 20% while idling still. Any ideas on what these readings may point to as the problem?

Thanks

 

have you checked your air filter or MAF sensor??

 

we need a wot fuel rail pressure reading.

 

I hate to ask but what is the easiest way to do the WOT test myself and can I do it without risk of damage knowing how my truck is running?

Thanks

 

Watch your Edge and hold it to the floor.

 

The fuel rail pressure is 15-16 when in park and full throttle. The fuel rail pressure on the road with full throttle is 25-26.

What range should my injectors be reading at on the Edge controller? Currently I have one cylinder at 4.0 and the others all between 2 and -2.2.

Last, what is the best, most thorough way to clean the injectors? The dealership did a flush and I do use additives but I am not confident in how clean they are especially after the really bad fuel incident that started this whole mess.

 

its 15-16kpsi at idle?

the balance rates spec for the injectors are +/-4 in park and +/-6 in gear with your foot on the break.