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Help! GMC Sierra 2500

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  • Help! GMC Sierra 2500

    I have a 2005 GMC Sierra 2500HD. I have been getting about 145 miles per 10 gallon fill. I recently put bigger wheels and tires on my truck and my millage went down to about 125 per fill; which is understandable. That millage only lasted for 2 fills and now I'm only getting about 95 miles and when I fill up I'm only able to get 8-8.5 gallons. When before I was able to get 10-11. As soon as I get to about 80 miles the truck cuts out and has no power if it goes above 2700 RPM. If I keep the RMPs low it runs ok. This happens while driving or standing still and reving the motor. (it cuts out above 2700 RPM) If I have full tanks it's fine. Anyone know what's going on??? Thanks!

  • #2
    Re: Help! GMC Sierra 2500

    What system do you have? More than likely the calibration in your computer is having issues with updating itself.

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    • #3
      Re: Help! GMC Sierra 2500

      I have a Technocarb conversion. I took it to a local shop, CNG Technologies, and he hooked up his computer and said he did a factory reset, but it didn't help and nothing has changed.

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      • #4
        Re: Help! GMC Sierra 2500

        check the high pressure filter it running good when full sounds like it could be pluged and when full it is forcing it through and at lower pressure it is not getting a pressure drop across the filter to where the motor is running at to low a pressure just a guess my dodge did the same thing a new filter fixed it good luck

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        • #5
          Re: Help! GMC Sierra 2500

          opps it is getting a pressure dope across the filter

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          • #6
            Re: Help! GMC Sierra 2500

            I'm assuming that truck is bi-fuel (correct me if I'm wrong).
            Does it run normally on gasoline? If so the problem is probably with CNG (redundant, yes but question needed to be ask).

            Check the cng pressure under load at the inlet to the HP regulator. If it not the same as tank pressure, you have a restriction (tank valve, filter or pinched line)

            check the pressure at the mixer/injectors whatever (Not familiar with the system). If it is not within specs start looking for restriction in LP filter or pressure regulator, LP lock-off (if equipped).

            Filters should have very little pressure drop if they are not plugged or restricted.

            After the above checks start looking at injector/mixer problems, then check timing advance (scantool time).

            Does it go above 2700 rpm int he lower gears??

            Just a few basic questions and ideas

            Larrycng

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            • #7
              Re: Help! GMC Sierra 2500

              Thanks for the tips so far, appreciate it. Changing the high pressure filter sounds like the easiest fix so I'm going to start with that.

              Yes, it is a bi-fuel truck, and yes it runs normally on gasoline.

              It runs great on CNG until the tanks get low. Once they're low it cuts out above 2700 RPM and the lower the tanks get then it starts cutting at 2500, then 2400...2200...etc... Doesn't matter what gear I'm in, even in Neutral.

              I go to fill up and I only get 7.?? to 8 gallons when I used to get 10+.

              I'll try the high pressure filter and report back. This may be a dumb questions but where is the filter located?

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              • #8
                Re: Help! GMC Sierra 2500

                More than likely there is not a filter on your system. What type of tank valve do you have? Did you buy the conversion, if so where from?

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