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MY 2,000 GMC 2500HD Bi-Fuel - Gas works, CNG doesn't

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  • MY 2,000 GMC 2500HD Bi-Fuel - Gas works, CNG doesn't

    Looking at a GM 2500 HD. Think it's an IMPCO installation.
    The truck runs great on gas. You switch over to CNG and it shows that it is switched on the remote panel. However, after 100 miles the gas pressure does not drop and the regular gas gage does.
    After reading some of these forums it looks like some kind of filter clog? Possible pressure switch?
    I'm a pretty handy guy, is this something I can get a manual for and fix or do I take it to a certified installer.

    thanks
    Jim from Phoenix

  • #2
    Re: MY 2,000 GMC 2500HD Bi-Fuel - Gas works, CNG doesn't

    OK,
    Just had this diagnosed and the guy says the Fuel Management Unit is faulty. They want $1,000 to rebuild. Anyone know where I can get one of these or get it rebuilt for cheaper?

    Jim

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    • #3
      Re: MY 2,000 GMC 2500HD Bi-Fuel - Gas works, CNG doesn't

      Fuel Management Unit? What is the guy who diagnosed it talking about? There are two components that are the basic electronic controls of the system. The first is the AF-ECU (and that can't be rebuilt). The other is the GMS/MCV which (by qualified persons who are members of this fourm) might be able to be rebuilt. There are also several sensors and subsystems that feed data to the AF-ECU that could also cause problems. Your technician needs to understand how the system should operate, what checks it does before cranking the engine and what he should see as far as data is concerned during the start attempt. He needs to have the CNG supplement to the service manual to understand these things. How did the technician come to the conclusion he did? Did he have a GM Tech2 scan tool? If he did not, he can not see the AF-ECU or any codes or parameters of the CNG system. You need to get a list of DTC's from the Tech2 and then look at the requested and acutal gas flows reported to the AF-ECU to find out where your problem(s) are. No aftermarket scan tools will read the AF-ECU or even know it exists. Without the Tech2 you are shooting in the dark.

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      • #4
        Re: MY 2,000 GMC 2500HD Bi-Fuel - Gas works, CNG doesn't

        This was done at Tech ONe Auto here in Phoenix, a certified dealer.
        Yes they used a computer hook up tool. I am asking him more questions when he calls me back based on your answer above. Will get back to you. thanks,
        Jim

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