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need technical advise please

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  • need technical advise please

    I just bought a 2001 ford F-150 (7700 series) it has the cng, gasoline conversion. When i bought the truck it ran fine on both systems. Recently the cng has been acting up what it does it starts up only if i hold the pedal down a third of the way once started if i release the pedal to an idle it will stall out. The only way i get to use the cng is if i go on highway running gasoline and shut the truck off and switch the button to cng get it started and put it in drive. Once running on the highway on cng it runs fine any suggestions or similiar problems anyone has occured to. Thanks

  • #2
    Re: need technical advise please


    Not sure where to start on this one, but, larrycng has just posted some of the older E,F-150 Ford manuals in the Library. They might help. I don't think the systems changed that much.

    Sounds like something to check would the the throttle position sensor? Just a thought.

    Good luck,



    • #3
      Re: need technical advise please

      I'm in exactly the same boat; and worse, not living in Utah I have only one Ford dealer skilled in CNG. I wasted a day w/ these guys, who said all they could do was pull the plugs, injectors and start sleuthing for the cause....but just opening up the plugs/injectors would cost me $1200. no thanks.


      • #4
        Re: need technical advise please

        The throttle positiion sensor would be good and also the Manifold Absolute Pressure (MAP) sensor (the small vacuum line going to the GFI compuvalve) make sure open and has vacuum while the engine is running.

        Has the Compuvalve been flushed? Has the coalessing filter been checked?



        • #5
          Re: need technical advise please

          My first thought is one of the high flow injectors and possibly the low flow injectors are gummed up. The two black injectors in the Compuvalve are pulse width (they chatter when operating) and the 5 or 6 injectors on the side are high flow. They just open and close, they dont pulse. Each injector flows a specific amount of fuel through different size orifices. I have seen on some engines at idle, just one high flow injector and occasionally one low flow injector working, and on others, just the two low flow injectors. You can hear then snap open if the throttle is jabbed.

          My closest bet is to run some denatured alcohol through the Compuvalve. It is possible to do it on a running engine by partially filling the connection hose between the high pressure regulator and Compuvalve with denatured alcohol and starting the engine. It usually stalls pretty soon. It also helps to do it on a warm engine, it helps the alcohol to evaporate.

          Be careful of any spilled alcohol, it can take some paint off and is flammable.

          It sounds easy but can be a bear to do correctly. GFI used to supply an "inj-flush" program with their F-mon monitoring program.