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New to CNG. Help

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  • New to CNG. Help

    I just baught a 2001 cav bi-fuel and was told the CNG doesnt work on it. How do I check that? The light on the switch does not turn on when I try to switch it over. Im not savy on these vehicle's at all. Help?! How do you tell if the map sensor is bad?

  • #2
    Re: New to CNG. Help

    You need to go through the old posts on this site to get an idea of how the system on the Cavalier is supposed to work. There is no "changeover" that can be controlled by the driver. The system runs on CNG until it runs out and then switches automatically. The switch on the dash is just to check the fuel level in the gasoline tank when running on CNG. If the lamp in the switch is not on and you are running on gasoline then the fuse labeled "CNG" under the hood is blown (or someone removed it), or the bulb is burned out, or the AF ECU is kaput. When you first turn on the ignition, if the fuse is good and the AF ECU is working properly, then the bulb should light for a couple of seconds and then go out. When running on gasoline, the light should be on, when on CNG the light should be out. A simple scan tool can read the MAP output and you can compare it to local pressures with the engine stopped and watch if it changes when the engine is running. If you are going to be working on your own car, you need to pick up the GM service manual supplement for CNG cars. Any supplement you find for 1998 through 2002 Cavaliers will be generally correct for your car. Working your way through any problems you have with the vehicle will be a lot easier. Welcome to the site and the world of CNG .
    Last edited by siai47; 12-18-2009, 07:35 AM. Reason: added info about supplement MY


    • #3
      Re: New to CNG. Help

      Right. You can't "switch it over" from inside the car. The only way to make it switch to gasoline if there is CNG in the tank is to pull the fuse under the hood (assuming everything is working properly). The pushbutton inside the car will show you the gasoline tank level while running on CNG, but it takes forever to move (on our 2003) and so is not very useful.
      02 GX
      01 GX
      03 Crown Vic
      06 GX
      Home Fueler


      • #4
        Re: New to CNG. Help

        Originally posted by freedml View Post
        The pushbutton inside the car will show you the gasoline tank level while running on CNG, but it takes forever to move (on our 2003) and so is not very useful.
        The pushbutton works properly when the vehicle is stationary for several seconds. I try to check it when I'm in park. It cycles in about 15 seconds. If you are driving on CNG and want to check the gasoline level it takes several minutes and never really seems to work properly. It's the same as when you're driving along and the vehicle switches from CNG to gasoline. It takes a couple of minutes before the true gasoline level is shown on the gauge.

        This attachment is from the 2001 GM alternative fuels manual but they didn't put the same wording in the 03/04 Manual.
        Attached Files
        2004 Toyota Avalon bi-fuel
        2013 Tesla Model S 85


        • #5
          Re: New to CNG. Help

          Thanks for the replies people! I really appreciate it! I now have a 1202 code with the check engine light on. I was told it is a gas mass flow sensor. Is that the same as a map sensor? The dealer said the part is $2500.00. Is there a place to get parts cheaper? Thanks again!


          • #6
            Re: New to CNG. Help

            The short answer is "NO"--it is not like a MAP sensor. On top of the engine is a rectangular box with a large hose going into it along with a couple of electrical connectors. This is the GMS/MCV--the $2500 part the dealer quoted you--and the part that DTC P1202 is refering to. This is a problem child for all Cavaliers and pickups built until the design was changed to fuel injection in the later model years. The GMS part of the assembly is a gas mass sensor. Basically, it measures CNG flow going through it. If the requested CNG flow from the computer isn't being reported back by the GMS, then the computer cancels CNG operation and sets the code. Although a failure of the GMS/MCV is the usual cause for this code appearing, it is not the only cause. A couple of questions are in order. First, has the car ever run on CNG since you owned it? Even with the check engine light on, does the car run on CNG (even if it is only once in a while)? Did the check engine light come on while driving or did come on when trying to start. When trying to start on CNG does the car fire for a second or two and then crank until it changes over to gasoline operation? Have you been able to clear the code (with a scan tool or by disconnecting the battery) and then been able to resume CNG operation? The reason I ask is sometimes this code is set by lack of CNG getting to the GMS and with no gas available to measure DTC P1202 is set. The most common problem causing this scenario is a failure of the low pressure lockoff valve to open. A proper scan tool can show the requested CNG flow from the computer and the actual flow reported by the GMS to nail down the problem. If the GMS/MCV has failed choices are getting one off E-bay or having one of the members on this site try to clean your existing unit. GM considers the unit non-servicable and would really like to drain you pockets of $2500--they have a lot of debt to repay !