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won't run on cng

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  • won't run on cng

    I have a 2002 bi-fuel cavalier. It started sputtering on me when running on natural gas and the check engine light came on. I pulled over and pulled the cng fuse and it runs fine on unleaded with no engine light on. I tried to run it a couple of more times on cng with the same result, it even died on me once. Checker auto diagnosed an O-2 sensor. Does this sound right? Any ideas on what else it might be?
    Thanks Shane
    Last edited by shane.olsen; 02-16-2008, 11:38 AM.

  • #2
    Re: won't run on cng

    Where is the CNG fuse in case I ever need it?


    • #3
      Re: won't run on cng

      No, it doesn't sound right.

      Your problem sounds like cng fuel starvation.

      First off, Checker's scan tool is probably just like my actron OBD2 generic scanner, which means it is incapable of pulling CNG related codes. At least it is on my model year 1998 cavalier.
      I purchased a genisys recently, a professional level scan tool, and IT was incapable of pulling out CNG related codes. Hence, I am on a mission to find a GM Tech2 (GM dealer scan tool) at a reasonable price, so that I have access to the scan data and codes for these systems.

      You may have an o2 sensor code in history due to the drivability problems on cng. The cng system still uses a closed loop feedback system utilizing the o2 sensor signal to maintain air/fuel ratio.

      I would be curious to see if my actron or genisys can pull codes on later model years, as I am told the menu structure is broken out a little differently in the Tech2, as later model years are labeled "alt fuel veh".

      The 20A cng fuse is located in the engine compartment fuse relay center.


      • #4
        Re: won't run on cng

        Removing the 20amp CNG fuse is one way of turning off the CNG system. This disables the alternate fuel ECM by shutting off the power to it. In addition the red light on the fuel switch will be out and no check engine light will illuminate. Another way to turn off the CNG system is to remove the power to the lockout solenoid. This is accomplished by taking apart the small single wire connector located in the trunk between the rubber boot covering the lockout valve and the trunk liner. This is useful if you want to purge the fuel lines from the tank to the engine or check that the lockout valve is working correctly. After the connector is removed, start the engine---the fuel gauge will immediately plunge as the pressure in the line is used up. The engine may transfer to gasoline or stall. In any event, the engine can be restarted on gasoline. The red light on the fuel switch will be on along with a check engine light. Once the connector is reattached, the vehicle can be restarted on CNG. The check engine light will go out after 3 sucessful starts on CNG.


        • #5
          Re: won't run on cng

          Yoy may need to look at the LPR (Low pressure regulator) and maybe the Gas Mass sensor, what other problems were you having before this stuttering issue, was there any Natural Gas smell? any other check engine lights?Kevin


          • #6
            Re: won't run on cng

            I agree with Kevin on this one. It has to be the low pressure regulator or the gas mass sensor. These are the round and rectangular boxes on top of the engine.
            Mountain Green, Utah
            2003 CNG Cavalier
            2003 CNG Silverado 2500HD