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2002 Cavalier runs rough on CNG

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  • 2002 Cavalier runs rough on CNG

    I noticed all the great information about the '03 Cavalier misfiring on CNG. I have a similar problem with a 2002 that I just received.
    When I first put CNG in it I had a little trouble getting it to start on CNG so I pulled the fuse and forced it to gasoline and re-inserted the fuse and it was able to start on CNG but ran very rough.
    I took it for an emissions inspection and it passed on gasoline but FAILED emissions on CNG. CO level was 2% too high.
    I drove it on CNG and it seemed to run pretty well until it was supposed to switch to gasoline but it seemed like it didn't switch.
    This morning it started on CNG but backfired a couple of times and I couldn't get it to run at any speed above idle on CNG. I pulled the fuse and ran it on gasoline.
    I had a spare low-pressure lockout solenoid so I switched that out and filled it with CNG again. This time it still ran rough and after a couple hundred yards of driving it switched to gasoline (CNG was now full). I could restart it on CNG but it kind of surges at idle and when I'm trying to hold a steady speed.

    Any ideas?
    2004 Toyota Avalon bi-fuel
    2013 Tesla Model S 85

  • #2
    Re: 2002 Cavalier runs rough on CNG

    How old are the spark plugs and plug wires? Check that the wires are in good condition and clean. Inspect the wires carefully where they all pass through an area where they are behind a bracket on the drivers side of the cylinder head just before the coil packs. There should be a piece of plastic wire loom there to keep them away from the bracket and head. Make sure the wires are in their holders and aren't touching any metal. CNG operation requires a perfect ignition system on these cars and although they may run alright on gasoline they act goofy on CNG. If you don't know the age, change the plugs and wires---not expensive and can't hurt anything. Next, do you have a check engine light? If so, what is/are the code or codes? Backfiring is usually from a lean fuel condition which could be a number of things from electrical problems, low fuel pressure, air leaks in the intake system to a bad gas mass sensor. After the car switches to gasoline, does it ever attempt to go back on CNG after driving for a while? You said "when I first put CNG in it" you had a problem. Is the car new to you and did it ever run correctly on CNG? Do you know it's history---i.e. do you think it has been run on CNG regularly or not? Hint: if its an ex government vehicle it might never have been run on CNG. If plugs and wires don't help, we'll keep chipping away at it until it gets running right--there are only about 500 things that could be wrong--and nothing that can't be fixed .

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    • #3
      Re: 2002 Cavalier runs rough on CNG

      I would check the pressure on the LPR, the high side is checked in PSI the low side in inches of water column. This would eliminate a HPR problem.
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      • #4
        Re: 2002 Cavalier runs rough on CNG

        Thanks for the suggestions. I bought it at an auction a month ago and I didn't get any maintenance records. I looked around the engine and noticed that the airbox had broken - probably from previous backfires. So I guess I'll start by eliminating air leaks and then look at the plugs, etc.
        2004 Toyota Avalon bi-fuel
        2013 Tesla Model S 85

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        • #5
          Re: 2002 Cavalier runs rough on CNG

          The air box for the CNG and gasoline Cavaliers are the same part---usually available from junk yards for a couple of bucks. Check the lower box where the cover screws attach, sometimes that part also gets cracked. Check the clamp that holds the gas distributor adapter (intake hose) to the throttle body on the rear of the engine. Also check the condition of the hose that connects the low pressure regualtor to the gas mass sensor (round thingy to the square thingy) and the gas mass sensor to the gas distributor adapter (square thingy to the intake pipe). These are very special hoses and if cracked, need to be replaced with a dealer part only. If the intake hose is full of dirt, very common after a backfire as the air cleaner element is bypassed) remove the rubber hose that connects the air cleaner to the gas distributor. Look inside the distributor with a flashlight and you will see where the CNG enters the airstream. If this area is coated with oil and/or dirt you need to remove the adapter from the engine (two more clamps and two bolts by the regulator) and clean it out with engine degreaser or scrubing bubbles bathroom cleaner. Rinse it well with water and blow it out with air before reinstalling. Backfires occur in these cars during starting even when they are running correctly. If you experience a backfire, check out the air cleaner afterward. GM even made a air cleaner modification with pop off valves to protect the air cleaner during backfire---trust me, the air cleaner still blows apart to protect the valves .

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          • #6
            Re: 2002 Cavalier runs rough on CNG

            Great info for preventive maint

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