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CNG 2001 Chevy Cavalier revving up and down whether in park or drive?

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  • CNG 2001 Chevy Cavalier revving up and down whether in park or drive?

    Hi,
    My 2001 Chevy Cavalier, whether running on cng or gasoline revs up and down. It is a high rev and sounds like someone is stepping on the gas pedal and let off. It happens 1 or 2 times a second. What started the whole thing is my car backfired (sound came from drivers side under the hood). I pulled over and saw the air filter cover was completely cracked, so I took out the cng fuse and drove off. About 30 seconds later it felt like my gas petal was stuck because I kept accelerating. I pulled over and put it into park and the car was throttling up and down in regular intervals (as described above). I think there may be a vacuum leak? I changed the Mass air flow sensor and bought a MAP (Manifold absolute pressure sensor) but I don't know where it is located? There is no "check engine light" on. Can someone tell me if I should change the MAP or is there something else wrong with the car?

    Thanks.
    Last edited by midijeep; 09-15-2012, 07:28 PM.

  • #2
    Re: CNG 2001 Chevy Cavalier revving up and down whether in park or drive?

    A backfire can cause hidden damage. Erratic idle most likely is caused by a air leak in the induction system between the engine and the throttle body. The intake manifold could have a crack in it or a gasket could be blown out of position. I would remove the air intake system (from the aircleaner to the throttle body) and then the throttle body itself. I would then inspect the manifold carefully for any damage along with it's gaskets. The MAP sensor is located on the manifold right next to the throttle body. I must be getting old as I cannot remember these cars having a MAF (mass airflow sensor) on the gasoline side---where did you find it is located? The other thing to look at is the IAC (idle air control) valve located in the throttle body. Remove it and clean the port that it goes in and check for damage to the rubber plunger attached to the end of the valve. I know all this stuff is on the back of the engine so it is had to see but these are the areas you need to inspect.

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    • #3
      Re: CNG 2001 Chevy Cavalier revving up and down whether in park or drive?

      Thanks. I will be try all of what you indicated today. Should I turn off the gas cng valve in the back of the car before removing the throttle bogy?
      Also, I original asked for the Mass air flow sensor but the dealer said he could find one for the cng car but referred me to the MAF. It is located between the air filter and the engine (the large oval rubber rubber coming off the air filter cap).

      I suspect that I have have some kind of air leak for a while. For about 6 months, running in cng I would get a loud backfire when trying to accelerate too quickly (the computer is running the cng too lean). I think yhis last event did something as you describe. I'll be posting back what I find today.

      Thanks!!!!

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      • #4
        Re: CNG 2001 Chevy Cavalier revving up and down whether in park or drive?

        I removed the large diameter plastic tube that attaches to the throttle of the gasoline carb.. I found a piece of plastic that was blown off during the last backfire. It came from some where in the intake and was lodged between the butterfly of the throttle, Took it out reassembled and the car started right up and was idling perfectly on gasoline. Took it for a drive and drives great.

        I did find a vacuum hose half way on underneath the cng intake box. I reconnected it, I haven't put in the cng fuse but I'm hoping this solves the backfiring when I stomp on the accelerator in cng.

        Thanks for your help!!!
        Last edited by midijeep; 09-16-2012, 06:10 PM.

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