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2001 Cav rough on CNG, but great on Gasoline

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  • 2001 Cav rough on CNG, but great on Gasoline

    Hi,
    I just started having this problem today. I have no power and my car runs very rough/sputters on CNG. I can floor it and it revs a bit but doesn't move much and then surges. I couldn't go faster than 50 on the freeway. I stopped to pull the fuze to run on petrol and the car runs fine with all the power it needs. I got my scan tool and checked to see what code was coming up and it was a P0133 O2 sensor code that didn't make much sense. I cleared the code and tried to run it again on CNG and it came up instantly with P0134 O2 code error. I am not sure what is wrong. Has anyone else had this problem?

    2001 Chevy Cavalier CNG/Gasoline
    Plugs and wires less than year old. Has run great since Oct. of 2008 when I purchased it.

    Thanks,
    Wayne
    Last edited by wacbass; 03-16-2010, 11:03 PM.

  • #2
    Re: 2001 Cav rough on CNG, but great on Gasoline

    P0133 is generated by the Powertrain control module (PCM) and P0134 is generated by the AF ECU. Both relate to the pre-converter O2 sensor operation. Other then providing power to the sensor, the PCM is out of the picture and does not generate P0134. Without being able to see how the O2 sensor is acting, (and the fact that you got two codes) my first guess would be the sensor performance is degrading--most likely by age or contamination. I would change the sensor and see what happens. What's going on is the O2 sensor feedback sets the engine mixture setting. Although failure of the sensor affects both systems, this may be more noticable during CNG operation than it is on gasoline operation due to the fact that the engine produces less power on CNG. If you contiune to have problems, there is possibly a connection problem between the sensor and the PCM or the AF ECU.

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    • #3
      Re: 2001 Cav rough on CNG, but great on Gasoline

      Thanks for the reply. I have plugged in the scan tool to see the O2 voltage and I get two options for DTC. 17 and 10. I assume one is the normal DTC and the other is the AF DTC? I am getting normal reading on the O2S11 and O2S12 in what I assume is the normal DTC. I assume the AF one (given the sputtering when putting the fuse in) is the one with O2S21 normal but the O2S22 is not showing up at all. It is at 0.0 V on the scan tool live data. So, is this the O2 sensor for the AF? Or is the AF computer not seeing the connection? My understanding is the sensor, if bad, would be at .450 V and not vary. I am scared the AF DTC has an issue with a perfectly good 02 sensor since the normal DTC registers the proper voltage. Which O2 sensor would this be? The one in the manifold or the one at the CAT?

      Anyone else have this issue? Anyone know of a certified, trustworth, honest and reliable shop that can help me out?

      Thanks for your help. This is a bugger to troubleshoot.


      Wayne
      Salt Lake City, UT

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      • #4
        Re: 2001 Cav rough on CNG, but great on Gasoline

        Wayne, try Lancer Automotive - 801-268-8863.
        -Derek
        www.CNGUtah.com

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        • #5
          Re: 2001 Cav rough on CNG, but great on Gasoline

          We are talking about the pre-convertor sensor which is located in the exhaust manifold. Unless you are using a Tech2 scantool, you might not be seeing the AF ECU data correctly. You are right, this is a bugger to troubleshoot as other things like the MAP sensor and the TPS can contribute to this. However, I still think with limited information, that the O2 sensor is the problem and for the cost of the part, it might be worth a try to change it. If the sensor is contaminated, it will read a high voltage (rich) causing the ECU to lean the engine causing the conditions you report. The same sensor feeds data to both the PCM and the ECU.
          Attached Files

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          • #6
            Re: 2001 Cav rough on CNG, but great on Gasoline

            Okay, now I get a check engine light for P0420....man when it rains it pours. Could this be associated with the same issue I am trying to fix?

            Thanks in advance.

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            • #7
              Re: 2001 Cav rough on CNG, but great on Gasoline

              Looks like it is the rear O2 sensor with that flow code. Gas is fine except that cat code about flow. Now with the new O2 sensor at the manifold I still get the same error with CNG but now the P1202? Gas Mas sensor low voltage....Is this from the after market O2 sensor?

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              • #8
                Re: 2001 Cav rough on CNG, but great on Gasoline

                No, you have two different things going on here. First does the car now run on CNG and gasoline after changing the O2 sensor at the manifold? The P0420 code relates to the after cat O2 sensor and is really determining if the cat is performing correctly. CNG cars are really hard on cats due to the heat generated when running on CNG. Also, on the earlier Cars (pre 2003) ignition problems usually caused by bad wires caused mis-fires that destroyed the cat--melting and plugging parts of it. I don't know the mileage on you car, but cat replacement might be getting close. If someone somehow put leaded gas in the car or some other fuel additive, the sensors (and the cat) could be having issues. If it is running correctly with no other codes, the P0420 code might go away after the new O2 sensor settles in. The P1202 code is GMS low frequency---which is telling you it is not seeing the AF ECU's requested amount of CNG actually being read by the GMS. A lot of times this is set when starting the car and the CNG flow hasn't reached the GMS for several different reasons one of which is the low pressure lockoff valve not opening during cranking. This code can be cleared and should go away on its own after three sucessful ignition cycles. A little code reader with code clearing button is a nice gaget to have in the glovebox on a CNG Cavalier!

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                • #9
                  Re: 2001 Cav rough on CNG, but great on Gasoline

                  Saia47,
                  Thanks for your knowledgeable input. I forgot to mention that the car is still acting the same and the O2 sensor (one of them) isn't showing up in my scanner. I have an ODBII scanner and have been clearing the codes to see what is up. I replaced the O2 sensor like you mentioned ahead of the Cat. Still acting the same. I am thinking of replacing the second one since I got those CAT codes. I have 136123 miles on the car. I have put 15 thousand on it myself. It was a government vehicle out of CA. I am worried that the ECU for the AF is not seeing the O2 sensor behind the CAT but the Gasoline one see's it fine. It appears to still run good on gasoline but as soon as I put the 20 amp fuse in for the CNG it starts to choke and act lean and the codes pop up. This is frustrating yet very rewarding because I am learning a lot. I am just worried crap is hitting the fan with all of these codes.

                  Thanks in advance.

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                  • #10
                    Re: 2001 Cav rough on CNG, but great on Gasoline

                    Sorry to hear you are still having problems. Without a Tech2 scan tool it is impossible to see what is going on with the AF ECU data. Because some of the sensors are used in both fuel system operations, the generic OBD II scan tool will report their outputs with incorrect data. You are correct in assuming that the AF ECU isn't seeing the HO2 post cat sensor--that is because the post cat sensor isn't connected to or used by the AF ECU. DTC P0420 is generated only by the PCM on the gasoline side. Seeing how the only oxygen sensor used in the AF side of the system is the pre cat O2 sensor (and you had a code) it was logical to look at that sensor as being bad. With the Tech2 you can see what is going on relating to NG flow through the system and O2 sensor performance as it relates to and controls fuel mixture. Without it, you and I are shooting in the dark. At the mileage you have on the car, the Catalytic converter perfomance is most likely suspect. I also would guess that the converter is at least partially plugged. You stated in your first post that plugs and wires were changed. If this was as a precautionary move to prevent a misfire, fine. If it was a response to a misfire then cat damage could have already been occuring. A lot of problems show up first when running on CNG as the engine produces less horsepower on CNG and therefore is working at higher throttle openings then on gasoline. There may be mechanical problems starting (intake leaks, cam wear, valves, etc.) or other common sensor issues (map, tps, temperature, etc.) that might be failing and will soon show up during gasoline operation. I know this isn't easy to do but you need to get your hands on a Tech2 and look at the fuel flow and air/fuel ratio data streams while the car is being driven to see where the problem is.

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                    • #11
                      Re: 2001 Cav rough on CNG, but great on Gasoline

                      Okay,
                      I finally took it into Lancer Automotive. They put a Tech 2 scan tool on it while it was running rough and found that the CNG computer was getting a different reading about the TEMP than the Gasoline computer. They called me up to inform me that I needed a new thermostat. Said they had a similar situation with a cav a while back and that is what fixed it. It didn't make sense to me since there are no wires to the thermostat that would lead into the computer......

                      Well what do you know, that is what appears to have been the problem. They replaced the thermostat and CNG is working great again...

                      Am I crazy to think the thermostat doesn't make sense?

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