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Cavalier with clogged catalytic converter

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  • Cavalier with clogged catalytic converter

    A bit of background, I bought my 2004 cavalier about a year ago and it didn't run on CNG because one of the injectors was bad. I took it to a CNG mechanic who replaced the Keihin blue 52cc injector with a Keihin orange (63cc) injector. He didn't tell me he used the wrong kind of injector and I didn't discover that until June. Anyhow, I ran it on CNG for a while in that state, until it stopped starting on CNG. It also became less powerful on both CNG and gasoline and started giving me a P0420 code which normally indicates a bad catalytic converter.

    Now, I've replaced that injector, and it runs on gasoline OK except that it's less powerful than normal, and on CNG it misfires a bit if the engine is cold and it's idling - after driving it 20 feet it's fine. Could this be caused by a clogged catalytic converter? I've looked at the spark plugs which look good and replaced the ignition coils. I don't know how to weld so I can't replace the catalytic converter myself and the mechanic I've taken it to said he wouldn't replace it because it was misfiring.

  • #2
    A misfire can toast a catalytic converter--particularly on CNG because of exhaust gas temperatures. The mechanic is correct to be hesitant to change the converter while you have the misfire. I wouldn't worry about it if the misfire is as you describe. The damage to the converter occurs under high load and high temperature operation which melts the converter internal structure. The restriction from the damaged converter is much worse than the possibility of damage to the replacement. After the backpressure is reduced the idle problem may clear up. If you can find someone with a Tech 2 scan tool, I would suggest you perform the "injector balance" test to make sure all your CNG injectors are flowing the same amount of fuel.

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    • #3
      My 03 had what I thought was a minor misfire that I didn't take care of, and sure enough, melted the converter core. Being in California, the replacement C.A.R.B.approved converter would have been about $700 installed. I sold the car as is with disclosure of the problem. The buyer was in Utah, and got a new non carb converter for about half .

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      • #4
        Thank you for the replies. I got the catalytic converter replaced, ran it on gasoline for a while to get the engine warm, then put in the alternative fuel relay and started it on CNG. It was misfiring badly at both low and high RPM so I stopped the engine. It seems very odd to me that replacing the catalytic converter would have made the situation worse. After a bit more driving on gasoline a pending OBD code appeared, p0171 but it has not been confirmed. Any thoughts on what to do?

        Does anyone have a Tech 2 who would be willing to help me figure it out? I'm in Provo UT and at this point am willing to drive some distance to figure this out.

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        • #5
          Originally posted by dkane View Post
          Thank you for the replies. I got the catalytic converter replaced, ran it on gasoline for a while to get the engine warm, then put in the alternative fuel relay and started it on CNG. It was misfiring badly at both low and high RPM so I stopped the engine. It seems very odd to me that replacing the catalytic converter would have made the situation worse. After a bit more driving on gasoline a pending OBD code appeared, p0171 but it has not been confirmed. Any thoughts on what to do?

          Does anyone have a Tech 2 who would be willing to help me figure it out? I'm in Provo UT and at this point am willing to drive some distance to figure this out.

          Lancer Automotive ?

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          • #6
            The code you are getting says the air/fuel ratio is so lean that it is out of range of the "fuel map" and cannot be corrected by fuel trim. I would suspect that one of your CNG fuel injectors is at fault. Again, someone with a Tech 2 (possibly Lancer) could find out in a hurry if you have an injector problem. Don't run it on CNG until this is resolved. The reason the problem seems to have gotten worse is that in reducing the backpressure in the exhaust system it is leaning out the already lean fuel supply system. The engine is now passing more air through it but the fuel amount is unchanged. Clear the DTC and see if it returns on gasoline only operation. Again, the Tech 2 can show you where the fuel trims are at. BTW--this could also be a result of an intake leak. Be sure there isn't any problem where the CNG injectors plug into the intake manifold gasket/adapter. It is rare that this code would show up on both fuels NG or gasoline as the systems are entirely independent of one another except for the air side.

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            • #7
              I think I found out what the problem was. I did a simple test, although not a completely precise one (I used a balloon as a slightly compressed air source and measured the volume of water displaced on the outlet) and found that the new injectors flow half as much CNG as the old ones. I probably ought to do the test more carefully. I think I'll use a short 12V pulse with the injector in the fuel rail on my car. Does anyone know how the rated 52cc for the injector is determined? Is that with some standard pulse duration?

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