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Misfire on CNG

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  • Misfire on CNG

    My car, a 2003 Cavalier, 110,000 miles, is running rough on CNG. The check engine light is on and I had it checked, it comes up with a misfire code on cylinder no. 4. As soon as it runs out of CNG it runs fine and the light goes off. The TRAC OFF light also came on, and that goes away also when back on gasoline. The spark plug was fouled out as well. Any ideas?

  • #2
    Re: Misfire on CNG

    Did changing the plug clear this up at all? Could be a bad coil pack, however that would usually cause a misfire in two cylinders, not just one (coil pack is shared). It is possible that the #4 CNG injector is stuck or not fully closing and is allowing gas flow after it is switched off. This would cause a rich condition. This can be checked with a scan tool during a cylinder balance test. It can detect a bad injector. Very unlikely, it is possible that the cylinder #4 relay that switches between gasoline and CNG is not energizing when on CNG. Each cylinder has a relay that controls which injector (CNG or gasoline) is to be used dependent on fuel selection. However, a failure in this system should set an additional code other then misfire and the problem would only exist until the gasoline fuel pressure ran low and you would then have a misfire from a dead cylinder. The relays are located behind the drivers side headlamp. There is a relay for each cylinder and a small black box located there called the fuel injection module (FIM) which controls the relays. Check the relays and wiring to see if there is a problem. Use a test lamp and probe the wire going to the CNG injector on the #4 cylinder and make sure it flashes when on CNG. If it doesn't, check the gasoline injector using the same method. The control module switches the injector ground lead so one lead should light the test lamp at all times and the other should flash. A bad injector should be your problem, and any dealer should be able to find it with Tech2 scan tool in about 5 minutes. The injectors are still available but cost $$$ . If you get the injector out, you might be able to clean it.

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    • #3
      Re: Misfire on CNG

      Thank you for the information, I will get that checked out. I also had the coil pack changed, along with new plugs, and it did not clear up the problem.

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      • #4
        Re: Misfire on CNG

        It almost has to be a defective injector. Any dealer (or anyone with a GM Tech 2 scan tool) can do this test even if he has no knowledge of the CNG system. You attach the scan tool to the car, and get to the alternative fuel screen. There is a section called "special functions" and one of them is the injector or cylinder balance test. The technician enters the test mode and the scan tool primes the fuel system by pressurizing the CNG fuel rail to between 35-45 PSI. The tool then starts with the #1 cylinder injector and pulses it for a predetrmined time after which the technician records the pressure drop in the fuel system. Usually the drop is a few PSI. The scan tool then re-primes the system and tests each of the remaining injectors. The pressure drops must be the same for each injector to pass the cylinder balance test. If the #4 injector is leaking or sticking, the pressure drop will be much higher then the other three injectors. I hope this helps you nail down the problem.

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        • #5
          Re: Misfire on CNG

          To run the test, does the car need to have CNG in the tank, no CNG, or it doesn't matter?
          And if the injector is bad, can it have CNG in the car to change the injector?

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          • #6
            Re: Misfire on CNG

            Yes, the car must have CNG in the tank to run the test. You can fill it and then remove the CNG fuse in the fusebox located next to the battery to get the car to wherever you are going to have the test run. You will need to re-install the fuse to run the test. After running the test, remove the fuse until repairs are made. The injector can be changed, if necessary, with CNG in the tank.

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            • #7
              Re: Misfire on CNG

              So you changed the plugs, but what about the wires? I had the same problem, turned out I had a bad wire. My wires were new, but when the guy replaced them, one of them was running near, touching the engine. I didn't know that this was bad, but it ruined the wire, and it caused some serious rough running, and backfiring. I would assume that you already checked/replaced your wires, but if you didn't, do that before you assume it is an injector.

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              • #8
                Re: Misfire on CNG

                This is grimmy's dad. His brother-in-law is trying to run the test and he called and said he needs to connect a fuel gage in order to see what the pressure is doing when the injector is fired. Would you happen to know the location of a port to be able to do this? He said he found a place on the gasoline side but can't find one for the cng. Any help would be appreciated.

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                • #9
                  Re: Misfire on CNG

                  The test can only be done with the GM Tech 2 scan tool. Any dealer or any shop that has a Tech 2 can do it. The Tech 2 uses the CNG fuel pressure transducer located on the engine fuel rail for reference. It also sends the required pulse to the injectors to run the test. There is no other way to do this without tearing the fuel system apart. The other test port you might see under the hood (on the low pressure regulator bolted to the transmission) is an intermidiate pressure test point which would not be of any use for diagnosing your problem.

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                  • #10
                    Re: Misfire on CNG

                    Kirk, from what I have been told, this car doesn't have plug wires. It has insulators that run off the coil pack, and that was all changed, and the problem did not go away.

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                    • #11
                      Re: Misfire on CNG

                      There are, in fact, no traditional plug wires in the Ecotec engine. However, there is a small spring inside each of the rubber boots that go between the coil pack and the spark plug. This is the "plug wire". If this is missing, the engine will miss also . I assume this stuff is all intact as you changed the plugs after the problem started and found the fouled #4 plug. Again, I stress that the use of the GM Tech 2 is the only easy way to verify if an injector is the problem you are seeing.

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                      • #12
                        Re: Misfire on CNG

                        Well, I had my cylinder balance test. All cylinders tested to 42 psi. #1 went down to 32, #2 went down to 32, #3 went down to 27, and #4 went down to 8. Obviously #4 is bad, and I ordered an injector for that, but is #3 going bad as well? How common is it for these injectors to go bad? Are they getting dirt inside, or do they just wear out?

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                        • #13
                          Re: Misfire on CNG

                          Yes they do wear out, however dirt might be more of a problem. You might want to keep the #4 injector and try to clean it. At the same time you are doing this work, change the fuel filter located under the car just inside of the left rear wheel well. The filter is a water and dirt trap. To drain it, unplug the connector in the trunk with the single wire on it located to the left of the CNG tank. This will shut off power to the high pressure lockoff valve. Start the car and wait for it to switch over to gasoline. This will drop the intermidiate fuel pressure from about 125 PSI to zero so the filter drain bolt and then the housing can be removed to replace the element. This is a standard Parker-Hanifin fliter and if the dealer can't get the element, a Parker dealer can. The injectors are sensitive to dirt and the filter is only used on the '03 and '04 Cavaliers. This should be the end of your problems. The #3 injector may continue to work. The computer has a "fuel trim" function that should adjust for injector imbalance that is not too far out of spec. When you reconnect the HPL connector in the trunk you may get a check engine or service light for 3 start cycles (or you can clear it with a scan tool). It's just letting you know there was a problem with the HPL circuit from when you disconnected it. Good luck with the car and come on back to the forum with any other info about your experiences---good or bad .

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                          • #14
                            Re: Misfire on CNG

                            I have a 2003 Cavalier that is misfiring on cylinders 3 and 4 at around 4000 rpms. I have been watching this discussion closely for I know the problem is either spark plugs, coil packs, or injectors. I have not had the problem on gasoline. I don't think the problem is the coil packs because one pack controls cylinders 1 and 4 and the other controls cylinders 2 and 3. With it not missing on gasoline, I don't think the problem is the spark plugs, plus my car only has 24,000 miles on it. That leaves the injectors. Can the injectors be cleaned? If so, how difficult is it remove them and clean them?
                            Jared.
                            Mountain Green, Utah
                            2003 CNG Cavalier
                            2003 CNG Silverado 2500HD

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                            • #15
                              Re: Misfire on CNG

                              We remove the injectors, flow test , clean, test fire them and replace them for $190.00. Or if you want to pull them your self we can do it for $115.00
                              Lancer Automotive Group
                              3687 South 300 West
                              Salt Lake City UT 84115

                              801-268-8863

                              http://lancerautogroup.com

                              Visit our Facebook pages
                              https://www.facebook.com/pages/Lance...68469439838785

                              https://www.facebook.com/alternativefuelcenter?ref=hl

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