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  • CNG leak while engine is running

    This is my first time posting here, so if I miss some etiquette, please forgive me..

    I have a 2002 Chevy Bi-fuel Cavalier. Factory Installed. When the engine is running on CNG, there is a small leak. I only get about 25 mpg on CNG, so I guess it is leaking about 10-15% of the CNG as it is running. I don't have any experience with these engines and am trying to see if I can get it fixed on my own before the expensive 95$/hr diagnostic and repair.

    Does anyone have any suggestions to troubleshoot this? Manuals for determining what is happening or a parts list so I can figure out what part goes where? Thanks for any help.

    Mike
    Kearns, Utah

  • #2
    Re: CNG leak while engine is running

    Believe me, if you were "leaking" 15% of your CNG, you'd smell it. It's the same for a gasoline engine...poor fuel economy. Start with the usual suspects.

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    • #3
      Re: CNG leak while engine is running

      Well, now that I got it registered and actually drive it, I have realized that it doesn't smell that bad all the time. It is mainly when the car starts up on CNG (no problems with normal gas). After that, I can't smell it any more really. I may have overguessed the leakage.

      I don't know what the usual suspects are. I can't currently identify the car parts that comprise the CNG delivery aspect. Anyone have a good pointer to where I can learn about those?

      I have tested the lines from the tank to the engine at the joints with dish soap and didn't find any leaking joints.

      Comment


      • #4
        Re: CNG leak while engine is running

        Here is a rundown of the CNG components on the engine of a 1998-2002 Cavalier.

        Most likely the "smell" is coming from the low pressure regulator. You will notice a vent on top of the regulator. A slight smell of NG is normal during start up. It should go away when the car is running.
        Attached Files
        Jared.
        Mountain Green, Utah
        2003 CNG Cavalier
        2003 CNG Silverado 2500HD

        Comment


        • #5
          Re: CNG leak while engine is running

          25 mpg isn't unusually low fuel economy. Depends on how you drive--is this city (short trips) or highway? How does the car do on gasoline and how are you calculating your CNG usage? If you are fueling at a station, record the number gallons used over several fillups and compare that with total mileage driven on CNG. It takes few fillups to even out the variables (pump pressure, temperature) to get a more accurate read. With a bi-fuel it should be close, but not quite as good as gasoline fuel economy .

          Comment


          • #6
            Re: CNG leak while engine is running

            Usual suspects-the same reasoning behind low fuel economy for gasoline will also apply to CNG. Still uses the same air filter and spark plugs. Agreed with siai47, 25mpg isn't necessarily low for an '02.

            If there is a leak, it's likely the LPR...the kit will run you $80-100. You'll find sources by doing a search on this board.

            Comment


            • #7
              Re: CNG leak while engine is running

              Ok, I can handle the small CNG smell when it starts up.

              Another issue is that the car will sometimes be hard to start and eventually start in gas mode when there is CNG available. This throws a troublecode. If I stop it and restart it, it has started up in CNG just fine.
              I have gotten some troublecodes off of the car. They are as follows:

              1138:
              High voltage during decel fuel cutoff
              The ECM has detected the catalyst monitoring O2 sensor was more than 650 mV for 10 seconds during a decel feul shut off condition
              Probable cause:
              1. Rich air/fuel ration
              2. Internal short in the O2 sensor
              3. Poor sensor ground in the ECM or circuit

              0202:
              Fuel Injector curcuit condition- injector number 2
              The powertrain control module monitors the guel injection electrical circuit for opens, shorts and system overload conditions.
              Probable cause
              1. Open or shorted fuel injector(s)
              2. Open or short in eletrical circuit
              3. High guel pressure-pressure regulator defetcive-fuel return line blocked.
              4. Check TPS and circuit for failure

              1202:
              Gas mass flow sensor low frequency
              Vehicle operating in CNG mode and the ECM detected teh actual gas flow was lower than expected
              Probable cause
              1. Failed GMS/MCV assembly

              The guy at the store didn't have any idea what exactly to do, so I turn to you guys for help. Anyone have experience with this? Thanks,
              Mike

              Comment


              • #8
                Re: CNG leak while engine is running

                Mike,

                First, understand that sometimes these Cavaliers ('98-'02) are hard to start. The CNG system works off of a vacuum. It takes several cranks to create the vacuum to "suck" the NG into the engine. Cheverolet knew this so after 8 seconds (or so) of cranking, the car will start on gasoline. I am unaware that when this happens it throws some troublecodes.
                Jared.
                Mountain Green, Utah
                2003 CNG Cavalier
                2003 CNG Silverado 2500HD

                Comment


                • #9
                  Re: CNG leak while engine is running

                  Highmarker is right.

                  These cars take some serious cranking in order to start on CNG, expect to crank from 3-6 full seconds before it will start (if the car has been off for a period of time. If the car has only been off for a few minutes, plenty of CNG should still be present as it hasn't had time to 'bleed out', and the car should start easily). Six seconds seems like an eternity.

                  IF you only crank for 1 or two seconds and then get nervous and stop trying, you will get a trouble code about 50% of the time (engine light comes on), this will force you to start on gasoline on your next try. This code is only temporary, and resets itself after three successful starts where the engine runs for a specified time/number of miles. I've never checked to see WHICH code(s) show up when you wimp-out on the cranking, so it could be that you are experiencing another problem, I just know this from personal experience.

                  In short, what you are experiencing is totally normal for this car.

                  P.S. The next time my wife wimps out while cranking, and the engine light comes on, I'll grab my OBDII reader and see what code(s) show up.

                  Comment


                  • #10
                    Re: CNG leak while engine is running

                    Highmarker is correct---sometimes these cars are a little hard to start for the reasons stated. However, a DTC code should not occur due to hard starting. The only code you have pulled that relates to CNG operation is the P1202 code. The description of the code is not very clear on what is going on and the failure of the GMS/MCV isn't the only thing that will set the code. In fact, if the car finally starts on CNG the GMS/MCV must be working. What the code really says is that there isn't enough NG passing through the gas mass sensor which causes it to report a flow lower then the lowest limit it is designed for. Because the flow is converted into a frequency output for the ECU to decode, low frequency means low flow. Now that we have that worked out, you need to know why there is no gas getting to the sensor. I already went through this on another thread, but one of the problems leading to hard starting is the failure of the low pressure lockoff valve to open during cranking. The valves sometimes get a little sticky and with the lower voltage the valve receives during cranking, it just doesn't open with a result of P1202 being set. If the car runs normally after you finally get it started on CNG, then I would look at the gas supply (lockoffs, regulators, etc.) to resolve this.

                    I feel a little guilty giving advice on Cavaliers now that one of my own is giving me fits. I think I have it worked out (will know tomorrow) but so far a lot of head scratching .

                    Comment


                    • #11
                      Re: CNG leak while engine is running

                      Mileage sounds about right. I see 23-25 with AC on during city driving and 25-26 with AC off. Gasoline mileage runs about 29 in the city. All highway driving and I have achieved 27-28 with variations from the pressure differences at fill.

                      I had a similar gas smell at start up coming from low pressure regulator LPR. We checked the LPR for leaks with a small cup of 1 part dish soap and 5 parts water. Start the car in CNG mode and use a small brush or sponge to dab the solution on the small vent that looks like a screen on the top of the LPR. With a little math, you can estimate the rate of gas loss by the amount of bubbles being released. Mine made a stink, but did not really impact mileage. There was a hole the size of a straight pin in the diaphragm allowing the leak to occur. It made a lot of bubbles steam off of the vent screen. The rebuild of the LPR is not very difficult. I purchased mine from Alternate Fuels.

                      Cranking is described well by others in this post. Post your success when done so we can learn how you resolved this issue.

                      Comment


                      • #12
                        Re: CNG leak while engine is running

                        If we turn the car on and let it crank the trouble codes don't show up. So that works just fine.

                        I have had the LPR tested and it was the source of the leak. I have the rebuild kit. Ran about $83 from a local fork lift parts dealer in SLC. Frontier. Good guys to work with. Is there anything special that I need to do before rebuilding it? Can I just let it sit for 5-10 minutes and then work on it straight? Do I need to let it run out of CNG before the rebuild? Will pulling the CNG fuse plug and then running it for a minute clear any CNG? What is needed to make sure that I don't uncork a high pressure line?

                        I do have a good tool set and feel reasonably comfortable, just want to know if there are any risks. Thanks,
                        Mike

                        Comment


                        • #13
                          Re: CNG leak while engine is running

                          You can rebuild the LPR in place and on the car. That way you won't have to remove any high pressure fuel lines or mess with the big hose (Gann vapor hose) coming out of the regulator. Remove all the parts of the regulator following the instructions in the kit. A little brake cleaner will clean up any oil residue that you might find in the base of the regulator. Make sure you understand and follow the instructions as they relate to the bending of the regulator arm included in the kit. Compare the new arm to the existing arm to get an idea of what it should look like. Take your time and you will be rewarded with a perfect working regulator . BTW--you don't need to remove any fuses, or run it out of fuel. The LPL isolates the LPR from the rest of the system. Removing the 20 amp CNG fuse still traps CNG in the high and intermediate stages of the fuel system. If you must drain out any residual CNG (if this makes you feel safer when working on it) remove the plug going to the HPR in the trunk. This is a single orange wire with a connector on it located by the drivers side of the tank. Start the car with the connector disconnected and when it switches over to gasoline, you have emptied the fuel system past the tank. This will set a check engine light that will clear after three ignition cycles.
                          Last edited by siai47; 10-26-2009, 09:31 PM. Reason: Added note about relieving CNG

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                          • #14
                            Re: CNG leak while engine is running

                            I would like to express my gratitude to those of you who gave me the knowledge and the courage to attempt this repair on my own. I have succeeded.

                            It wasn't without some struggles.
                            First I took the screw off the top of the spring retainer housing and that dropped into the top area of the engine. That was pretty easy to take care of. Then the spring retainer housing was stuck fast to the gasket. I had to pry it open and when it finally popped off, the spring sprang out and fell between the engine and the firewall. It didn't make it to the ground so I looked around down there and couldn't find it. Eventually I jacked the car up to search from the bottom and found it caught on a brake line. Whew...

                            Next I cover was stuck fast like the spring housing and it took a bit to get that off.

                            I got it disassembled down to the primary cover. Even with all the screws off, I couldn't get it off. I put a small nail into the pin holes and pulled up with a hook on the nail, I still couldn't get it off. I reasoned that the top diaphragm was probably my leak and just forgot about replacing anything deeper. I put it back together using the new parts and bending the fulcrum as directed.

                            Once put together I started it up and it ran on CNG just fine. I soaped up the vent on top of the LPR and found there to be very minimal leak. After 10 seconds only a small soap bubble had started to form where as before it was blowing bubbles like my son in his milk. =] I suspect a little leakage is to be expected and am happy to not have any CNG smell in the cab anymore.

                            Thanks everyone.

                            Comment


                            • #15
                              Re: CNG leak while engine is running

                              Our 2002 cavalier had a leak in the CNG line under the hood where it passed the air inlet on the top right rear of the engine (standing facing the engine) before it reaches the low pressure solenoid. The difficulty with finding this was that the line was up against something and it did not leak constantly. I recall that there were error codes, but the service tech didn't find the leak. I found it at home. I believe they actually replaced the low pressure lockoff solenoid by mistake. It cost me $900 to replace the entire line.

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