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2004 Cavalier. CNG quit working any ideas?

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  • #1
    Re: 2004 Cavalier. CNG quit working any ideas?

    Thunderstik,

    The "ping" of the tank when you turn your key on, is the electronic solenoid valve. I would start there, but I'm not sure of the reason for leaking gas.
    Jared.
    Mountain Green, Utah
    2003 CNG Cavalier
    2003 CNG Silverado 2500HD

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    • #2
      Re: 2004 Cavalier. CNG quit working any ideas?

      When you apply pressure to the fill port in the Cavalier, it will pressurize the fuel system lines regardless of the operation of the high pressure lockout valve. This may be why it will run for a few seconds when you fill it. The fact that it runs for a short time and that you stated that the ping from the tank did not sound as loud as you remembered might make you think that the solenoid valve in the tank is plugged or otherwise not fully operating. What does your fuel gauge do during the short time that it runs on CNG? If it reads full when connected to the gas pump and then falls rapidly after you start on CNG, then the high pressure lockout valve is not operating. There is a single pigtail connector located by the round rubber cover on the left side of the tank. One end of the connector comes from the ECU an supplies power to the solenoid valve in the lockout valve assembly. You should have battery voltage at that connector anytime the vehicle is trying to start on or is running on CNG. Once it failes to start on CNG or is running on gasoline, the voltage will not be present. If the voltage is there and it fails to run on CNG, then there is a mechcanical problem in the valve. You need to check the voltage by poking the tip of your meter lead into the insulation on the wire. If you try to test it by disconnecting the connector, the ECU will determine that the lockout valve is not connected and will set a check engine light. After 3 sucessful starts on CNG that light will go out.

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      • #3
        Re: 2004 Cavalier. CNG quit working any ideas?

        The solenoid valve is located inside the assembly that is screwed into the tank on the passenger side of the vehicle. The fact that it works intermittently seems to suggest that the valve is sticking. If there was an electrical failure of the coil in the valve it wouldn't work at all. I do not have a part number, but the assembly is still available. There is a special wrench called out in the manual for removing the valve, but a large adjustable wrench should remove it. You are going to have to remove all the lines and connectors from the assembly and then roll the tank back to get it out. One thing you might want to try before doing all this would be to apply 12 volts to the valve by disconnecting the single connector in the trunk located next to the rubber cover on the lockout valve assembly. Be positive that you are on the valve side of the connector, not the ECU side before applying power. If you cycle the valve several times with 12 volt power it is possible that you can free it up without taking it all apart.

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        • #4
          Re: 2004 Cavalier. CNG quit working any ideas?

          If I understand this right,
          1) You came out started the car.
          2) And it started on gasoline.
          3) You recognized it started on gasoline.
          4) You became suspect of the tank valve due to no CNG tank "Ping" on ignition.

          I would be suspect of other things not the tank valve. The tank valve doesn't have to open for the car to try and start on CNG. For example you can unplug the tank valve and start the car on CNG and run the lines dry of pressure and the car will switch to gasoline like normal. (This is how I purge my CNG system prior to any CNG repair.)

          There are a few things that can cause the car to not start on CNG.
          The temperature sensor on the tank is reading less than -10F degrees
          The pressure sensor on the tank is reading less than 200psi
          The CNG main fuse or relay not functioning
          Also certain CNG fault codes cause the system not to start on CNG (once cleared it will try again)

          Sorry I can not point to a specific item to replace. Both the Pressure sensor and the temperature sensor can be verified with a volt meter. However if both are operating correctly it takes a MUCH deeper troubleshooting to find the specific problem. Also with it being intermittent it will most likely only be troubleshootable while not functioning on CNG.

          -Harp

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          • #5
            Re: 2004 Cavalier. CNG quit working any ideas?

            A faulty pressure transducer will cause the symptoms that you described. If the pressure transducer does not send an accurate signal that there is gas present in the system the car will not run on CNG.
            Last edited by ashtonmotors; 06-01-2008, 02:13 AM.

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            • #6
              Re: 2004 Cavalier. CNG quit working any ideas?

              I still feel that it is the Lockout valve sticking---but it is hard to diagnose via the internet. The fact that it will start on CNG just after it has been attached to a gas dispenser and runs for several seconds as the fuel gauge falls rapidly shows that the pressure transducers, HPR, filters and everything else downstream of the fuel tank itself is working correctly. The fact that you have no check engine light verifies that the coil in the lockout valve is still connected electrically. The fact that the car will occasionally operate on CNG shows that it is sometimes getting fuel from the tank. The fact that you hear the valve operate but it does not sound quite as loud as you remember it also points to the valve. In any event, if you are not comfortable with trying to actuate the valve with a temporary power source you should also not try to remove the lockout valve from the tank. You need to find someone who can do the repair for you and verify what is actually wrong. A wrong connection when trying to power the valve could cost you the ECU which is $1200-$1500 to replace. The tank is heavy and could be damaged by the bracket if not supported properly when removed to service the valve. When you refuel (if the station hasn't been maintained properly) all kinds of gunk from compressor oil to moisture in the gas stream can get into the tank. It all has to go through the valve before it goes anywhere else. The little screen on the fill port only stops the chunks. BTW--I have vaporized a few multimeters in my time

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              • #7
                Re: 2004 Cavalier. CNG quit working any ideas?

                If the valve sticks shut and the Pressure transducer is functioning properly then the car will crank for 8sec before failing over and starting on gasoline. If it is immediately starting on gasoline then and not trying to start on CNG then you have one of two things.
                A) pressure transducer is intermittently failing
                B) tank valve is sticking and you have a CNG leak between the low pressure lockout and the tank valve. (this is the only way the tank valve can be sticking and get immediate start on gasoline.)

                The pressure transducer measures the pressure between the tank valve and the motor not the tank valve and the tank. If you don't believe me disconnect the tank valve, start the car on CNG and watch the pressure drop as the lines run dry, and the car will switch to gasoline.

                I would be more suspect of the pressure transducer unless you know you have a CNG leak.

                To check the pressure transducer you have to measure the output voltage convert it to a pressure on the chart.

                To measure the voltage you have to put 2 pins through 2 wires and use a volt meter to measure the DC voltage between the 2 pins. If you are interested in doing this I'll get you a picture of the connector and the wires to put the pins into.

                Is the car parked in an enclosed area where you would know if you had a CNG leak?

                -Harp

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                • #8
                  Re: 2004 Cavalier. CNG quit working any ideas?

                  Originally posted by ThunderStik View Post
                  I have owned 3 multimeter's and blown them all up.
                  Get a Fluke with good fuses.
                  http://cgi.ebay.com/FLUKE-87-TRUE-RM...QQcmdZViewItem
                  http://cgi.ebay.com/FLUKE-73-III-MUL...QQcmdZViewItem

                  Never measure current unless you know what you are measuring.
                  You should have smelled the blown full-rectifiers in my EE class.
                  CH

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                  • #9
                    Re: 2004 Cavalier. CNG quit working any ideas?



                    That is the Pressure transducer aka pressure sensor. The GM P/N is 52369194 $420.67 shipped from gmpartsdirect.com I have found a after market replacement however the OEM one is 4000psi (custom order requires 1000s of pieces and long lead time) and the after market one is 5000psi. What will happen is that; if you install the after market it will never read full again however it will read correctly as the pressure approaches 0psi. The after market one is $120 shipped.

                    My FPS (Fuel Pressure Sensor) is on its way out the door. A week ago or so it shifted it readings down meaning I would get 3000psi fill and it would only show ~2000psi. Then it will also switch over when there was still 1000psi in the tank. I purchased the after market one with the intent to replace mine however the day before it arrived my OEM one it corrected it's self. (I suspect that the 3900psi fill I got heated the sensor up enough for it to temporarily re-bond to the glass in the sensor) I expect it will fail again completely someday and I will install the new sensor at that time.

                    -Harp
                    Attached Files

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                    • #10
                      Re: 2004 Cavalier. CNG quit working any ideas?

                      Originally posted by ThunderStik View Post
                      So the pressure transducer/pressure sensor and fuel pressure sensor are all the same thing? Is this correct?
                      YES, the repair manual calls it Fuel Pressure Sensor (FPS)

                      Originally posted by ThunderStik View Post
                      So, you stated the fuel pressure sensor (FPS) measures pressure between the tank valve and the motor. But does the FPS have the ability to tell the the tank valve to close/shut down the system?
                      No, only the computer can close or open the tank valve. Regardless the computer always tries to power the tank valve when the ignition is switched on to check the tank pressure.

                      Originally posted by ThunderStik View Post
                      You stated earlier;
                      "If the valve sticks shut and the Pressure transducer is functioning properly then the car will crank for 8sec before failing over and starting on gasoline." If this is correct then why would the car try to start when no pressure is there to be applied. Or does the pressure come from the residual in the lines?
                      The lines are still pressurized if you don't have a leak and the FPS is reading this line pressure therefore the car thinks the tank has pressure and tries to start.

                      Originally posted by ThunderStik View Post
                      Where is you source for the FPS, also I don't know if dovh told you but I found the HPR's for $161.72. I bought 2 at that price as I have 2 cars...Dammit! I HAVE 2 CARS!
                      The 5000psi sensor is made my MSI like the OEM, however it isn't the same series. It also doesn't have the packard connector on it. Rather it has 2' of wires hanging out. I am currently trying to find a packard connector source to make it drop in replacement (in case I am not satisfied with the never full fuel readings; I won't have cut the connector off the harness) If you are interested in purchasing it at risk (aka I haven't tested mine) I'll pm you the source and part number. However until I have tested it on mine I don't want to post it on the forum.

                      -Harp

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                      • #11
                        Re: 2004 Cavalier. CNG quit working any ideas?

                        This will be my last word on this. After reading all the replies, I still am leaning to the HPL mechanically sticking. The key to this thought is that the fuel gauge reads full and the car will start and run on CNG for several seconds after being connected to a fuel dispenser. The fuel gauge falls quickly as pressure in the fuel lines between the fill port and the LPR falls. The vehicle senses that it is out of fuel and switches to gasoline. You can create this same scenario in a normal Cavalier by turning on the ignition key, pulling the Packard connecter in the trunk and starting the engine. (trying this at home will get a check engine light as the ECU senses there is no HPL solenoid connected). Trying to fill the car with fuel charges the fuel lines which is burned off when the engine is started with the HPL stuck shut. If you had a Fuelmaker attached it would probably idle until the fuelmaker reached max pressure in the line and shut off. The tank is full but the pressure is sensed after the lockout valve. Let the group know what you finally do to correct this problem.

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                        • #12
                          Re: 2004 Cavalier. CNG quit working any ideas?

                          We have had pretty good luck getting our local chevy dealer to diagnose cng problems with our cavaliers. Even though they aren't familiar with CNG cars their computer will generate diagnostic codes. You can still fix it yourself if you want to. Just make them explain the diagnostic codes clearly. The cost for me is around $60.00 dollars per error code generated.

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                          • #13
                            Re: 2004 Cavalier. CNG quit working any ideas?

                            It is replaceable and serviceable but I don't know if it is repairable. The valve is part of the assembly that is screwed into the tank. Without looking at the service manual, I don't believe that it can be removed without removing the tank. However, removing the tank straps and sliding the tank to the passenger side of the car might give enough room to remove it. It seems like the problem comes and goes and with each movement of the valve some cleaning of it may be going on. Each time the ignition switch is turned on, the valve operates. Listen each time and if the "ping" doesn't sound right, turn off the key and try again. With luck and with some use it might clear up on its own. It also might start to work better as the weather warms up---I can only guess

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                            • #14
                              Re: 2004 Cavalier. CNG quit working any ideas?

                              You have to completely run the tank dry of CNG then vent the last 200psi off. (NOT JUST THE LINES) You will need the CNG service manual it says you have to remove the tank from the mount. Also it shows the tank comes out the trunk and NOT into the passenger compartment. I would not be comfortable removing the tank without the brackets they show to roll it on. Removing the tank without the brackets risks damaging the carbon fiber exterior of the tank. I don't know the valve part number, it is called the High Pressure Lock-off (HPL) in the manual. It is the whole assembly that the FPS is screwed into.

                              -Harp

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                              • #15
                                Re: 2004 Cavalier. CNG quit working any ideas?

                                There is a hex screw in the valve that is used to vent the lines and lock the tank off (on the end of the HPL.) Once you have driven the car time the tank is dry, then you remove the line going to the HPR off the HPL. Then you attach a vent line to where the HPR line was. Then you remove the whole hex assembly not just the screw. Apon removing the hex assembly the tank will vent out the vent line you just attached.

                                -Harp

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