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98 Chevey Cavilier CNG WILL NOT WORK

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  • 98 Chevey Cavilier CNG WILL NOT WORK

    Vehicle: 98 Chevy Cavalier Bi-fuel

    Gas mode: (by removing cng fuse from under hood) runs fine
    CNG MODE: When we try to start the vehicle with the fuse in we can get it to start for about 2 seconds at a time in between many cranks and only about half the time.

    Here is what we have done (I'm writing in definitions just in case I am wrong as to their meanings):
    We have verified that gas comes (with fair pressure) all the way up to the low pressure regulator(the round box under the hood just before the square computer box)
    After attempting many tests we finally toke a known good low pressure regulator, and a known good computer box (square box under the hood with large wire connector that plugs into it) and a known good wire (the wire that connects to 2 places on the computer box and solenoid valve (the one just before the low pressure regulator). We have also changed the spark plugs and verified that they are firing during our tests. Any suggestions?

  • #2
    Re: 98 Chevey Cavilier CNG WILL NOT WORK

    I have heard (although I haven't experienced it myself) that the shared fuel injectors in Chevrolet Bi-fuel vehicles can often become "clogged" in a manner which seems to affect CNG flow, without affecting gasoline flow. The end result is what you are describing, a car that runs in gasoline mode, but doesn't run well or at all in CNG mode.
    The person who told me that he experienced this problem said that he corrected it by removing the fuel injectors and cleaning them.

    I don't know any more than that, and the person who told me this may have been full of it, so take it with a grain of salt. Anybody else out there know about this?

    On the other hand, my 2001 cavalier once started running very rough, and eventually got to the point where it wouldn't run at all (or at least not more than a second or two) while on CNG, but it seemed ro run fairly well (a little rough) when on gasoline. Upon replacing plugs and wires, the car ran beautifully. I suprised that a bad plugs affected CNG mode so much more dramatically than gasoline mode. Although it looks like you've already covered your bases when it comes to spark, so I would look closely at injectors.



    • #3
      Re: 98 Chevey Cavilier CNG WILL NOT WORK

      First on cavaliers there is only 1 injector, it is located in the air intake its called the Gas distribution Adapter (GDA). It just fuel lines with drilled holes to evenly distribute the natural gas in the intake air.

      Second I would be suspect of the Gas Mass Sensor/Mixture Control. (Square box next to the low pressure regulator) Here is the description of it from the service manual:
      "The gas mass sensor/mixture control valve (GMS/MCV) combines a hot wire anemometer and a magnetic rotary valve. The GMS/MCV is located between the LPR and the gas distribution adapter (GDA). The GMS measures the mass fuel flow and outputs a frequency to the AF ECU. The AF ECU processes this signal and determines the amount of fuel entering the engine. Comparing this with the optimum fuel flow rate required to obtain stoichiometry and using feedback from various engine sensors, the AF ECU modulates the MCV in order to maintain a stoichiometric air/fuel ratio."
      This all means to me that it makes sure your air/fuel rate isn't too lean or to rich. Natural gas requires a very specific air/fuel ratio to ignite. If your GMS has failed it will cause the CNG to run too rich or too lean (no ignition). Someone on the boards was able to take their GMS apart and found water had corroded the circuit board. They repaired the corroded solder joints and it fixed their problem. Repair thread
      Originally posted by tallan View Post
      The GMS/MCV assembly has two circuit cards in it. One is the AC7-20419 Digital Assembly, I have not found any problems with this PC board yet. The other one is the AC7-17967 Analog Assembly, on this card i have found some bad solder joints. If you take the black cover off you will see the Digital PC board, remove all of the screws then lift the one side of the PC board, it is tied to the Analog card with a flat flex cable soldered on both cards. There are several components that are stuck to the aluminum housing for a heat sink, be careful and work them loose, then the whole assembly will come out. The latest unit I worked on looked like there had been water about ¼” deep sitting in bottom where the flow element sits; it had what looked like hard water deposits and it corroded the hot wire element and the RTD sensor and some of the PC board.
      Un-solder the connections and clean up the corrosion and re-solder the components back in place. Use a Ohm meter to verify connections are ok.

      On a different unit the GMS/MCV was ok but the butterfly valve was missing the 2 4-40 bolts and it was binding against the housing. If that is the case then clean up the area where it was binding and replace the bolts and use a small amount of lock-tight.

      I hope this helps.
      Please post your results as we find many people document their problems and don't post the fixes.


      • #4
        Re: 98 Chevey Cavilier CNG WILL NOT WORK Need Wiring Diagrams

        Okay, so after some further work last night we determined the problem. The valve just before the intake in the computer box is no being turned because the voltage delivered is reading 2-3 volts!. What we really need now is a wiring diagram does anyone have some pdfs or resources? Thanks!


        • #5
          Re: 98 Chevey Cavilier CNG WILL NOT WORK Need Wiring Diagrams

          Is this the part you are talking about? That part is the Low Pressure Lock-off (LPL) the GM part number is 52368527. $75 shipped from This part should be receiving a 12V supply when it is activated. To verify the part is bad disconnect it and check the DC voltage in the supply connector while cranking the engine. This line is only active while cranking. If you see 12VDC while cranking and the valve is not turning on then I would be suspect of it, however this valve only receives 12VDC while cranking to start on CNG or running on CNG. If the vehicle is not running on CNG or is running on gasoline this part will be deactivated.
          Attached Files


          • #6
            Re: 98 Chevey Cavilier CNG WILL NOT WORK

            Yes, we checked the voltage while cranking, it measured 2-3 Volts. Obviously not enough to turn it. If we can get a wiring diagram or insite on why its showing that low. Our guess is a bad relay or a short but we have no idea how to go about it since we don't know the wiring.



            • #7
              Re: 98 Chevey Cavilier CNG WILL NOT WORK

              I'll post you schematic tonight however for sanity check I would apply hot wire 12V to the Soleniod while cranking and see if it starts up for ya. I'll post the schematic with what connections it goes through tonight.



              • #8
                Re: 98 Chevey Cavilier CNG WILL NOT WORK

                Thank you so much in advance!


                • #9
                  Re: 98 Chevey Cavilier CNG WILL NOT WORK

                  WHOOOOAAAA back the trolly up here fellas. ARE thier CODES present?
                  code 12 and 13 are normal but have you others?
                  Turn the key to the run position and within 10 seconds press the CNG button 3 times....
                  you should see the litle green light flash count the flashes they will give you the codes 3 times in a row then move to the next code.
                  WHAT are the codes?

                  Read the sticky Its very usefull to start with I can get the code flow charts as needed
                  Ken's Master Auto Care

                  1998 Cavalier CNG

                  1972 Chevy Blazer Twin turbod Duramax in it, 12.8 @105MpH 505 rwhp 1109 rwtq

                  Still in the works
                  1969 Camaro Cstm blt 87 vette frnt susp 6 pot 13" frnt brks rack and pinnion Dana 60 w Cstm Lateral-Dynamics 3 link rear susp 4 pot 12" rr brks 18X10 frnt 18X12 rr Bozeforged Speedsters, on the list still is
                  Gforce T56 6speed gen3 427 Ci Twn turbo


                  • #10
                    Re: 98 Chevey Cavilier CNG WILL NOT WORK

                    We've been jimmy rigging the solinoid valve just before the low pressure regulator and we have now removed the plate from the butterfly valve. So these codes may have relation to that but here are the codes we pulled from a obdII reader p0335 & p0342. So at this point we know that we are getting gas up to the intake. The vehicle still will not start though. Again we have placed a known good low pressure regulator and a known good maf computer box. Somehow I'm guessing our mixture isn't right but how?


                    • #11
                      Re: 98 Chevey Cavilier CNG WILL NOT WORK

                      You may not be getting the proper pressure from the high pressure regulator. In the service manual it tell you to hook a pressure meter to the low pressure diagnostic port on the line before the LPL. It has you verify the pressure that it has you start the motor, continuously watching pressure to verify it stays over a minimum pressure. Then they have you power brake the motor for 3 sec and verify it can hold the pressure high enough; if any of the pressure readings fall to low they have you replace the high pressure regulator. I can get you the pressure numbers tonight if you want, however I don't have pressure meter that I can hook to the line, if you have one respond and I'll post the numbers tonight. Otherwise you may want to swap the HPR with a known good one (seems you have a running car you are swapping parts off with).

                      Madspeed, I tried the method of getting the codes through the gasoline light however I didn't get any flashing. According to the PDF you posted it appears you have to plug the OBDII reader in to receive the codes.
                      Last edited by Harpoon; 05-15-2008, 07:39 AM.


                      • #12
                        Re: 98 Chevey Cavilier CNG WILL NOT WORK

                        You need to replace the "plate" that you removed before you go any further. The AF ECU controls the opening of the butterfly valve you removed to control the amount of CNG that is mixed with the incoming air to provide that proper A/F ratio to provide combustion during all aspects of the operation of the engine. Without the "plate", it is impossible to get the proper mixture. The engine will be extremely rich in CNG and will not run properly--if at all. When you re-install the "plate" make sure that it fits properly when closed and after you tighten and loctite the screws, check the operation for binding. Don't make the mistake of creating more problems then you already have by removing or modifying components. If what you did didn't cure the problem, then the solution lies elsewhere. Replace or return anything you modified to the origninal condition before moving on.