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Cavalier AF FPR (Fuel Pressure Regulator) - Car switches to gas before CNG is empty..

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  • Cavalier AF FPR (Fuel Pressure Regulator) - Car switches to gas before CNG is empty..

    So I need some input...

    I have a 2000 Cavalier. AWESOME runs great, except for the fuel gage. We all know that they aren't accurate on a non liquid fuel, but this problem extends beyond the accuracy I guess. Although it use to register to the FULL mark and now it doesn't (when full).

    I can fill the cng tank completely up with a 3300psi pump and when my fuel gage drops to just around a 1/4 tank reading if I dare turn the motor off it's done on CNG....it will only operate in gas mode (obviously it's thinking the tank is emptier than it really is)....however, if I don't turn it off I can drive for a good 40+ more miles on CNG before it switches to gas. So as of late I'm trying to not have to turn the engine off if I know I am approaching the 1/4 tank mark. This doesn't make sense why it will run clear down to below E (check gages light comes on) as long as I continue to drive it...but if I shut that engine down and try to start back up again it will not operate back in CNG mode until I've refueled.

    Previously at my BEST fill I've only ever been able to get 4.3GGE. So I got curious about this system. What I did was disconnect the Fuel pressure regulator from the cng tank and turn the key on...then the fuel gage registers completely FULL and VOILA...CNG mode it will start and operate. I could then turn the motor off, reconnect the regulator and my gage would represent the aprox. 1/4 tank ....however, it will sucessfully register CNG and I can start it in CNG 4-5 times in a row no problem, but then it goes back to gasoline only again. I can repeat this process and the cng works again and then doesn't work again. I am certain it has to do with the regulation of fuel being told to the system to operate on gasoline because it feels it's almost empty on cng.

    So with all that being said I decided to just drive around until I completely ran out and it switched over to gasoline....then I disconnected the regulator and it starts back up again on cng...from that point I must have drove an additional 40 miles or so before it switched to gasoline. Now, obviouslly with the regulator disconnected I can't see what my CNG level is but when it switched to gasoline the gage worked just fine for gasoline mode.

    So after giving this information my questions are as follows:

    A) Any reason one couldn't just leave the fuel pressure regulator disconnected and get more milage out of their tank? Is there any reason this should cause damage to the system? I don't think it would.

    and...

    B) Why the heck does it not let me start in CNG mode again at a 1/4 tank? Is the possiblity there that my regulator is just fubarred? Would replacing it make much of a difference?

    I could give a crap less about seeing a fuel gauge on CNG mode...I can reset the trip odometer and know about when I'm empty, and if you do run out that's why it's BI-FUEL to begin with.

    I am looking forward to EVERYONE's feedback and input on this matter.

    Oh, and after I disconnected the fuel regulator gage from the CNG tank when it finally ran out I was able to pump back into the tank 5.2GGE, this is nearly a gallon more than I ever have gotten on a good day. I sure like the extra distance, filling up every 3 gallons to 4 gallons is no fun, but it's a small price to pay for the big discount at the pump!

  • #2
    Re: Cavalier AF FPR (Fuel Pressure Regulator) - Car switches to gas before CNG is emp

    I have a '98 service manual bi-fuel supplement. It says to check the following for unwanted switch to gasoline above 1/8 tank:

    search for service bulletins
    inspect crank sensor (CKP)
    inspect for intermittent ignition input to ecu
    verify system pressures
    inspect the gas distribution adapter (GDA)for leaks
    inspect for intermittent ngo enable circuit
    inspect for a skewed fuel pressure sensor(FPS) reading
    inspect for faulty mixer control valve (MCV)

    The FPS sounds like the thing I would check. This involves measuring the voltage at the FPS. and comparing to actual pressure. I wonder if you could do it at the filling station before filling? If you are interested I can post a wiring diagram and a chart example 1.25v=750 psi,2.5v=2000psi.

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    • #3
      Re: Cavalier AF FPR (Fuel Pressure Regulator) - Car switches to gas before CNG is emp

      Previously at my BEST fill I've only ever been able to get 4.3GGE. So I got curious about this system. What I did was disconnect the Fuel pressure regulator from the cng tank and turn the key on


      I have a very similar situation to yours. My 2002 Cavalier will only hold about 4.1GGE with a 3600psi fill up. When the gas gauge reads about 1/3 tank CNG, the car just switches to gasoline . . . no red "check gauges light" or empty indication. All other CNG functions work fine. Where is the Fuel Pressure Regulator you disconnected? Is it the black and grey wire harness connector in the back left corner of the trunk coming out from the bottom of the tank cover? I disconnected this and my gauge read full, car started in CNG, and then sputtered as it switched to gasoline.

      Thanks for your assistance. I would rather fix this problem myself, than go to a dealer. Last year the car would hold about 5.1GGE and the quantity gauge would read empty with the red "check gauges light" on before switching.
      Last edited by haagm; 06-24-2008, 09:29 PM.

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      • #4
        Re: Cavalier AF FPR (Fuel Pressure Regulator) - Car switches to gas before CNG is emp

        haagm...

        This is mounted directly on the left hand side of the tank. If you remove the plastic cover (2 nuts hold it on) that has a rubber seal around it...you will see a connecter that is connected to a silver spark plug looking item..this plug has 3 wires going into it (at least it does on mine).... Once I disconnect this my fuel gage registers full and my cng will start and I can drive around for many miles before it switches over to gasoline.

        I believe what you're disconnecting may NOT be the gage only, you might be unplugging more than that! The fuel pressure sensor has a purpleish rubber colored ring on the connector at the base of it before it plugs into the sensor. Check that out and see if you can see what I'm describing, if needs be I can post a picture, easy enough to do.

        I'm still searching for the possibility that running the tank out continuously with the fuel pressure sensor disconnected would cause damage...I really highly doubt it would, mine has switched to gasoline just GREAT and then when I fill up it runs out and switches to gasoline again...no problems, but now I can get 5.2GGE instead of 4.3GGE.

        Hope this helps you.
        Last edited by krnorth; 06-24-2008, 11:16 PM. Reason: revise

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        • #5
          Re: Cavalier AF FPR (Fuel Pressure Regulator) - Car switches to gas before CNG is emp

          KNORTH

          Thanksfor the prompt reply. Does anyone have the correct do-it -yourself diagnosis and fix for this problem. I have read several ides on various posts, but figure that this problem is fairly specific. I am sure some guru has the answer. . . Please pass it on!

          Thanks for your help,

          HAAGM

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          • #6
            Re: Cavalier AF FPR (Fuel Pressure Regulator) - Car switches to gas before CNG is emp

            haagm..

            Your first do-it-yourself check is to disconnect your fuel pressure sensor. If your car will start back on cng after you've disconnected it then it's very viable that your fuel pressure sensor is not working properly. This can also be checked by voltage, if you look at this post karlhafen has posted some voltage displays that can help you test this.

            Karlhafen...can you post a more detailed voltage readout from top to bottom?

            I think someone down the line has posted the voltage here somewhere also. You could do some searching through forums and you'll be able to find it.

            Disconnect that sensor and figure out if you can run cng for awhile longer. It's a good start.

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            • #7
              Re: Cavalier AF FPR (Fuel Pressure Regulator) - Car switches to gas before CNG is emp

              You could also have a leak which causes your pressure sensor to read below what is really in the tank. For mine I have to turn the car on once then turn it off and then back on again before it will start on CNG. I might have two problems though.

              A picture would be nice. If this works without damaging the system it could be possible to run the pressure sensor voltage to the front of the car and have a cheap voltage sensor at the front and use that as the NG gauge. Since you never know how much pressure you are going to get here in UT it would be nice to have more than just the mileage to rely on.

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              • #8
                Re: Cavalier AF FPR (Fuel Pressure Regulator) - Car switches to gas before CNG is emp

                Just an FYI, I'm having the same problem with my gage. I suspect it is just the gage though because I refuel at home and I believe the 3600psi compressor controls the shut down when 3600 psi is reached. So I feel confident that my tank is full. I also have to either start the car twice or turn the key to power everything and wait a minute until my cng panel light goes out before starting. I will drive out a full tank this Friday so I'll know then if the gage is a false reading. BTW it only shows approximately 3/4 tank after I refuel.

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                • #9
                  Re: Cavalier AF FPR (Fuel Pressure Regulator) - Car switches to gas before CNG is emp

                  Originally posted by rguzek View Post
                  ... BTW it only shows approximately 3/4 tank after I refuel.
                  Pressure transducers can give you bad info... I needed to replace mine on a 2000 Tahoe conversion... The original transducer would allow me to run the tank empty (below 100 PSI) and showed a full when it was full, but that xducer eventually died open (electrically open, I didn't leak CNG ). The new xducer will register full only above 3500 PSI (according to the analog gauge under the hood). and worse, it switches over to gasoline at about 700 PSI . Switching over at such a high level is a real PITA when you only have a puny 4 EGG tank!
                  2008 GX (extended range, trunkless version)
                  Polished Metal Metallic 2012 Civic Natural Gas
                  Fuelmaker FMQ-2-36 (since 2001)
                  Previously owned: 2000 GX (11 years), 1995 Bi-fuel Sonoma, 2000 Bifuel Tahoe, 2000 Bi-fuel F150

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                  • #10
                    Re: Cavalier AF FPR (Fuel Pressure Regulator) - Car switches to gas before CNG is emp

                    Keep the sensor off and use a little more CNG between fill ups.
                    Was thinking of doing it myself. I'll let you know how it goes

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                    • #11
                      Re: Cavalier AF FPR (Fuel Pressure Regulator) - Car switches to gas before CNG is emp

                      It would be nice to put a switch towards the front of the car and then we could switch the sensor off once it starts getting low and then we would still have the benefit of the gas gauge before then we would have to switch it off.

                      Post your pic and then I'll start playing with it and I'll find where the wire is up front.

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                      • #12
                        Re: Cavalier AF FPR (Fuel Pressure Regulator) - Car switches to gas before CNG is emp

                        Originally posted by dovh49 View Post
                        It would be nice to put a switch towards the front of the car and then we could switch the sensor off once it starts getting low and then we would still have the benefit of the gas gauge before then we would have to switch it off.

                        Post your pic and then I'll start playing with it and I'll find where the wire is up front.
                        Sounds like an excellent idea to put a switch in for sure! It's kind of an extra hassle to get in the trunk to disable & enable.

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                        • #13
                          Re: Cavalier AF FPR (Fuel Pressure Regulator) - Car switches to gas before CNG is emp

                          What exactly do you mean that the cng didn't work for you?

                          Once you unplugged the fuel pressure sensor you got absolutely NO starts on CNG?

                          This should not be the scenario. Once you unplug the sensor your gas gage should register full and cng will run fine until your tank runs out. Your'e absolutely positive you're unhooking the correct thing right? I am going to unhook my brothers today and I will post a picture tonight or tomorrow morning of this sensor.

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                          • #14
                            Re: Cavalier AF FPR (Fuel Pressure Regulator) - Car switches to gas before CNG is emp

                            I have been wondering about any problems that can occur with running the tank "dry". I noticed in the Cavalier repair manual that it states that if you are filling a totally empty tank (such as when replacing the tank) you must not use a fast fill. Instead, you fill to a low pressure (I think it was 500 psi) and then stop. After waiting you can fill again to a higher pressure. This only involved a quick fill and there was no problem with filling from zero pressure with a time fill. Is there some stress problem with the tank during a quick fill from zero psi? If not, why is the restriction listed in the repair manual?

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                            • #15
                              Re: Cavalier AF FPR (Fuel Pressure Regulator) - Car switches to gas before CNG is emp

                              It doesn't say anything about that in the 2003 owner's manual supplement.
                              02 GX
                              01 GX
                              03 Crown Vic
                              06 GX
                              Home Fueler

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