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How low of pressure in tanks will the vehicle still run?

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  • How low of pressure in tanks will the vehicle still run?

    Almost ran out of fuel today for the first time. I made a mistake on my morning route and had to double back a considerable distance. I tried to exercise every fuel saving measure I could think of once I realized my error, but I barely limped into the Hurricane station. After I turned the valve to fill and the pressures equalized, the pump read 280 psi. That got me wondering, when are you "out of gas"? When pressure drops below fuel rail pressure?

  • #2
    Re: How low of pressure in tanks will the vehicle still run?

    You're probably right. Fuel rail pressure, or somewhere in the neighborhood of 100psi is probably the lower limit. The one time I ran out I discovered that they don't run out like a gasoline car. I was able to restart the car and idle in gear a little bit further at 1-2mph. I could have walked faster, but if I gave it gas it would die again. So unless you're 30-50ft away from fuel when it dies... probably isn't worth the time or effort to mess with that.

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    • #3
      Re: How low of pressure in tanks will the vehicle still run?

      Originally posted by pb4ugo View Post
      That got me wondering, when are you "out of gas"? When pressure drops below fuel rail pressure?
      Precisely. The injectors are driven by rail pressure.
      Curt, what psi does the Civic fuel rail normally run at?

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      • #4
        Re: How low of pressure in tanks will the vehicle still run?

        It's not specifically the rail pressure, but the tank pressure necessary to get enough flow past the regulator(s) before the rail. Rail pressure is (I think) preset at 43psi in the GX but it will take over 100psi to get enough flow past the regulator(s).
        02 GX
        01 GX
        03 Crown Vic
        06 GX
        Home Fueler

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        • #5
          Re: How low of pressure in tanks will the vehicle still run?

          For the Cavalier, changover to gasoline is around 200 psig with 600 psig required in the tank to resume CNG operation (this prevents fuel switching from occurring when you near the lower limit). Rail pressure is around 35 psig--although the book says it's higher. I would assume the Civic is something close to this.

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          • #6
            Re: How low of pressure in tanks will the vehicle still run?

            What has been said here is essentially correct and vehicle specific.

            In general terms, as the high side system pressue drops, the HP regulator will stay open longer to get the volume of gas necessary to build up pressure and close (regulate) low side pressrue. The HP regulator will eventually stay open all the time because there is not enought pressure build up in the low side to overcome the spring tension on the diaphram and close the regulator inlet valve.

            When you get to the fuel rail, as pressure drops the injector pulse width (injector open time) will increase to get enought fuel into the engine cylinder to produce power. The O2 sensor will catch the engine going lean. Also as the injector on time increases the short term and then the long term fuel trim will increase to compensate for the change in injector pulse width. Also during the above operational condition, the engine will start loosing power due to lack of fuel (the injector can't stay open long enough to get the required fuel into the cylinder). Remember that injectors are pressure/time measuring devices; a given amount of gas will pass through a given size orfice per given amount of time. We hold the pressure constant and vary the injector on time.

            Given enought time or engine revolutions (depending on the computer program) a check engine light could be set for things like fuel trim, lean mixture, or mis-fire.

            The above will generally take place unless there are any computer algorithms in the program to to cause a fuel change over or shut down the engine.

            Hope this helps

            Larrycng

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            • #7
              Re: How low of pressure in tanks will the vehicle still run?

              Dang, Larry. I'm surprised that your head hasn't exploded from the wealth of knowledge you have stored up there. You are a never ending source of detailed information that never ceases to amaze me... specially when you consider that I can't even remember what I had for breakfast most days. You are the man!

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              • #8
                Re: How low of pressure in tanks will the vehicle still run?

                Im wondering this my self. New to the forum big problem went to the fueling station yesterday and for some reason it will not refuel. Only have little less than a 1/4 tank left i have no Idea of what the problem is.

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                • #9
                  Re: How low of pressure in tanks will the vehicle still run?

                  Originally posted by amoparacer View Post
                  Im wondering this my self. New to the forum big problem went to the fueling station yesterday and for some reason it will not refuel. Only have little less than a 1/4 tank left i have no Idea of what the problem is.
                  Please follow the discussion here:
                  http://cngchat.com/forum/showthread....ILL-NOT-REFUEL

                  Now, back on topic of minimum psi...

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