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Filling Question

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  • Filling Question

    I'm still a bit newbish here, with only 3 weeks under my belt, but I've noticed (or imagined?) an apparent issue with my fillups.

    It seems that when I fill, I'll get x PSI of gas, depending on the pumps. It typically gets me somewhere between 1/2 - 3/4 of a tank, occasionally pushing just above 3/4. Now, I drive away from the station and stop elsewhere (work, store, whatever). When I come back out later, it seems the gauge on the truck indicates slightly less gas than I would expect it to have. It's not a significant amount, and may be attributable to gauge inaccuracy, but I wonder...

    I thought I heard or read somewhere that the act of fueling causes the gas to heat up, and once it cools, would this result in the effective PSI in the tank being lower? If so, does it make sense to return to the pump an 30 minutes later and try to stuff some more gas in?

    Also, is it normal to occasionally catch a smell of NG? I don't believe I have a leak, as the truck can sit all weekend and still have about the same gas as when I parked on Friday, but want to make sure I don't have an issue... and I do get occasional whiffs of the smell.

  • #2
    Re: Filling Question

    If the vehicle is running rich you might smell the gas in the exhaust, but odds are if you smell the natural gas, you probably DO have a leak. The odorant in NG is very strong (for a reason) and a VERY small leak will give most people the odor. I'd take it to a local CNG shop and see what they say, odds are they'll soapbubble all the joints to see if they can narrow down the location.
    1997 Factory Crown Victoria w/ extended tanks ~~ Clunkerized!
    2000 Bi-Fuel Expedition --> ~~ Sold ~~ <--


    • #3
      Re: Filling Question

      To answer the other part of your question, you would be able to add a bit more fuel after the tank cools down. It's probably not a big deal in the terms of your day to day driving, but if you need the additional range... it would probably give you near an additional gge. Other factors that might help are time of day; is the storage being drawn down by heavy daytime use? If so, try early or late top-offs to see if it makes a difference. You didn't mention what you're driving, the tank pressure your vehicle calls for, or the pressure of the station you're using. For example; if you're driving a Civic GX (3600psi) and filling at a 3000psi station, you'll never get a complete fill. If you're driving a 3000psi vehicle and a 3000psi station, you should be able to top off when you hit it the 2nd time, unless the stations pressure is low.


      • #4
        Re: Filling Question

        My understanding is compressing the gas heats it just like air compressors get hot when running. This expands the gas creating more pressure than when cooler and limiting fills. I am fortunate, because I work nights and am able to go to the pumps in the middle of the night when there is almost no use.
        These are some of the things I tried and I do. (Keep in mind, I don't mind going to fill up, I actually like it, and the station is just over a mile from where I work on empty streets).
        I have a bi-fuel F150 with what I guess to be about a 12.5 gallon or so tank. When I fill up, I only turn the nozzle part way on to slow the flow a little. I don't know if this helps, but I've not seen a problem with it. If my tank is low, the fill nozzle gets frost on it, then as the tank fills, it goes from freezing to where it just barely feels warm when it kicks off. I also let it sit for about 30-45 seconds after it stops. Most of the time, the pressure will build back up and it will start pumping again. The repeats a couple times, but I really only get about another gallon and a half or so at the most.
        When I fill up, my fuel gauge shows just under full, then a few hours later, it can show as much as a 1/3 tank less. This is from the CNG cooling down and the pressure dropping in the tank. Then, on my way home, I stop and top off, usually only getting 3-4 gallons. The most I have put in my tank this way is just a touch over 11 gallons with several hours between fills, and only driving about 2.5 miles between fills.
        The most I have gone on a fill is 190+ miles, on a trip climbing hills going through mild mountain ranges. The low fuel light was on and the guage showed empty, but it hadn't switched over.
        Another thought I had, but don't know if it really helps, is I try to fill up from 1/4 to 1/2 tank instead of almost empty. I am guessing this little difference in the amount of cng being pumped with allow a little fuller fill, but not sure. The pump pressures vary somewhat around here, usually between 2700-3400 psi, but generally it's above 3000. I have a 3600 psi system.
        I don't know that all I do helps, and know it would drive people crazy filling up like I do, but it's something I enjoy doing.
        I figure if I am on a trip, I'd try filling up, then buying something from the station and hang around eating it for about 15-20 minutes then try filling again. I really dont' know how long it takes for the CNG to cool down and the pressure to drop.



        • #5
          Re: Filling Question

          Sounds like you have low pressure at the station, so it will not 'fill' the tank all the way no matter what. The little bit you can add an hour later would not be worth the drive.

          A little gas is released when you disconnect the fill hose.

          Also, if there is a small leak 'downstream' of the electric shut-off valve, it will leak then stop until the key is turned back on.
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