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How much cng can my three tanks hold?

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  • How much cng can my three tanks hold?

    I have a 97 Ford E 250 Dedicated Natural Gas. It says that it holds 59.9 gallons of water. So my question is, how much natural gas should it hold?

    I have one long tank on the drivers side and then two small tanks on the rear under neath.

    My usual fillup is around 5 to 8 gallons each time.

    I'm thinking my rear tanks might not be filling up, but I don't know why?

    Any advice?

  • #2
    Re: How much cng can my three tanks hold?

    Starting with a new tank, at 0 to 1 psi of cng , not air, and slow filling to 3000 psi at 70 deg and sea level, this is about 14.5 GGE, Laboratory conditions . Real world 70 degrees, fast fill with 3300 psi should get you about 11-12 gge from almost empty tank(s).

    Sounds like one of your tanks is not filling, or is full and not sending fuel out, or the station has very low pressure.

    Do the tanks have electric solonoid valves ? , if so, one is prob. bad.
    They are about $900 from Ford, However, may used ones can be found for less than half of that.

    P.S. A 1997 , with original tanks are about to the expire date, if not already.
    What is the "Do not use after" date on the tanks ?


    • #3
      Re: How much cng can my three tanks hold?

      When I am really, really empty on my E350 and the station has good pressure, I expect to get anywhere from 9 to 10 gge's on a fast fill.

      You need to ignore the water capacity figure because it tends to skewer the learning curve when it comes to CNG. You will never get the rated gge capacity with a fast fill. The process of fast filling heats the gas up, it expands and lowers your capacity.

      Without insulting your intelligence, because you may already know this, another caveat that snares some people is the concept of GGE's. In order to get an equivalency, (ie: apples to apples) what they did is they took a gallon of gasoline and measured the amount of BTU's in it and then said, okay .. we need this much BTU's of natural gas and we are going to call it a Gas Gallon Equivalent.

      That being said, if you OCD on MPG and stuff like that, you would do well to ignore computing MPG for individual fillup's because you will not always get a consistent fillup like you can with a liquid. It's best to look at weekly trends or even monthly trends.

      Now about your tank problem. The way Ford wired the up the tanks, there is a master fuse in the engine bay, from there it goes the inertia switch located behind the kick panel on the passenger side. From there it fans out to the tanks.

      When you do a KOEO (key on, engine off) you should hear a series of clunks. One clunk is the Fuel Pressure Rail (FPR) solenoid opening to pressurize the fuel rail. It is located at the back of the engine on top of the intake manifold. The other clunks should be the fuel tank solenoid's opening. IIRC, they stay on for eight seconds or so for KOEO and then close.

      What you need to do is make sure that you are getting voltage to the tank to open the solenoid's. The quickest way to see which ones are opening is to disconnect all of the leads to the tanks and then, one at a time, reconnect a tank, start the engine and see how long it runs. If it runs for a couple of minutes .. that tank's solenoid is opening. If it dies, it didn't open.

      From there, you make sure that voltage is getting to the plug. If voltage is getting to the plug, the simple act of the mechanical disconnecting/reconnecting may solve your problem. If not, your solenoid's have gone to the great solenoid in the sky and that, my friend, complicates matters.

      At least for your pocketbook ...

      Hope that helps.
      Last edited by jblue; 10-23-2009, 08:17 AM.

      ?Innovation is driven by having access to things.? -- Gleb Budman, CEO of


      • #4
        Re: How much cng can my three tanks hold?

        toper et al,

        To add to the discussion I've attached two .jpg files that we gave as handouts to students at WVU. I'll also put them in the library.

        The first is a general formula to calculate GGE based on the length and diameter of a cylinder at various pressures (settled). The second is a matrix (I used to call these charts, but I learned a new word while at the NAFTC) for range given a MPG and tank size.

        Hope this helps,


        p.s. I'd be interested if either of these aids pass the 'real world' test.
        Attached Files


        • #5
          Re: How much cng can my three tanks hold?

          Thank you all so much for your help. This answers my question. I was wondering why I was only going about 100 miles on a fill up. I kept thinking I should be getting more gas and better mileage. I only hear two clicks when I start up.

          I will do as you guys said and see if I can get it figured out. I'm hoping it's only a fuse.


          • #6
            Re: How much cng can my three tanks hold?

            Also, the Tanks expire in 2012.

            If the van continues to run well, I plan to just replace the tanks.


            • #7
              Re: How much cng can my three tanks hold?

              I know this is a old post but have the same type of problems. Was wondering what you found out and how you solved the issue. I am in OKC and would appreciate any help.


              • #8
                Re: How much cng can my three tanks hold?

                Wow, this was a really old post. You can tell I'm bored tonight.

                I couldn't figure out how to do the math exactly, so the way I measured was the old fashion way... I ran the tanks down as far as I could, then went to the best station in Tulsa (TGT) and filled up. I did find that I had a solenoid out on this vehicle... but found out after I sold it.

                My current van holds 18 gge but it has 4 tanks. Bulky yes, but who cares. I would never use the space for anything else.