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2 tanks only fills up with 7GGE

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  • 2 tanks only fills up with 7GGE

    2010 Chevy impala I have two CNG tanks but only fill up with around 6to 7GGE?

  • #2
    How large is each tank? How much does it normally take for a fill? If each tank is supposed to hold at least 7 GGE, then it's possible that one tank's lockoff valve isn't working--if both tanks use an electric valve. Two, 7 or so GGE tanks in a 2010 Impala seem a little large, but I don't know how they are installed. I have seen this before in vehicles with multiple tanks, all you have to do is look at your vehicle range and if it about one half of normal, this this is where to look first at the problem.


    • #3
      I don’t know how big each tank is. I will take the back seat off and look at them. I know it’s one big tank and one smal one.


      • #4
        It probably has one 2.5 GGE tank and one 8.3 GGE tank. That was the configuration for many of the Dallas taxis and Oklahoma fleet vehicles converted in 2010. These are nominal values, so fast-filling will provide about 85% of this and time-filling will provide up to 100% of this. As siai47 indicated, the smaller tank's valve is probably stuck closed. These cars have OMB valves with electronic solenoid lock-offs and manual closing knobs. Turning on the key will energize both valves, and you should hear a "clunk" sound at both tanks. If you only hear one, then it is likely a stuck valve.

        To confirm this:
        1. Fill the car.
        2. Manually shut the small tank's valve by turning it all the way clockwise.
        3. Drive the car 100 miles or so. This should consume fuel only from the large tank, and the small tank will still have 3600 psi in it.
        4. With the key on, SLOWLY open the small tank valve manual knob counterclockwise.

        If you hear a hissing sound then our assumption is incorrect, because this indicates the solenoid valve is indeed working: fuel pressure will equalize between the two tanks and the hissing will stop after a minute or so. And note the valve has a safety flow limiter, so don't quickly open the valve all the way or the hissing may be a very short period of time before the valve shuts flow due to the limiter.

        Given that hissing = both tanks are operable, your problem is likely that of having oil and other liquids pooling in the tanks. This reduces the capacity for gaseous CNG fuel.

        If there is no hissing, the simple first step is to see if power is getting to the OMB valves on both tanks. If power is getting there when the key is on, then you will have to have a professional NGV shop replace the valve because the solenoid is not opening up for the gas to flow out. This can be a dangerous process, requiring proper fuel venting and grounding. Procedures are here:

        If you decide to vent and replace, then the NGV shop might as well do both tanks and valves at the same time. This will provide an opportunity to drain out fluids, and replace the fuel filter... after 8 years in taxi service it probably hasn't seen many drain intervals and is letting a fair but if compressor oil to blow by into the tanks.

        Good luck, and let us know how you get along.
        Last edited by John Mitton; 05-11-2018, 09:26 AM.