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CNG LEAK??? - 99 Cavalier

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  • CNG LEAK??? - 99 Cavalier

    I had about 75 miles on my tank when I got to work this morning and the needle for my CNG was somewhere just below half. When I left for work tonight, I started up on gasoline. I didn't even notice until I was halfway home. When I pressed the gas button, the needle dropped all the way to empty for CNG. I've never even been able to get my tank that low before, so I thought it might be a fluke. After checking it again a few more times and finding I couldn't get started back on CNG, I'm assuming it is accurate. The question is how I got to empty while sitting in the parking lot. I filled up the tank two days ago, so I'm fairly certain that whatever happened, happened fairly quickly today rather than over those few days.
    I think I remember smelling a whiff of natural gas as I walked away from the car this morning, but I'm not certain - it was very light if I did.
    So, what do you guys think? Is there anything I should check before refilling again tomorrow morning? Would love some input.
    Thanks.

  • #2
    Re: CNG LEAK??? - 99 Cavalier

    You may have a sticking or inoperative HPL (high pressure lockoff) or a failed TPS (tank pressure sensor) The picture you paint looks correct---just below half tank at 75 miles seems normal. When you went to restart something went wrong. If you fill with a fuelmaker this will be eaiser to check but can also be done with a fast fill. Connect the fill nozzle and then when fuel flow starts (you won't have much time if it is a fast fill) turn the ignition on at look at the fuel gauge. If it reading 1/2 full and going up it would varify the TPS is working. Also, the red light should be out indicating CNG operation is available. After filling try to start the car while watching the fuel gauge. If the gauge start to fall rapidly, the HPL failed to open. Sometimes just refilling can fix a intermittent TPS or a sticky HPL. Check back !

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    • #3
      Re: CNG LEAK??? - 99 Cavalier

      Originally posted by siai47 View Post
      You may have a sticking or inoperative HPL (high pressure lockoff) or a failed TPS (tank pressure sensor) The picture you paint looks correct---just below half tank at 75 miles seems normal. When you went to restart something went wrong. If you fill with a fuelmaker this will be eaiser to check but can also be done with a fast fill. Connect the fill nozzle and then when fuel flow starts (you won't have much time if it is a fast fill) turn the ignition on at look at the fuel gauge. If it reading 1/2 full and going up it would varify the TPS is working. Also, the red light should be out indicating CNG operation is available. After filling try to start the car while watching the fuel gauge. If the gauge start to fall rapidly, the HPL failed to open. Sometimes just refilling can fix a intermittent TPS or a sticky HPL. Check back !
      It looks like you were right on. This morning, I started right up on CNG without refilling. The gauge showed 1/2 tank again. It was okay again on the way home, so was this just a fluke? How much will a TPS cost if I need one?

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      • #4
        Re: CNG LEAK??? - 99 Cavalier

        The TPS is expensive and seems to be a bit troublesome. So is the HPL. However, if it isn't bothering you too much (at least in the case of the TPS) filling the tank usually unsticks it or moves it off a dead spot. Try to keep on eye on the fuel gauge when you start the car each time. If the gauge is about where it was when you shut off the car and then it rapidly falls to empty, as soon as you start, the HPL is sticking. If the gauge is on empty before you try to start, the TPS is intermittent.

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