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2000 Cavaler not starting on CNG

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  • 2000 Cavaler not starting on CNG

    I have a Cavalier that was not run on CNG for a number of years, everything was kept up to date (fleet vehicle). I put a partial fill up in as I was unsure it would still work, It worked initially as I remembered but the 3rd time went to start only started on gas. I had battery disconnected for another reason and it started on CNG the first time and reverted back to regular gas. From what I am reading, would the Gas Mass Sensor be the issue?

    ?Any thoughts?

  • #2
    When you try to start it on CNG, and you turn the key to run (not crank) do you hear the tank valve open (clunk)? Before you crank it, is the orange light on the fuel switch on or off? What is the gas gauge level reading? Is it over 1/4 of a tank? During cranking, is there any attempt to start on CNG--i.e. and long crank time with no start and then a start on gasoline? When it starts on gasoline does the fuel level change to what you think is the gasoline level and what does the orange light do? Do you have a check engine light on? A 2000 Cavalier that hasn't run for years on CNG could have a lot of things causing the problem and if you aren't into tinkering with it, it most likely isn't worth your time to fix it. If you have a check engine light, a normal scan tool won't read the AF-ECU DTC's so unless you can score a GM Tech2 scan tool you really won't have a clue about what the system is trying to do. Only the Tech2 can access the AF-ECU trouble codes and provide you with data about what the fuel system is doing. The fact that the battery disconnect seems to allow a start attempt on CNG makes me think that a soft code is being set that you cannot see without the scan tool.


    • #3
      I listened and heard the valve in back open (tink sound) - I decided to unhook battery momentarily like before and it started on Cng like 10 times. I then read your response so before cranking with key on, the orange light is lit, then when cng tank valve opens (tinks) the light goes out and it proceeds to start on cng as it should. I filled up and in the next few day put about 150 miles or so on, ran it down close to empty as the next fill up was 5.1 gallons. Everything functioning as it should, until after last fill up, I then drove 30 miles and stopped at store, upon restarting this relief valve in rear (in picture) started popping off, drove 1 mile home like that still running cng - fuel gauge registered E. Then when trying to restart when turning key on, while orange light is lit valve relieves cng, light goes out when cng valve opens and it quits until engine is started (starts on cng) then relief valve commences to relieve itself while it is running. No check engine lights on. Has this "control" valve gone bad? or the pressure relief valve? worth fixing? I appreciate the info


      • #4
        The thing in the picture is the high pressure regulator---which is sticking. The "POP" sound you hear is an intermediate pressure relief valve opening when the pressure exceeds 250 PSI. This is to protect the low pressure regulator on the engine which runs on about 175 PSI from damage. The regulator is a piston regulator and could be sticking from gummy oil or corrosion problems. If you can find one, replace it. Impco makes one that is similar but the coolant ports (the rubber hoses) are in a different position. You can find them on E-bay from time to time but the one you are looking for is a 3600 PSI regulator marked GMC at the end of the part #. If you are handy, take off the feed line to the regulator (which is in the trunk on the drivers side) move the line out of the way, remove the "O" ring face seal, spring and filter. You might need to put something sticky like gum on a stick and press it on the filter to remove it. Then, put a shot of WD-40 or Breakfree down the hole into the regulator. Reassemble, test for leaks, and try to see if the "POP" goes away. The spray lube will help loosen up any deposits on the regulator piston and might get you going again on the cheap.


        • #5
          Thanks so much for the details and what to do to give it a try