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  • cng cavalier doesnt start and stay running

    i bought this 1999 cavalier about 3 months ago, the guy said he was driving and the car died. the car sat for a year before i bought it. i brought it home, put a battery in it and it fired right up! I drove it around the block for a few days just visiting friends house's and such and then about 3 miles from my house heading home it starting acting like it was running out of gas (it was running on natural gas at the time) and i barely made it home and the car then would start up and die. so i replaced the fuel pump (i had one lying around) and it did nothing. but then i hotwired the fuel pump and the car ran fine. now what im thinking is that the switch that converts it to natural gas and regular gas is bad! so a month ago i brought it to the local shop b/c the guy new about these cars. i stopped by today and talked to the guy and he said it started up and ran fine, i then turned on the key to start it and i heard the fuel pump go on (it didnt before). so i drove it home no problems, so i decided to put some natural gas in it. the nearest station was about 30 minutes away and i had no plates on the car so i trailered it and put 90 cents in it. i dont know how these pumps work, maybe i screwed up, but that didnt sound like alot to me! so i brought the car home and it was doing the same thing allll over again! i do not know whats going on here, im about ready to set fire to the car!!!!!!! please someone help me!

  • #2
    Re: cng cavalier doesnt start and stay running

    Please don't set fire to it because it most likely has a full tank of CNG . On the CNG side there is no fuel pump--it is only used for gasoline operation. Did the car come with an owners manual? If it did, read about what should happen during CNG operation. Big thing to check first is the little red light in the fuel level select switch. It should come on for a couple of seconds for a bulb check and then go off. If the bulb check is OK and the light stays on after you are running the engine, then it is running on gasoline. If the light does not come on at all (no bulb check) you are probabally running on gasoline. Is the check engine light on? What in the world did you do to "hotwire" the fuel pump? You could smash the switch in the dash with a hammer and it wouldn't affect CNG/gasoline operation--so that's not the problem. If the bulb doesn't come on at all, check the 20 amp fuse marked CNG under the hood and see if it's alright. If the gasoline fuel pump isn't running there are other fuses to check and a relay that could be causing problems. Get that hotwire off the fuel pump as soon as possible as it could cost you real problems down the road.

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    • #3
      Re: cng cavalier doesnt start and stay running

      there is no check engine light on. all i did to hot wire the fuel pump was run a wire from the positive on the battery and run it all the way to the fuel pump wires and did it like that, i ran it for about 10 seconds and that was it. i do have a owners manual but it really doesnt say much about it, they never really do. as for the switch, someone cut all the wires off of it and i soldered them back on, and there is no light at all that comes on. i have checked about every fuse there is on the car, i have yet to see one that is marked "cng" though.
      but i will go back out to it now and recheck everything to make sure.

      now when you say "If the bulb check is OK and the light stays on after you are running the engine, then it is running on gasoline. If the light does not come on at all (no bulb check) you are probabally running on gasoline." what do you mean by that?
      thanks for you help.
      Last edited by baascavy; 10-21-2009, 05:32 PM.

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      • #4
        Re: cng cavalier doesnt start and stay running

        There are two engine ECU's on the CNG Cavalier. In the 1999 model, the gasoline ECU is in front of the right wheelwell. The CNG ECU is behind the drivers side headlight. The bulb in the switch on the dash receives it's power from the CNG ECU. If the bulb is burned out or the CNG ECU has failed, or the ECU isn't receiving power, the bulb won't light---Therefore, you are not running on CNG. If the gas gauge is showing consumption of fuel and the level in the gasoline tank is going down, you are not running on CNG. There is no reason for anyone to cut the wires on the dash switch, so I would assume someone who had no idea how the system works has tried to "fix" it . If the CNG ECU has failed, or does not have power, no MIL (check engine light) will come on as it gets power from the CNG ECU. The CNG fuse I am talking about is a 20 amp fuse located in the fuse box next to the battery. The cover has a chart on it showing the position for the fuse. Some one may have removed it during their "fixes". Also check the Maxi fuse (in the same fusebox) labeled "cooling fan" and make sure it is good, and it is a 40 amp not a 30 amp fuse.

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        • #5
          Re: cng cavalier doesnt start and stay running

          okay, i found the cng fuse and pulled it! now the car actually runs!!! its runs on regular gas. the light from the switch blinks twice and then stays off. there is a check engine light on but i cant find any codes (i have a code reader). im thinking tomorrow im going to check the ECU's and see what i come up, b/c it would be really great if i could get it running on both gas's.

          i'll keep ya posted

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          • #6
            Re: cng cavalier doesnt start and stay running

            I am going to send you a PM

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            • #7
              Re: cng cavalier doesnt start and stay running

              I had a similar occurrence on my 1999 Cavalier. It supposedly had run out of CNG but it seemed a little soon as the distance didn't seem as high as it should. Then after running on gasoline it quit. It acts like it's out of gasoline even after I poured a gallon into the tank. It acts like it is attempting to start and sounds like it runs on a couple of cylinders for a second and then dies. Any clues on what the problem is? Where can I get an electrical schematic for this car?

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              • #8
                Re: cng cavalier doesnt start and stay running

                Believe or not my Cavalier is acting like the one Baascay has. It was running along on gasoline and I'd driven probably 250 to 300 miles on a full tank of gasoline. No problems and running fine. Then it slowed down like it was out of gasoline. After numerous attempts to try to start it I called a towing service and had it towed a 100 miles. This morning I tried again to start it and it continued to cough. Next, I let it crank for a while. (Normally, I don't allow one to crank for a long period.) It started and would run at an idle and after several attempts it ran fine. Now, I'm wondering if it was attempting to start on CNG even though the red light was on and if I'd allowed cranking for more than 8 seconds it would have started yesterday. Then, again, that doesn't make sense because it stopped while running on gasoline. Possibly, I had water or trash in the gasoline and it finally flushed its' way out. Maybe draining the gasoline tank and replacing the fuel filter would be the next step to take.

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                • #9
                  Re: cng cavalier doesnt start and stay running

                  Does the car run on CNG? If it does, then the first place to look is the long neglected gasoline fuel filter located near the rear axle. If changing that doesn't work, check the fuel pressure and if it is low, the fuel pump might be worn out. They usually fail far from home on a dark and stormy night . Now that we have Ethanol, water in the gas is pretty rare and if its enough to stop the car, you have a lot of it. You might ask the last gas station you were at if they have had other customers with a problem (but if they did they wouldn't tell you anyway). I think baascay's car might have other things going on. Electrical schematics are found in the service manual, CNG specific schematics are in the CNG supplement. '99 thru '02 are the same. Before you dig too far into the electrical, let me warn you that without a scan tool that can provide both DTC's and data streams, these cars are a little had work on.

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                  • #10
                    Re: cng cavalier doesnt start and stay running

                    Yes, I'm replacing the fuel filter. It does run on CNG but the red light was on indicating the CNG had run out. When the CNG tank had some in it the light would come on before it was empty for the the first trip of the day and then not be on the next time it was started. It's strange the way it stopped as there was no rough running and it just slowed down and stopped going up hill. It runs fine, now. Nothing like an intermittent problem to drive you crazy!

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                    • #11
                      Re: cng cavalier doesnt start and stay running

                      One thing I noticed this morning was when I first started the car it seemed like it was trying to start on CNG even though the light was on. Could it be trying to run on CNG with the light on? After it cranked about 8 seconds it stated.

                      Right before the problem started (car stopping) the gas gauge must have failed because it indicates the tank is full of gasoline. It also indicates a small amount of CNG in the tank.

                      The person I bought the car from said that sometimes you have to reseated the CNG computer from time to time because the connections get corroded. Is that a possible cause of the problem?

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                      • #12
                        Re: cng cavalier doesnt start and stay running

                        I finally had time to change the gasoline fuel filter. The old one did not appear
                        clogged. Car runs fine but I've only been driving around the block before braving to drive to get the CNG tank filled.

                        One thing I had noticed shortly before the car stopped was that the fuel gage showed full even though the tank was approximately 3/4 empty. It has never indicated correctly since I've had the car. I was wondering if the fuel gage and the fuel pump use a common ground that might cause the fuel to run intermittently.

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                        • #13
                          Re: cng cavalier doesnt start and stay running

                          It might be time to go through the vehicle's ground system since grounds can be a source of many problems. Make sure that the other end of the battery ground cable (besides the battery end) is clean an tight. You also might wand to run some redundent grounds from the battery to the body and engine, use 10 gauge wire.

                          Also sometime you may get moisture in the body connectors, you may want to spearate them and give them a look; make sure you disconnect the battery ground cable first

                          As to your question on common grounds, get a wiring diagram for the vehicle; that will answer your question. There are a load of small grounds to the body in a vehicle, as you find them you may want take the screw lose clean them and body contact area then put a "shake proof" (star washer) on the screw when reattaching them.

                          Hope this helps

                          Larrycng

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