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  • New to CNG, about to give up

    I bought a CNG Cav with all of the best intentions. This thing is quite funny, it ran great for awhile and now is having all kinds of issues. First it suddenly started having issues firing on CNG. I would shut the car off and wait awhile and then it would fire on CNG. Kind of erratic. Then it started dying when slowing down or cornering, sometimes it would just lull and then go again, sometimes it will die completely. I just bump into neutral while coasting and refire the car and away I go. Just today I had to wait for a meeting so I was sitting in the car and noticed really loud popping noises in the back of the car. It didnt do it long enough to find where the popping was coming from. I did notice the HPR was frosted, I thought it was supposed to have coolant pumped to it so I was going to look at it later. One thing I did notice when it lulled and then took off again the gas gauge seemed to drop off and then come back up. I am going to keep driving it to get more of an idea of when and why it acts up, but thought I would post and see if anyone had any similar issues.

  • #2
    Re: New to CNG, about to give up

    mdemco,

    Let's take one thing at a time. Start with the 'frosted HPR'. Check the coolant level in the radiator then check that the lines to and from the regualtor are clear. It' sounds to me like the HPR is freezing up and slowing or stopping gas flow.

    Report back...and we'll go on to the next item.

    Don't give up.

    afvman

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    • #3
      Re: New to CNG, about to give up

      I would agree with Afvman on this one. In cold weather, before the coolant warms up enough to heat the regulator, moisture in the NG can freeze causing the regulator to stick and open the relief valve. It is also possible to overcome the ability of the regulator heating system to prevent freezing if the tank temperature is very cold and you are driving at high speed (withdrawing NG at a high rate further cooling the fuel). Even on a hot summer day, at high speed, the tank temperatures will drop below freezing.

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      • #4
        Re: New to CNG, about to give up

        First, if you need to get somewhere, pull the 'Alt Fuel' fuse in the engine compartment and it will run on gasoline regardless of whether there is CNG in the tank. What year Cav?
        02 GX
        01 GX
        03 Crown Vic
        06 GX
        Home Fueler

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        • #5
          Re: New to CNG, about to give up

          I believe the fuse is marked "CNG" in 98-02 Cavaliers and "alt fuel" or "af" in 03-04 Cavaliers. Pull it and the car doesn't even know it has a CNG system. At least it will get you around on gasoline.

          That HPR relief valve popping can take you by surprise. I've only had it happen to me once with a car that had been parked for a few weeks and was nearly completely buried in drifted snow. Once it warmed up the the HPR was fine.

          Where are you located? If you're nearby I could recommend a few local shops. We may even have a spare HPR out of a parts car if you need to replace it.

          -Derek
          www.CNGUtah.com

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          • #6
            Re: New to CNG, about to give up

            The cav is a 98 with 151k on it. This weekend I will pull the coolant lines from the HPR and check coolant flow to it. I actually rebuilt the top end of the engine and did not even notice lines running back to the HPR. When I was reading on here I was surprised to read about it. I will post back my findings, I love this little car even if it is giving me issues and will pursue it until corrected.

            This morning I ran over to one of our shops, went back to the office and when I started it 5-10 minutes later it was running the worst it ever has. Starting out if I gave it gas it would sputter and when I slowed down it was doing the same. Again I noticed the gas gauge dropping and it finally died around a corner. I fired it right back up and it was running fine!! Funny little thing.

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            • #7
              Re: New to CNG, about to give up

              This thing is an interesting little car. I have not heard the popping noises for at least a month and I get great mileage when the car works. It seems like now that it is warming up outside the car is getting worse. I am constantly bumping the car into neutral and restarting it to switch to CNG. If I am on the freeway and need to pass I can not do it because I switch to gas, if I pull away from a light or pull into traffic I have to ease into it or it switches to gas. If I neutral the car it always starts back up on cng. I have noticed the occasional wiff of gas from the LPR but nothing I am to concerned about, I soaped it an no bubbles.Also it seems like the power is down at the top end of the rpm's like it is leaning out under throttle. From reading every post in this forum it seems I may have a GMS issue, whatever it is it will not be good. I have already put about 5k on the car and do not really want to lose it but some of these parts are worth more than the car.

              Comment


              • #8
                Re: New to CNG, about to give up

                With 151K on the car you might want to check the CNG fuel filter located in the top of the HPR. If this is plugged (or starting to) you may notice the type of problem you are reporting. It may be something else, but this is worth a look. You need to know what you are doing to relieve the pressure in the system before you start, but the repair/inspection is simple.

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                • #9
                  Re: New to CNG, about to give up

                  Hmm, I thought only the newer Cav's had the CNG filter, not the 98-02 models? If I am wrong correct me and I will check it tonight.

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                  • #10
                    Re: New to CNG, about to give up

                    mdemco,

                    All the Cavaliers have a filter just before the HPR. The manual recommends cleaning it or switching it out for a new one every 60k miles.
                    Jared.
                    Mountain Green, Utah
                    2003 CNG Cavalier
                    2003 CNG Silverado 2500HD

                    Comment


                    • #11
                      Re: New to CNG, about to give up

                      Ok I will go and try and find a procedure for pulling the filter and then go from there. I will report back!!

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                      • #12
                        Re: New to CNG, about to give up

                        Hope this drawing helps you out on this. You are dealing with high pressure CNG here so if you don't feel comfortable with this---don't do it. The HPR is bolted through the floor pan near the drivers side of the vehicle in the trunk. To access the area that you need to work on, lower the rear seatback, remove the clamp at the end of the rubber boot coming from the tank at the floor. Roll the boot back (like rolling a sock) to gain access to the fuel line. At this point, there is high pressure CNG in the line. There are several ways to bleed this fuel. You can close the high pressure lockoff valve with an allen wrench or remove the single connector that is exposed in the trunk to shut off power to the HPL. Either way you need to start the car on CNG and watch the fuel gauge fall to empty and switch over to gasoline. At this point there is still about 200 psi in the line. That can be bled by loosening the fuel line at the low pressure lockoff valve under the hood until the remaining pressure bleeds off. Back in the trunk you will find a single line going to the center of the HPR. This is the line you want to remove. Hold the nut at item six in the attached picture and remove the tube nut. There is a small "O" ring face seal located in a recess in item six. Don't loose it and if you do, don't replace it with a "O" ring--they are different. Next remove item six from the HPR. You will then take a small pair of tweezers and remove the spring item four. The filter itself is item three which is a porous metal filter. The easy way to remove this is to turn the car upside down on its roof and shake it until it falls out . If you want the next easiest way, try putting a little chewing gum or clay on the end of your tweezers and sticking it to the filter and then lift it out. It may take a couple of tries, but it will come out. The filter can be cleaned with carb cleaner and reinstalled or you can buy a new filter kit from the dealer. When you reinstall it, make sure the closed end of the filter is up and when you drop in the spring, make sure it stays standing straight up. Before you re-install the nut, item six, check the "O" ring item five for condition. Put some vasoline on the "O" ring and on the face seal before screwing item six back in the HPR. Finally re-install the tube nut making sure the face seal remains in the groove. Both item six and the tube nut need to be tight, but you don't want to overtighten them---the "O" rings are the seal. Reconnect (or tighten) the tube nut at the low pressure regulator--use vasoline on the seal if you removed it. Connect the power connector at the HPL and/or open the shutoff valve. Start the car, check all the connections you removed or loosened with soapy water and make sure there are no bubbles. If all is OK clamp the rubber boot back around the regulator base and you are done. Hope this helps and solves the problem. If it doesn't fix it, report back and we work work our way through it untill it either runs right or you get tired of listening to me .
                        Attached Files

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                        • #13
                          Re: New to CNG, about to give up

                          OK, last night I pulled that filter out. It was a little dirty, I ran about a half a can of brake cleaner through it. Then I pulled out on the highway and held the throttle wide open until I hit third gear and it did not switch to gas. I have to commute in the car today so I should be able to see if it is any better.

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                          • #14
                            Re: New to CNG, about to give up

                            Well, since I have cleared the filter out the car has not once switched back over to gas under throttle. I am not trying to jinx myself but I do not downshift when going up hills as much and the power seems to be much better. The popping from the back of the car returned today. I was outside and had just drove the car 20 miles and it popped and popped for probably 10 minutes. It seemed to be under the car in the back, I didnt have a jack but I assume it was the hpr, what causes this?

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                            • #15
                              Re: New to CNG, about to give up

                              It's the HPR failing. The popping occurs when the intermediate pressure exceeds a preset amount and causes the PRD (pressure relief device) to open. You can see where it is mounted on the HPR if you look at the picture I posted in this thread. I works just like a safety valve on an air compressor. If your HPR is messing up, it could cause a lot of your issues. To much pressure when it isn't needed and to low pressure when needed (such as under load). Usually caused when the HPR freezes up during cold weather it can clear itself up after the cooling water heats the HPR and thaws it out. Like any regulator, it can fail or stick--you really need a method of monitoring the test port pressure when the car is being driven to diagnose the issue. Best thing to do is just change it for one on E-bay---they are there most of the time. Although the Impco HPR 3600 will work, look for the HPR 3600-GMC which has the ports in the proper position.

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