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Help with Chevy /Impco bifuel van

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  • Help with Chevy /Impco bifuel van

    My van rolls over to gas before the CNG is out (around 1/2 a tank) while I am on long road trips. Around town, it does not happen, only on long trips. But if I stop and shut it down, when I start up again it starts on CNG (because there is some there). My mechanic (who is not certified...there are none in my area- he has the manual and studies it everytime he works on my van) says that it could be the pressure sensor in the tanks- on long drives where I am draining down the tanks, the drop in pressure cools the tanks and contracts the fuel, dropping the pressure even more and he thinks that it is giving a false "empty" read to the pressure sensor causing the brain to roll over to gas. Does this make sense? Anyone seen this before? Again, it only happens if I am driving straight for long periods of time.

  • #2
    Re: Help with Chevy /Impco bifuel van

    Just a wild guess here. Your mechanic is correct about the pressure drop due to the CNG cooling as the engine withdraws it from the tank at a high rate on the highway. If the fuel guage is reading correctly, (i.e. it is reading 1/2 full when the transfer to gasoline occurs) then the pressure and temperature sensors are most likely working correctly. Question--did this just start in cold weather or has it been happening in warm weather? If it is temperature related, you could be getting ice in the high pressure regulator shutting off the flow of CNG causing the changeover. The regulator is heated with cooling water which would thaw it out after running for a while on gasoline which allows the switch back to CNG you describe. If you think this might be the situation you need to check the water lines going to the regulator and make sure it is getting warm. Also, you have moisture getting into the tank from however you receive your gas supply.

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    • #3
      Re: Help with Chevy /Impco bifuel van

      Siai47, Good word, thanks. Can you coach me on how I would find those water lines and the regulator, and then determine if the coolant (heat-ant really) is getting out there?
      Matt



      Originally posted by siai47 View Post
      Just a wild guess here. Your mechanic is correct about the pressure drop due to the CNG cooling as the engine withdraws it from the tank at a high rate on the highway. If the fuel guage is reading correctly, (i.e. it is reading 1/2 full when the transfer to gasoline occurs) then the pressure and temperature sensors are most likely working correctly. Question--did this just start in cold weather or has it been happening in warm weather? If it is temperature related, you could be getting ice in the high pressure regulator shutting off the flow of CNG causing the changeover. The regulator is heated with cooling water which would thaw it out after running for a while on gasoline which allows the switch back to CNG you describe. If you think this might be the situation you need to check the water lines going to the regulator and make sure it is getting warm. Also, you have moisture getting into the tank from however you receive your gas supply.

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      • #4
        Re: Help with Chevy /Impco bifuel van

        and if it does turn out to be that it needs to be hotter cheapest fix could be changing the thermostat. having to do that on my chevy pickup just because the heater sucks in the winter right now it has a 160 in there. wondering if it would be wise to put a 190 in would this be too hot for the cng system? haven't seen this question on here before so had to ask.

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        • #5
          Re: Help with Chevy /Impco bifuel van

          The high pressure regulator is located just inside the left frame rail by the drivers door. You will see a stainless line coming from the rear running into the top of the regulator, there will also be another line running out it and connecting to the low pressure regulator. You will also see two brass 90 degree fittings with rubber hoses attached. These are the cooling (heating) lines. When the engine is warmed up and running the regulator and the hoses should be warm to the touch. If not, follow the lines up to the front of the van. The rubber hoses attach to two stainless steel lines that then attach with additional rubber hoses to a pair of "t's" which are inserted in the vehicle heater water lines. Make sure this is all still connected. When you call for maximum heat in the vehicle, most of the water is diverted through the heater and not much makes it to the regulator---just when you need it most . If the hoses or regulator aren't getting warm, the lines might need to be flushed or maybe someone disconnected the lines when replacing hoses. In addition, while you are crawling around the high pressure regulator, it might be a good time to change the inlet filter which is on the top. If you search the forum, there are some good descriptions on how to do this--I think a Cavalier is shown--however your regulator is the same. Have your dealer parts guy pull up sheet TG03-051 for a breakdown of the coolant lines and TG03-058 for a great look at the regulator and filter. Part 11 on the drawing is the filter itself and they sell a kit with all the seals and spring for the filter change. Make sure the spring sits in position to push on the filter as it can be dropped in sideways allowing crud to pass around the filter. You might want to drop the regulator out of the mount to make the job easier.

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