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Cng keeps shutting off and switching back to gasoline mode

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  • Cng keeps shutting off and switching back to gasoline mode

    I just spent 20 minutes making a detailed post and accidentally erased it so here is a short version.

    I have a 2004 f150 5.4 4wd factory bi-fuel.

    my cng was not working and it was running on gasoline, so i filled it up with cng and it took around 11 gallons. when i restarted it, the cng was still not working. i drove it 10 miles and it still didn't work. i turned it off for an hour then turned it back on and it was working so i drive it around 200 feet and it turned off. i parked it for the the night and when i turned it on this morning, it was working and stayed working all day for 147 miles. i filled it up tonight and it stopped working. i drove the truck 10 miles then turned it off for an hour and then restarted it and the cng was working. i drove it 200 feet and the cng stopped working and it automatically switched over to gasoline. i then turned it off and restarted it and the cng was working so i drove it again but accelerated very slowly to 35 mph and the cxit stayed working. i then pushed the pedal down around 2/3rd of the way and it stopped working. it seems like it the cng system is shutting off for a while after i fill up the cng tank and then shutting off the system under load if i accelerate too quickly.

    i read on this site that the high pressure regulator needs to be changed on some of these every 70k miles or so if it is in a cold climate but this has always been in so cal and that valve shouldn't cause this type of problem as far as i can figure.

    thanks for any suggestions as to the cause of the problem.

  • #2
    You may indeed have a pressure regulator problem, or a partially sticking tank solenoid valve. Both of these often show up in cold weather and / or after a fill up .Doesn't need freezing weather even in so cal can cause degraded components to act up. Forget the 70,000 mile component life. These components are often degraded by age and contaminants in the fuel, being station compressor oil carryover and excess water, both from poorly maintained fuel stations.


    • #3
      Concur that it is likely the pressure regulator, and it might be as simple as ensuring the coolant lines aren't somehow being blocked or clogged between the engine and the regulator. These lines are there to provide warmth to the regulator to prevent it from freezing-up as the pressure drops from, say 3600 psi to 100 psi. (Yes they will freeze even in balmy Southern California if the coolant isn't flowing).

      Are you getting a check engine light / codes?


      • #4
        Thanks very much for the replies.

        No check engine light and it just passed the emissions test.

        I just drove it today and it is colder than it was last night when I finally parked it and the CNG is working perfectly again. This makes 2 days in a row that the CNG wouldn't work right until the vehicle had been parked for many hours, and since it was colder today then yesterday, it again suggests to me that this is not a valve problem, but I have no idea how these valves work. It certainly eliminates the possibility of it being a contaminated valve though.

        I am more than happy to change any parts it may need, however, I can't afford to just start changing parts without knowing which ones are actually bad.

        I think I will just fill it around 2/3rd full next time it needs CNG and see what happens. If it works fine, then it seems like it would likely be the regulator or solenoid unless there is a high pressure shut off solenoid, in which case, that could possibly be the problem.

        Does this sound like reasonable plan?

        Thanks again for the help.


        • #5
          My 2001 BiFuel has a similar issue. it runs on CNG when I start the engine cold, fine acceleration and all. Once temperature goes up it switches to gasoline. When I then stop the engine and turn ignition on, I see the ALT-light trurn green for 1sec and off without cranking the engine.
          I checked the regulator it seems not to be frozen - any way to diagnose it?
          Last weekend I changed the filter and when I removed the old one it was torn - could there be a piece of it stuck in the gasoline? but why would it run when cold?


          • #6
            I had a similar issue with switching to gasoline about every 5 miles or so on a 100 mile trip. On the way back the CNG quit completely. After a while it occurred to me to check the fuses. Yep, burned fuse holder. Check the fuses. (Prins after market system)


            • #7
              Thanks, I did check the fuses (the two mounted on the side of the battery) and they are not blown.
              Also, truck starts up on CNG when cold.
              Last edited by OhMyGas; 1 week ago.


              • #8
                I think it may be a filter that is plugging I dont know what kind of hp regulator you have but some regulators have a small stone filter in the hp fitting such the impco hp 3600 took me a lot of looking to find that one I would get the manual and find and change all filters and look for kinked lines as they can act as a orfice and cause freezing problems on 100 degree days good luck