LOP back fire Intermittant issue.

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  • #84498

    Hi,

    Got an issue which I am having trouble solving.

    32/2300 running LOP at approx 13.7 GPH. I will get a back fire randomly on the RH engine. Some times it won’t do it at all, some times it keeps going every 5 minutes. The only way to fix it is to run ROP, it never does it when ROP. It will do at at 1500 through to 25000ft.

    I have changed plugs, Mags O/H due 500 hours last annual ( 2 months ago approx 50 hours), swapped VAPC RH to LH problem stayed. I never had GAMI injectors but installed them today and problem persists.

    If I am LOP and advance the LH MP the fuel flow stays constant. if I do the same with RH engine the fuel flow will increase( this stayed the same after swapping VAPC). I am not sure if this is related.

    When it back fires the MP then starts fluctuating, causing fuel flows to rise and fall and the only solution is to run ROP till it settles down.

    Any suggestions, comments etc

    Thanks

    Andrew

    #100871

    If the engine backfires, I’d expect the manifold pressure to drop. Basically you’re not getting the power you need to keep the turbo producing boost briefly, so it will hiccup.

    The right engine on the 310 did the same thing prior to overhaul. A few things that can contribute:

    -Plugs (you did those)
    -Mags (you did those – are they Slick or Bendix?)
    -Ignition harnesses (you didn’t mention condition, but it’s worth looking at)
    -Fuel servo/controller
    -Water hiding in fuel

    How lean are you WRT peak? How many hours on your engines? Any fuel system issues?

    If you’re getting different behavior between the two engines advancing the throttle WRT fuel flow, that makes me think a fuel system issue. Remember that when LOP, you are much closer to the engine shutting down due to lack of fuel. The backfire/hiccup basically means you’re not getting sufficient fuel to keep the engine running. So GAMIjectors wouldn’t fix this problem, and I’d also expect that it goes away running ROP. Basically, the hiccup is still going to be there, but when it hiccups it never goes lean enough to cause the engine to have a noticeable change in power.

    I’d look at the ignition harness first, though.

    #100897
    rwelsh
    Participant

      Andrew, hopefully you replaced the plugs with Tempest plugs. Even some new Champions have high resistance issues because Champion does not think anything is wrong with high resistance. When running LOP, every plug has to be in top shape. My engine will hiccup if one plug has resistance of over 2K ohms. The Tempest plugs do not go to high resistance because Tempest uses a different style of resistor and their’s is also sealed so no moisture can get in. One of my engine’s MP increases when I advance the throttle when LOP. I don’t know why, but I live with it, by reducing the MP when at final LOP position on the mixture control. If you have Slick/Unison mags and not Bendix 1200 series, then you could be getting flashover on the mag distributor block even if you have pressurized Slick/Unison mags. In my experience, it is usually the plugs if you have Bendix 1200 mags and can be the mags and or plugs with the Slick/Unison mags. Even Bendix 1200 mags can go bad after 500+ hours since new or a good overhaul.
      Dick Welsh

      #100957
      quote RWELSH:

      Andrew, hopefully you replaced the plugs with Tempest plugs. do you have Slick/Unison mags and not Bendix 1200

      Hi Dick,

      Yep got the tempest – massive.

      I’ve got the Bendix 1200 series.

      Andrew

      #100959

      [quote=
      -Mags (you did those – are they Slick or Bendix?)
      -Ignition harnesses (you didn’t mention condition, but it’s worth looking at)
      -Fuel servo/controller
      -Water hiding in fuel[/quote]

      Hi Ted,

      Bendix mags.
      Have checked the harness out visually, any tests I can do?
      Gues fuel servo would be an O/H?
      I am certain it’s not water.

      Andrew

      #100961

      Andrew,
      LOP is rewarding from a fuel consumption standpoint, but demanding on the ignition system. I would take another look at the harness. If it is >10 years old I would replace it with a new skytronics harness.

      I would also suggest (with mags off) opening the small inspection port and checking the mag teeth as someone pulls the prop around. Look for tooth damage.

      if you are running LOP, and getting backfiring then this generally means you have a cylinder that is too lean – the fuel does not combust in the cylinder all the time, and the excess burns in the exhaust.

      Have you done a GAMI lean test? what engine instruments do you have?
      For that cylinder, you would likely see CHT’s go down and EGT’s go up during misfire.

      #100963

      We’re the Mags given a full overhaul or IRAN? On mags, I prefer a full overhaul to make sure the spark-producing parts are fresh. I’d also agree with Eric that, if the harnesses are older, to go for a replacement.

      Fuel servo would need an overhaul, yes. I would focus on the ignition first. Although there are tests you can do on the harness (resistance, really) I don’t put a lot of stock in them as I’ve seen them miss a problem more than once. If the harnesses are old, just go for a new one.

      How many hours are on your engines?

      #100964
      rwelsh
      Participant

        Andrew, Rereading your original post I see your fuel flow was 13.7 at 32″/2300 RPM. That is really lean, probably 80 to 90 degrees LOP. For most engines, that is too lean. I would use 14.5 to around 15 GPH at that power setting. Your speed fall off will be less and you NMPG should be better at the higher FF. Do you increase the MP 3″ after you set the LOP power setting? I have found that 3″ recovers all the speed lost because of the LOP setting, and you can still lean it to around 15 GPH after increasing the MP the 3″. See my new post under the Operational Forum.

        #100980

        A few questions for you.
        1. How do the TIT/EGT compare engine to engine? (A difference could indicate timing problem, induction leak, or turbo issue.)
        2. When you say backfire do you mean a miss or an actual explosion?
        3. Once you installed the GAMI’s did you do the lean test. I recommend 10,000 feet as a good altitude. High enough to get the turbo working low enough that engine temps don’t get crazy out of control.

        You need an engine free of induction leaks. These can also occur around the upper deck pressurization lines, these are the lines that pressurize the injector to assist in good vaporization.

        One other possibility is a small speck of something in one of the injectors.

        #101021

        Hi Dan,

        quote DMOORE1:

        A few questions for you.

        1. How do the TIT/EGT compare engine to engine? (A difference could indicate timing problem, induction leak, or turbo issue.)

        The RH engine is about 100F hotter then the LH engine. Timing has been checked and I did the induction leak test. How would the turbo affect this?

        quote DMOORE1:

        2. When you say backfire do you mean a miss or an actual explosion?

        A miss.

        quote DMOORE1:

        3. Once you installed the GAMI’s did you do the lean test. I recommend 10,000 feet as a good altitude. High enough to get the turbo working low enough that engine temps don’t get crazy out of control.

        Yeh I did and also a LOP mag test.

        Hopefully this will work, here is a LOP test with mag check at 10,000 32/2300

        https://www.savvyanalysis.com/flight/408001/13790927-6426-4839-93bd-da7311307d8a

        quote DMOORE1:

        You need an engine free of induction leaks. These can also occur around the upper deck pressurization lines, these are the lines that pressurize the injector to assist in good vaporization.

        One other possibility is a small speck of something in one of the injectors.

        It was doing it with the original injectors and now with the GAMI’s as well. This has been happening for about 2-3 months now.

        Andrew

        #101022
        quote RWELSH:

        That is really lean, probably 80 to 90 degrees LOP. For most engines, that is too lean. I would use 14.5 to around 15 GPH at that power setting. Your speed fall off will be less and you NMPG should be better at the higher FF. Do you increase the MP 3″ after you set the LOP power setting?

        Correct, the LH engine runs fine at this. It happens from about 50LOP. I don’t have the AA intercoolers but do try to get about 2″ back. I used to be able to run fine this in only a new issue FYI. Engine about 1000 TSO.

        quote RWELSH:

        I have found that 3″ recovers all the speed lost because of the LOP setting, and you can still lean it to around 15 GPH after increasing the MP the 3″. See my new post under the Operational Forum.

        Yeh I have tried this from what you have said, mine just don’t feel to good at this.

        Hopefully you can see this data.

        https://www.savvyanalysis.com/flight/408001/e93effdb-ffcf-484b-b952-b7fcb6ac015b

        Thanks

        Andrew

        #101033
        rwelsh
        Participant

          Andrew, I don’t see anything wrong with the first Savvy printout, but I could not download the second Savvy you posted in the latter post. The first one is what mine and most look like. The spread in EGT and CHTs is normal from my experience. Se eit you can post the second Savvy.
          Dick

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