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  • Diagnosis and remedy of fouled spark plugs

    Jared's suggestion of maintenance topics struck home today. Folks who've been around piston powered aircraft, for a while, will have encountered fouled plugs. Sometimes carbon flakes lodge between the side and center electrodes. That bridge, over the spark gap, creates a path for the electrical impulse that normally produces a spark. Without a gap, there is no spark. Sometimes, it's not a carbon flake, but a small lead deposit (from the gas) that bridges the gap. Plug fouling is usually discovered during mag checks. A fouled plug won't fire and causes one cylinder to produce no power. The engine runs rough, when the mag, firing the affected plug, is selected.
    Frequently, the fouling can be cleared by a high power, leaned, runup. Carbon flakes usually get blown clear and the ensuing mag chck is good. That's a good day and you go fly! Lead deposits are not so agreeable. If no amount of high power and leaning make any difference, it's probably lead. Yesterday, after 2.3 hours of flying, giving rides, several starts and just before the flight to the hangar, I encountered a persistent bad mag check. I smiled to my wife, who was excited about the sunset flight, that we weren't flying anywhere. I'd fix it in the morning. This wasn't in a Bearhawk, but it was a Lyc IO540 and it happens to them all.
    This morning, I grabbed my IR thermometer, a spare spark plug and a plug socket. I started the engine and immediately selected the bad mag. I let it run (engine missing badly on the bad mag) for about 3 or 4 minutes. I shut it down and crawled under the cowl. Then I scanned the temperature of each exhaust stack and each lower spark plug. All but #2 stacks were above 350. #2 was about 180. Similarly, the #2 plug was about 50 degrees cooler than the rest. #2 wasn't firing. I pulled the lower plug, just because it was the easy one to access and noted a very small lead globule bridging the gap. A few more minutes to install the replacement plug and we were ready for a runup. A nice smooth mag check ensued. Problem solved. Pictures of the offending plug attached.
    It didn't occur to me that the process of troubleshooting the fouled plug was very noteworthy, until my partner asked how I was able to go directly to the fouled plug. It then occurred to me that something I'd done at least a dozen times, might benefit some Bearhawk builder / flier. If you've been burning kerosene for 25 years, you've probably never had a bad mag check. Tracing mag wires and pulling all the plugs is frustrating and not necssary. If you have a rough mag check that just won't clear up, run the engine a while on the bad mag. Then check the exhaust stacks to see which is cooler. If you don't have an IR thermometer, touch a piece of plastic to the stacks-hot ones will melt it and the cooler one won't. The cool stack is where your problem plug is. Then, you toss a coin and pull a plug, or pull both. Maybe you'll be lucky and pull the right one first. One will probably look like my picture. Clean the plug, check the gap, reinstall it and go fly!

    Bill

  • #2
    One time I found a fouled plug during a mag check with the family loaded up for a long flight. Back then it was an especially big production to get the little kids into their seats and everything. I taxied back to the ramp, hopped out and opened the cowl, removed the fouled plug and replaced it with a new one from my toolbag, and we were back underway in 10 minutes. An old mechanic friend recommended a crayon as the temperature tester, but he said in his experience it was almost always the bottom plug that was fouled.

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    • #3
      another "old school" variation I've seen---- run on the bad mag for 5 min or so (hood off)
      Shut down. Immediately---- take a sharpie pen and "wipe" the plastic base end across the exh. tube up close to the
      head...... The "cold" cylinder wont make a plastic streak like the normal temp cyls did.
      Tim

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      • #4
        Fouled plugs are a real nuisance...I got so tired of it years ago that I started using "fine wire" or "iridium" plugs. They last a lot longer, (2 to 3 times longer)... no lead fouling. bad news is they are more expensive... Collin

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        • Bdflies
          Bdflies commented
          Editing a comment
          You're right on all counts. I ordered a dozen Fine wire plugs yesterday. Hard to believe a set of spark plugs is going to eat up a thousand dollars..... I have a partner in that aircraft, though. I tell my wife that I get that stuff for half price!

          Bill

      • #5
        We use NGK and haven’t seen any fowling. Probably because we throw them out every 100 hours and buy new ones. 😊.

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        • #6
          My Patrol has a single magneto, firing the top standard REM38 plugs. A Pmag fires NGK platinum plugs, in the bottom of each cylinder. The platinum NGK's don't offer much area for deposits to lodge and the Pmag is so hot it would probably blow away anything that landed there. If a plug did foul, it would be a simple matter of tapping the screen, selecting the engine management page and simply seeing which of the 4 EGT's was down. Modern technology is great! Thinking it through, my original post was aimed at folks flying lower tech equipment, I guess. The aircraft that fouled the plug, Saturday, didn't have an EMS system and doesn't even have an EGT gauge. But it's CERTIFIED. That means it's more reliable......right?

          Bill

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          • Chris In Milwaukee
            Chris In Milwaukee commented
            Editing a comment
            That's what this year's $6000 annual said on my Maule. It's better, stronger, faster, and more reliable.

        • #7
          So when you're using auto plugs (presuming the NGKs are as such), do you have to use some sort of adapter or threaded insert for them? I can't imagine that airplane engines magically accept them. But then again, I've never tried.
          ​Christopher Owens, EAA #808438
          Project "Expedition"
          Bearhawk 4-Place Scratch Built, Plans #991
          Bearhawk Patrol Scratch Built, Plans #P313
          Germantown, Wisconsin, USA

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          • Flygirl1
            Flygirl1 commented
            Editing a comment
            Dennis and I had the engine built with the smaller holes with the idea that the area would hold up better against cracks and with the price of the NGK plugs, it seemed like a no-brainer.

        • #8
          Originally posted by Chris In Milwaukee View Post
          So when you're using auto plugs (presuming the NGKs are as such), do you have to use some sort of adapter or threaded insert for them? I can't imagine that airplane engines magically accept them. But then again, I've never tried.
          Well, yes or no. Brand new cylinders can be had with the heads drilled/tapped for automotive plugs. These require no adapters. Mine are for standard aircraft plugs, so adapters are used. I got the adapters when I got the Pmag.

          $6,000 annual, huh? I did the condition inspection of the Patrol in March. A couple hundred bucks, at most. But then, the Patrol doesn't meet the standards and safety requirements, set forth by the FAA..... HAHAHAHAHAHAHAHA!!!!!!

          Sorry. I couldn't help it...

          Bill

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          • Chris In Milwaukee
            Chris In Milwaukee commented
            Editing a comment
            Some day my prints will come... Wait, they're already here...

        • #9
          That annual expense is the very reason why I left the certified market. In my opinion an experimental is a much better compromise. A side benefit is that I got a much more equipped panel with less cost.

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          • #10
            Originally posted by tailwind View Post
            That annual expense is the very reason why I left the certified market. In my opinion an experimental is a much better compromise. A side benefit is that I got a much more equipped panel with less cost.
            Truth in that. A fella's gotta fly something while he's building But hey, I did get new spark plugs out of the deal. And a new exhaust on one side. And a freshly welded and repaired spinner backplate. And new landing gear bolts. And new cooling baffles. And an oil change. And the usual annual inspection stuff. All things that run up a heck of a nice labor bill for a 25yo airplane.

            On topic question: Do you all do spark plug resistance tests in your annual condition inspection as well? Or is that specific to a particular kind of plug? I had six of eight plugs replaced this year because the resistances were out of tolerance. I do admit that the plane starts like a dream now.
            Last edited by Chris In Milwaukee; 07-19-2018, 09:46 AM.
            ​Christopher Owens, EAA #808438
            Project "Expedition"
            Bearhawk 4-Place Scratch Built, Plans #991
            Bearhawk Patrol Scratch Built, Plans #P313
            Germantown, Wisconsin, USA

            Comment


            • #11
              "Resistance test"? Never heard of such. I know the 'R' means that it has an internal resistor, presummably to reduce radio interference, but I never heard of testing resistance of the plug. Doesn't mean it isn't done, just a new concept to me. How would it change?

              The massive electrode (REM38E) plugs wear considerably in a hundred hours, or so. I check gaps at annual and adjust them once. When the gap has opened up again, the center electrode is looking like a football (to some degree), instead of round. I toss em at that point. Long long time ago, an old plane guy told me that he changed plugs at about 150 hours. I've considered that a good practice.
              Now, I'm sure I'll hear from someone who puts 500 hours on massive electrode plugs. Not saying it's not doable. I really can't say it would be less safe. To me, it's just one less thing to consider, before heading out over 50 miles of swamp.

              since we're talking about spark plugs, do all of you guys check plug gaps right out of the nice plastic container - before installation? I can't tell you how many experienced, licensed mechanics have told me they never check gaps of new plugs. "They're gapped at the factory." Well, maybe so. But they're not all the same. They're set within a tolerance. I know I'm a bit strange (hey, I can see you all nodding!), but I set mine to .019". Not .016" to .021". I set them to .019". It makes a difference!

              Bill

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              • #12
                Resistance usually goes up with age, depending on the type of resistor. Tempest recommends replacement of plugs that exceed 5000 ohms. Too high and engine runs rough.

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                • #13
                  Maybe it's how my shop gets me to spend $$$ on spark plugs and associated maintenance... I'm still learning...
                  ​Christopher Owens, EAA #808438
                  Project "Expedition"
                  Bearhawk 4-Place Scratch Built, Plans #991
                  Bearhawk Patrol Scratch Built, Plans #P313
                  Germantown, Wisconsin, USA

                  Comment


                  • #14
                    For another data point, I recently read a book called Mike Busch on Engines. Mike says....

                    -When the plugs electrode erodes into a football shape and the short dimension is 1/2 the length longer dimension, its time to retire the plug. Go-no go gages are available
                    -Massive Electrode aviation plugs typically last 400-500 hours, fine wire plugs are good for at least 1500 hours.
                    -Mike does an Ignition Stress Test, aka an Inflight LOP Mag Check and uses an engine monitor, like JPI to evaluate his engine's electrical system.

                    I guess the Stress Test is pretty demanding on the plugs and magnetos, and gives him another data point to evaluate the plugs. Its more demanding than the Bomb Test, but can't be done in a maintenance hanger. The data is downloaded and evaluated. He's Looking for stable EGT's on each mag, an EGT rise when on a single mag. I think He would say, "if it passes the stress test, the plugs are airworthy."

                    Beware, I write this from book knowledge....not experience.. Mike will be doing seminars at Oshkosh. Check him out if you have not heard him speak. Ivy League graduate, Computer/math back ground with an A&P/IA and a boat load of data he has gathered to back up his viewpoint that not always status quo.
                    Brooks Cone
                    Southeast Michigan
                    Patrol #303, Kit build

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                    • Bdflies
                      Bdflies commented
                      Editing a comment
                      Man, I was joking about 500 hours!

                      Bill

                  • #15
                    The reason for the resistance check is that Champion delivered a huge number of massive electrode plugs with “defective” ceramic insulators that would fail very early in usage. The first visible symptom was high resistance when tested. Tempest plugs and the latest batch of Champions don’t suffer from that issue (at least out of the box). But the test is a good idea anyway...
                    Jim Parker
                    Farmersville, TX (NE of Dallas)
                    Patrol Quick-Build Serial # P312

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