Inflight Misfire

[RyanB]

Not sure if we can determine a smoking gun here or not, but it’s worth a shot.

Recently I’ve been experiencing what seems like a misfire or skip during flight. The run-up seems normal and everything is fine during high power (takeoff and climb), but after leveling off and reducing power to cruise, a misfire or skip-like episode occurs intermittently. I can tell that there seems to be slightly more vibration in the controls (pedals and yoke) than usual as well, like the engine just isn’t quite running as smoothly as it should be. To add, sometimes the skip will also be noticeable when reducing power to descend. I wish I could describe it better, but that’s the best description I can provide.

Today, the IA checked all eight spark plugs under pressure and found one that was bad and replaced with new, cleaned the remaining seven and rotated them from top to bottom. The problem still persists, so the plugs can be ruled out. The mags are the next item on the list to be examined. They’re 23yr old slicks and have 430hrs on them, so it’s about time (is time in my book) for the 500hr service. I did an in-flight mag check and the skip/misfire occurs noticeably, so I’m quite certain it’s an ignition issue.

If the mags come back and the problem still persists, what might the next thing be? Could a sticking valve cause what I’ve described? And has anyone else experienced something like this?

 

[Dana]

There are a few ADs and SBs on Slick mags, depending on date and/or serial range. Most are related to the points or distributor gear, which could cause an intermittent misfire. There is also a coil issue on some older mags... I was getting a misfire in flight, finally narrowed it down to the coil which was misfiring whenever the magneto case reached a certain temperature.

At 23 years and 430 hours it's probably time.

 

[RyanB]

I was getting a misfire in flight, finally narrowed it down to the coil which was misfiring whenever the magneto case reached a certain temperature.

I wonder if that’s not part of the issue I’m having. It doesn’t seem to act up until I level off at altitude for a few minutes, but it can be sporadic, almost as though it’s after the engine really warms up.

Did you just replace the ignition harness on both mags?

 

[Radar Contact]

How do you lean?

Also, I’d try an inflight mag check. May help narrow things down quickly.

*know some people here are terrified of it for some reason but in my opinion (what you paid and all) it can help identify a weak ignition systems.

 

[RyanB]

How do you lean?

I’ve got a single probe EGT on cyl #4 and lean via that. Usually I’m 75-100ROP during cruise.

Also, I’d try an inflight mag check. May help narrow things down quickly.

I did an in-flight mag check and the skip/misfire occurs noticeably, so I’m quite certain it’s an ignition issue.

 

[Sinistar]

Do you lean (to some amount) on your climb out as well?

If your engine is doing this often enough have you tried going full rich to see if it goes away.

 

[RyanB]

Do you lean (to some amount) on your climb out as well?

If your engine is doing this often enough have you tried going full rich to see if it goes away.

Yes and Yes. No change that I’ve noticed.

 

[Sinistar]

Earlier into "learning" our plane i would lean a bit too much on the climb out and it seemed to "tell me" just right after I leveled off and speed started to build up. I still lean a fair amount of the climb but not as aggressively now. It hasn't done it since that single change. Just thought I'd mention it in case its not mags and something more fuel related...probably not though.

 

[benyflyguy]

Carbureted or injected???

 

[Dan Thomas]

Make sure there isn't an induction leak in one of the intake tubes to the cylinders. One misfiring plug shouldn't cause that much of a skip, but a cylinder running lean will show up real quick when both plug can't fire the lean mixture. At full power the manifold pressure is high enough that not much air will be drawn past a loose coupling hose or broken gasket at the head, but closing the throttle reduces that pressure and a lot more air gets sucked in, leaning that cylinder into misfire territory.

23-year-old mags? I'd be doing them even if they only had 100 hours on them.

A flashlight and small mirror can reveal a cracked or broken gasket. The bolts might be loose.

 

[Daleandee]

Also, I’d try an inflight mag check. May help narrow things down quickly.

*know some people here are terrified of it for some reason but in my opinion (what you paid and all) it can help identify a weak ignition systems.

What???? After three pages of debate you still suggest that someone actually turn off a mag?

I guess it's easy for you to say cause you got a spare engine!

 

[Clip4]

Contaminated fuel, plugs, mags, ignition leads, leaking intake, sticking valve in that order.

 

[RyanB]

Make sure there isn't an induction leak in one of the intake tubes to the cylinders. One misfiring plug shouldn't cause that much of a skip, but a cylinder running lean will show up real quick when both plug can't fire the lean mixture. At full power the manifold pressure is high enough that not much air will be drawn past a loose coupling hose or broken gasket at the head, but closing the throttle reduces that pressure and a lot more air gets sucked in, leaning that cylinder into misfire territory.

23-year-old mags? I'd be doing them even if they only had 100 hours on them.

A flashlight and small mirror can reveal a cracked or broken gasket. The bolts might be loose.

Thanks Dan! Question, how do you determine when the spark plugs need replacement? The mechanic tested all eight today under pressure and said only one failed and he replaced, but the remaining seven are also 23yrs old. So do you replace by age? Time in service? Or if they test good, do you just keep using them?

 

[Dan Thomas]

Contaminated fuel, plugs, mags, ignition leads, leaking intake, sticking valve in that order.

Fuel is the first thing most people will blame, and it's the last thing that's is actually responsible. Ignition makes up about 90% of performance problems. So many folks spend awesome amounts of time and money fooling with carburetors or injection components, and find in the end that the ignition was doing it. It's the reason we have one carb but two magnetos and two sets of plugs. The fuel system also has at least three filters in it: one at the tank exit, one in the fuel strainer, and one in the carb inlet. That last one is so fine that water won't go through it if it's wet with fuel. A fuel injection system has one more in the injection distributor, another really fine filter. If the strainer is maintained properly and drained before every flight, and the tanks are sumped, water won't be a problem. When carbs quit, they tend to quit altogether. No fooling around.

The symptoms suggest an intake leak.

 

[Dan Thomas]

Thanks Dan! Question, how do you determine when the spark plugs need replacement? The mechanic tested all eight today under pressure and said only one failed and he replaced, but the remaining seven are also 23yrs old. So do you replace by age? Time in service? Or if they test good, do you just keep using them?

Keep using them if they spark at 135 PSI. Another test is to measure the resistance of the internal resistor: one ohhmeter lead on the contact at the bottom of the connector well, and the other on the end of the center electrode. Max 5000 ohms. And if they're 23-year-old Champions, all bets are off. They'll have a spring-loaded resistor that is famous for failing, and I've long suspected that they might be fine at room temperature but fail when hot. Hard to isolate that.

 

[Clip4]

Fuel is the first thing most people will blame, and it's the last thing that's is actually responsible. Ignition makes up about 90% of performance problems. So many folks spend awesome amounts of time and money fooling with carburetors or injection components, and find in the end that the ignition was doing it. It's the reason we have one carb but two magnetos and two sets of plugs. The fuel system also has at least three filters in it: one at the tank exit, one in the fuel strainer, and one in the carb inlet. That last one is so fine that water won't go through it if it's wet with fuel. A fuel injection system has one more in the injection distributor, another really fine filter. If the strainer is maintained properly and drained before every flight, and the tanks are sumped, water won't be a problem. When carbs quit, they tend to quit altogether. No fooling around.

The symptoms suggest an intake leak.

Fuel is the cheapest to trouble shoot and doesn’t require an AP. Most GA piston aircraft have maybe one fuel filters. The strainer and fuel inlets are screens.

 

[Flocker]

What kind of engine?

 

[Possum]

Had a similar experience with my 172. One problem was the engine would "diesel" when you attempted to cut it off by pulling the mixture out. Then we had a skip, sputter, rough running episode while at cruise. Had an A&P remove the plugs and run a compression check. Found out that the plugs were slightly fouled and compression was good. Then at annual the IA discovered that the carburetor was in his words "worn out". Simply put, the carb would not lean correctly and was running too rich. Replaced the carb and things are good. Hope that helps.

 

[RyanB]

Keep using them if they spark at 135 PSI. Another test is to measure the resistance of the internal resistor: one ohhmeter lead on the contact at the bottom of the connector well, and the other on the end of the center electrode. Max 5000 ohms. And if they're 23-year-old Champions, all bets are off. They'll have a spring-loaded resistor that is famous for failing, and I've long suspected that they might be fine at room temperature but fail when hot. Hard to isolate that.

Would you suggest continuing to run 23yr old plugs if they test fine under pressure? I hate to go with all new, if it’s not necessary, but also hate to troubleshoot deeper if it’s not really needed. Double edged sword.

When I did the inflight mag check, it ran ‘ok’ on both mags for a few seconds and then made the pronounced skip on R and L, but never dropped below 75rpm on either side. Seems like it has to be ignition related. My thought is, if the plugs are actually bad, wouldn’t I at least know it during the ground mag check? The misfire/skip doesn’t happen until after I get in the air. I can’t replicate it on the ground.

 

[Matthew Rogers]

Would you suggest continuing to run 23yr old plugs if they test fine under pressure? I hate to go with all new, if it’s not necessary, but also hate to troubleshoot deeper if it’s not really needed. Double edged sword.

When I did the inflight mag check, it ran ‘ok’ on both mags for a few seconds and then made the pronounced skip on R and L, but never dropped below 75rpm on either side. Seems like it has to be ignition related. My thought is, if the plugs are actually bad, wouldn’t I at least know it during the ground mag check? The misfire/skip doesn’t happen until after I get in the air. I can’t replicate it on the ground.

I was happy to spend $250 to replace my rusted and corroded (but still “fine”) champion plugs. Seemed like a fine way to ensure smoother running.

 

[Domenick]

Thanks Dan! Question, how do you determine when the spark plugs need replacement? The mechanic tested all eight today under pressure and said only one failed and he replaced, but the remaining seven are also 23yrs old. So do you replace by age? Time in service? Or if they test good, do you just keep using them?

First, if the plugs are Champions, replace with Tempests for the reason mentioned by Dan Thomas. The Champions with the "replaceable" resistors really did have a serious problem. It was eventually admitted by Champion, if not explicitly, then by changing their design.

There is age and there are hours. How many hours on the plugs?

...When I did the inflight mag check, it ran ‘ok’ on both mags for a few seconds and then made the pronounced skip on R and L, but never dropped below 75rpm on either side. Seems like it has to be ignition related. My thought is, if the plugs are actually bad, wouldn’t I at least know it during the ground mag check? The misfire/skip doesn’t happen until after I get in the air. I can’t replicate it on the ground.

If they are 23 y/o Champions, full cruise vibration may be "rattling" the internal, separate resistor.
See photo:

 

[RyanB]

There is age and there are hours. How many hours on the plugs?

About 430 hours.

 

[Dan Thomas]

Fuel is the cheapest to trouble shoot and doesn’t require an AP. Most GA piston aircraft have maybe one fuel filters. The strainer and fuel inlets are screens.

Screens are all any GA airplane has. Very fine screens have the properly of blocking water flow if they're already wet with fuel.

I have seen people send carbs or fuel servos in for rebuild because the engine doesn't run right, and the rebuilds don't fix the problem most of the time. Carbs and servos will continue to meter fuel as long as the screens are in good order, while magnetos, plugs and leads all deteriorate due to mechanical motion, heat, vibration, insulation breakdown and electrical erosion.

And, as I said, if fuel flow is reduced or stops, the engine usually just quits. Period. It doesn't just occasionally hiccup. Some of the metering jets in a carb are small enough that a drop of water won't pass through them. Surface tension and the repelling effect of the fuel do that. It stops the fuel flow.

 

[Dan Thomas]

Would you suggest continuing to run 23yr old plugs if they test fine under pressure? I hate to go with all new, if it’s not necessary, but also hate to troubleshoot deeper if it’s not really needed. Double edged sword.

When I did the inflight mag check, it ran ‘ok’ on both mags for a few seconds and then made the pronounced skip on R and L, but never dropped below 75rpm on either side. Seems like it has to be ignition related. My thought is, if the plugs are actually bad, wouldn’t I at least know it during the ground mag check? The misfire/skip doesn’t happen until after I get in the air. I can’t replicate it on the ground.

I would first rule out any induction leak. That's the cheapest thing and might fix the whole issue. I'd get those mags done anyway, AFTER I've checked for induction leaks. I'd want to know exactly what the problem was, and if the mags were being done while other troubleshooting was going on, I might never know.

Weak plugs often don't show up on a runup. At idle, when mag rotation is low and therefore spark intensity is low, leaning might discover something. In cruise, gradual leaning might get a steady misfire, with high cylinder pressures.

 

[RyanB]

In cruise, gradual leaning might get a steady misfire, with high cylinder pressures.

That’s more or less when the problem comes up. Reduce power to cruise setting and lean for cruise flight and it starts to skip. I know for sure it’s not too lean, so the ‘lean til rough enrich til smooth’ isn’t the problem. Often times it’ll skip even if the mixture is barely pulled back half an inch or so.

 

[Sinistar]

Are you okay with replacing those 23yr old plugs yourself? Its easy and no labor cost for that step. If it stops the small misfire on the way to your A&P shop you can just fly it back home. If not, tell him you did the plugs and the problem still persists. You could also fly it lower on fuel and then drain the rest out, run it through your car and put all new fuel in with no A&P labor labor there either. At the end of those two steps you have all new plugs and all new fuel. Probably not the right order and likely not the problem. But it gets you working towards the problem and both combined can be done in half a day or so including the flight test.

Also agree with others on getting the mags done soon'ish

 

[RyanB]

You could also fly it lower on fuel and then drain the rest out, run it through your car and put all new fuel in

I’ve ran plenty of fuel through the airplane, so I know this problem isn’t a contaminated fuel issue. Plus, I run 100LL in the airplane, so I’m surely not going to be draining it and then running it through my car. Leaded fuel in a modern day automobile is bad dojo. I definitely don’t need to add maintenance headaches to my car, that’s currently without.

Nonetheless, I appreciate your help!

 

[Clip4]

Fuel is the cheapest to trouble shoot and doesn’t require an AP.

Screens are all any GA airplane has. Very fine screens have the properly of blocking water flow if they're already wet with fuel.

I have seen people send carbs or fuel servos in for rebuild because the engine doesn't run right, and the rebuilds don't fix the problem most of the time. Carbs and servos will continue to meter fuel as long as the screens are in good order, while magnetos, plugs and leads all deteriorate due to mechanical motion, heat, vibration, insulation breakdown and electrical erosion.

And, as I said, if fuel flow is reduced or stops, the engine usually just quits. Period. It doesn't just occasionally hiccup. Some of the metering jets in a carb are small enough that a drop of water won't pass through them. Surface tension and the repelling effect of the fuel do that. It stops the fuel flow.

I am not suggesting an obstruction, just small amounts of water.

 

[Domenick]

That’s more or less when the problem comes up. Reduce power to cruise setting and lean for cruise flight and it starts to skip. I know for sure it’s not too lean, so the ‘lean til rough enrich til smooth’ isn’t the problem. Often times it’ll skip even if the mixture is barely pulled back half an inch or so.

How do you know it is not too lean?
In my life, I've known a number of things were "for sure" that didn't turn out to be true. First thing that comes to mind is "Til death do us part ..."

...You could also fly it lower on fuel and then drain the rest out, run it through your car and put all new fuel in with no A&P labor labor there either. ...

At the least, lead in a non-leaded car will kill the catalytic converter $$$!

 

[Dan Thomas]

At the least, lead in a non-leaded car will kill the catalytic converter $$$!

Real quick, too. 100LL has four times the lead that leaded Mogas had in it, and a tankful of leaded Mogas was enough to trash the converter.

 

[Jeff Oslick]

I'm in the camp of not wanting to fly behind 23-year old, 400+ hr mags. I'd start there, regardless of if that turns out to be the issue (very good chance it is).

 

[JAWS]

Sounds like something in the ignition system is heating up and intermittently having issues. Troubleshooting an intermittent problem is always fun!

I would suggest to pull one mag and get it overhauled/replaced. Good idea to have mag overhauls separated so that they don't wear out at the same time. On the same note, might also be good to replace half of the plugs now.

 

[MIA]

Sounds like something in the ignition system is heating up and intermittently having issues. Troubleshooting an intermittent problem is always fun!

Next thing that happens is the mag quits and ya get to roust out an A/P on a Saturday (or Sunday) to fix it. Suggestion: land at Mexia, TX airport if ya wanna try this. Not that I know anything about it...

 

[RyanB]

The mags are getting sent off for their 500hr inspection and we’re getting new harnesses and all new plugs. Fingers crossed that the problem will be solved after this is completed.

 

[Huckster79]

The mags are getting sent off for their 500hr inspection and we’re getting new harnesses and all new plugs. Fingers crossed that the problem will be solved after this is completed.

keep us posted

 

[Huckster79]

I had that in the Skyhawk on my first BFR - but leaning fixed it, unlike yours. Mine was fresh out of annual and the shop said they “gapped the plugs wrong”... but that doesn’t sound like it was yours. I couldn’t see it in the tach but I could hear it...

 

[RyanB]

As an update, the mags were sent off on Tuesday. Fingers crossed that they won’t be gone more than two weeks. New harnesses and plugs should be in the mail from spruce shortly.

 

[Timbeck2]

Where are you sending them?

 

[GaryM]

Real quick, too. 100LL has four times the lead that leaded Mogas had in it, and a tankful of leaded Mogas was enough to trash the converter.

I think it is the other way around...100LL has significantly less lead than leaded auto gas used to contain. Still enough to trash a converter, though.

 

[Dan Thomas]

I think it is the other way around...100LL has significantly less lead than leaded auto gas used to contain. Still enough to trash a converter, though.

100LL (Low Lead, the blue stuff)) was named that way because it has half the lead of the 100/130 (green fuel) it replaced. 80/87 (red fuel) had a quarter of the lead the 100LL does, the same as leaded Mogas.

In approximate numbers, 100/130 (later called 100) had 4 grams of lead per gallon. 100LL has 2 grams per gallon. Mogas and 80/87 (later called 80) had 0.5 grams per gallon. The numbers are reflected in the octane ratings. More lead equals higher octane. 100LL, with its four times higher lead of 80, results in the common lead fouling of sparkplugs most of us are used to seeing now in engines that were designed for 80.

https://en.wikipedia.org/wiki/Avgas Chart showing lead contents

https://www.autotrader.ca/newsfeatu...cted-reason-we-stopped-using-leaded-gasoline/ An excerpt:

...the US government finally saw air pollution as a political issue. In 1963, it passed the Clean Air Act, and formed the Environmental Protection Agency (EPA) in 1970. Three years later, that agency released a study confirming that airborne lead was, indeed, a health problem.

The EPA tried its best. Most refiners were adding 2.0 grams of lead per gallon, and the agency issued regulations that would gradually reduce it to 0.5 grams between 1975 and 1979. But the oil companies immediately filed lawsuits, stalling the regulations.