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Rare miss in flight

Manchu16

Active Member
I was hoping this group could help. Recently bought a 9. In flight at cruise, I get an occasional miss or stutter. So quick, my passenger doesn’t notice and my engine monitor does not catch it. Literally like it just doesn’t fire on cylinder on one revolution. It doesn’t always happen, but some flights it may do it 5 times over a 15 min period. It is noticeable enough to get me wondering what is going on.

O-320 160hp, one bendix mag (300 hrs since oh) and 1 PMag (300 since new).
Enriching mixture does not prevent it.
Running elec Fuel Pump doesn’t prevent it.
I have not done an extended single mag cruise test as it doesn’t happen that frequent and concerned about running on a single mag for 15+ mins.
EGT/CHT appear normal.

A&P checked;
Compression 75 cold across all cylinders. Borescope clean. No visible damage to plug wires. Mags timed. Plugs looked great after 40 hours since last cleaning. He is at a loss. Recommendation to get both mags serviced/overhauled. Then replace wires if that does not fix it.

Before I start sending parts off for overhaul and replacing items I was hoping this group may have other ideas on what to check or may have experienced something similar.
 
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Have you done the Lycoming valve wobble test (looking for possible sticking valve)?

No I have not done that yet. Wouldn’t I notice that starting as morning sickness? This is only noticeable after I am at cruise and normally after the first 20 mins or so.

Not dismissing the suggestion, just trying to learn more based on my limited experience.
 
I had one of the clips on the sparkplug wire crack and it would do the same thing you describe (I have dual P-mags). If the sparkplug boot feels like it doesn't completely snap on tight to the end of the spark plug, then you might have the same issue.
I bought a replacement sparkplug connector (West Marine link) and simply swapped the little clip onto the original end cap. You can cut off the boot, to access it, then slide on a new rubber boot.

IMG_5086-M.jpg


The another issue is that if you run the NGK BR8ES plugs for the P-mag, they have two different part numbers. You want the #3961 version with the solid end, not the screw on end. The screw on version #5422 can loosen and cause intermittent connection issues.
 
My miss

In my RV 4 with an O360 I had the same thing happen almost from the 1st day I bought it.

Just a miss that you couldn't see nor feel but it was enough to catch your attention at 12,500 heading west over the rocks. But only in cruise and might happen once every 10 hrs or so.

It was very intermittent and this went on for a couple of years until it started happening on climb out and now I'm really paying attention and grounded the plane as soon as I was home.

Even on my leak off tests all of the cylinders were in the high 70's. Then I noticed number 3 had started slipping to the low 70's over the course of maybe 50 hours. Now starting to get this narrowed down.

Did everything I could on my end and had to get an AME involved (with better hearing) and a few more tricks. He could hear it leaking near the top of the cylinder.

Soapy water onto the cylinder and had bubbles. So there was a problem with the jug.

I pulled it off and found a crack between the bottom plug and the exhaust valve. Impossible to see as it was in between the fins.

New jug and nothing since in 150 hours.

Tim
 
I had the same problem a few years back with a Lancair I was flying at the time. Had 2 A&P's that wanted to replace some VERY expensive items as their "guess" to what was happening. I subscribed to Savvy Analysis, did an in-flight mag check and sent data to them to analyze. They came back with back bottom plug on #2 cylinder. I replace one plug, flew smooth as glass from then on.
 
No I have not done that yet. Wouldn’t I notice that starting as morning sickness? This is only noticeable after I am at cruise and normally after the first 20 mins or so.

Not dismissing the suggestion, just trying to learn more based on my limited experience.

I've had two forced landings due to exhaust valves sticking open in flight. Both events were about 40 minutes into the flight without any hint of morning sickness. O320 E2A with around 350 hours on new Titan cylinders. Running on three cylinders really gets you attention.
 
Intermittent, very short duration (sub-second) misses are often ignition related, though can be very small fuel disruptions with fuel injection, but not carbs. Classic symptoms for sticking valves is poor running while cold, then better when hot, though can happen at ANY time. However, valves do not typically stick for fractions of a second and then release.

I would be looking for problems with ignition wires or possibly coils. Also verify the P lead is not inadvertently grounding itself. Pmag is notorious for problems with the connector loosening it's grip on the wires.

Next time in the air, drop the P mag for a second and turn it back on. If that is familiar to what you have been experiencing, start looking for problems there. Repeat with the mag. Dropping the Pmag will have a much greater influence due to it's greater advance. Pmags seem to have bearing issues and speculate that a failing bearing can cause intermittent shut downs, but have no experience with them. Pretty easy to check.

also, when near the edge of how lean you can get on your engine, intermittent misses happen. If I go too lean on my 540 in cruise, I get a tiny hiccup every couple minutes. I pull the throttle back a bit and richen up the mixture a bit (raise EGTs 10-20*) and problem goes away even though fuel flow is the same.

Larry
 
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Intermittent, very short duration (sub-second) misses are often ignition related, though can be very small fuel disruptions with fuel injection, but not carbs. Classic symptoms for sticking valves is poor running while cold, then better when hot, though can happen at ANY time. However, valves do not typically stick for fractions of a second and then release.

I would be looking for problems with ignition wires or possibly coils. Also verify the P lead is not inadvertently grounding itself. Pmag is notorious for problems with the connector loosening it's grip on the wires.

Next time in the air, drop the P mag for a second and turn it back on. If that is familiar to what you have been experiencing, start looking for problems there. Repeat with the mag. Dropping the Pmag will have a much greater influence due to it's greater advance. Pmags seem to have bearing issues and speculate that a failing bearing can cause intermittent shut downs, but have no experience with them. Pretty easy to check.

also, when near the edge of how lean you can get on your engine, intermittent misses happen. If I go too lean on my 540 in cruise, I get a tiny hiccup every couple minutes. I pull the throttle back a bit and richen up the mixture a bit (raise EGTs 10-20*) and problem goes away even though fuel flow is the same.

Larry

+1 - Everything he said.
 
On my IO320, I have had the "occasional miss or stutter" several times. Usually occurs five or so minutes after takeoff, while still at full throttle. My engine has a Slick mag and a PMag. I have noted the time of occurrence for reference to my engine monitor system. My engine monitor records at two second intervals. I have been able to trace the miss to a drop in exhaust gas temp on a particular cylinder. Each time, I have changed the automotive spark plug on the one PMag system and that resolved the miss.
 
Did you measure plug resistance when you had them out?

You may recall a period of time when Champion aviation plugs failed shortly after installation (they changed center electrode design). Perhaps you have a few plugs left over from that. I found 9 out of 12 with high resistance on my RV-10 (back when I was still using mags). New Tempest plugs and all was well.

Carl
 
Ive seen that on my RV-9A, O-320, carbureted. My IA said it could recur in five minutes or five months. I ignore it.
 
Also check all primer lines. I had one do that and only found it after pulling the engine for overhaul. A worn spot on a primer line in an unseeable spot that had the smallest that would get a blip of air when it flexed just right.
 
Thanks!

I am going to do some tests tomorrow while flying to see if I can isolate the issue further. The aviation and auto plugs are new and tested ok before I installed. Wires tested ok as well.

I appreciate all of the insight. Hopefully I can figure it out.
 
I had this problem except it was temperature related on climb out, high CHT's. I did have it happen once on cruise but I suspected the engine was running too lean. With help from others more knowledgeable I set out different causes. Cowling ramps and exhaust exit, baffle rubber, timing, ignition wires, spark plugs, compression, borescope, and then changing my carburetor Jet to enrichen it, which was a "mooney mod" associated with the 0360 (which provided more fuel for climb out and appears to have solved the high CHT's, now rarely exceed 400). The aircraft history of it sitting for several years with only flying a couple hrs a year or so I believe helped exacerbate this problem of which I would say was detonation, a lean mixture, heat related and possibly carbon deposits in the head or a lack of use. If you have a carburator, inspect the filter screen. Older aircraft hoses can have devastating effects, were talking fuel, where they flake, and you may drain your tanks and gascolator everyday but there is a last chance screen located on your carburetor. You can remove this and or remove your fuel line and with a very bright flashlight inspect the screen. I would remove the filter and bang it on a white cloth attempting to dis lodge any particles. Aircraft hoses have a shelf life.
 
Good ignition diagnostic tool.

I had the same problem a few years back with a Lancair I was flying at the time. Had 2 A&P's that wanted to replace some VERY expensive items as their "guess" to what was happening. I subscribed to Savvy Analysis, did an in-flight mag check and sent data to them to analyze. They came back with back bottom plug on #2 cylinder. I replace one plug, flew smooth as glass from then on.

The inflight mag check along with a good analysis of recorded EGT and CHT like above is a good method to determine if it is caused by ignition. My IO360M1B will have a tiny, almost imperceptible, miss nearing the lean limit and will wander in and out in flight. An increase in FF of only 0.1 will eliminate it. It is not specific to one cylinder/plug.

Misfires caused by weak ignition are amplified with lean mixtures and if specific to one cylinder/plug, then it can be identified this way, but if not . . . then it is likely mixture related.
 
Check mag coils

Dad had an almost identical fault years ago on his Stinson. An occasional blip exactly as described, a failure to fire on a single compression stroke. It didn’t occur when cold.

It turned out to be a faulty mag coil that had a broken wire that showed up once the coil had warmed up. Most of the time the spark jumped the break in the coil wire and plug gap, but occasionally it didn’t. The coil checked ok with an Ohmeter when cold.

Auj4
 
Update 1

I was away wearing the green suit for two weeks. I appreciate all of the input. I still don't have the root cause, but I was able to get a few tests in today.

I am still experiencing the occasional miss. I was able to borrow a logging device for my engine monitor. Unfortunately, I didn't realize it didn't capture the first flight until we packed up.

1. The miss is still present if the electric fuel pump is on or off. While that doesn't rule out fuel delivery, it mitigates my concern the lower pressure of the engine driven pump causing the problem.

2. The miss is still present regardless of the mixture (ROP vs Full Rich). Still doesn't rule out fuel delivery, but helps rule out I am running too lean or too rich.

3. I did a short inflight test of each mag, but I want to perform the test again because I was not able to capture it in the log. The engine ran rougher on each single mag as opposed to both, but I noticed the miss in all 3 configurations. Ceilings were low and I wasn't comfortable performing the test under 2k AGL.

I have added two screen captures of the last flight. One is during the run up and the other during cruise and initial arrival. I placed comments on the screen captures. You can see the dips (which I believe represent the miss I am feeling) in all phases of the flight. Please let me know if there is a better way to share these.

I am wondering if this could be an ignition switch going bad (I have the ACS key type Off/L/R/Both/Start type). I am going to remove and tape off the leads to the mags before my next flight to see if I can confirm/disprove the issue.

As always any thoughts are greatly appreciated.
 

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I was away wearing the green suit for two weeks. I appreciate all of the input. I still don't have the root cause, but I was able to get a few tests in today.

I am still experiencing the occasional miss. I was able to borrow a logging device for my engine monitor. Unfortunately, I didn't realize it didn't capture the first flight until we packed up.

1. The miss is still present if the electric fuel pump is on or off. While that doesn't rule out fuel delivery, it mitigates my concern the lower pressure of the engine driven pump causing the problem.

2. The miss is still present regardless of the mixture (ROP vs Full Rich). Still doesn't rule out fuel delivery, but helps rule out I am running too lean or too rich.

3. I did a short inflight test of each mag, but I want to perform the test again because I was not able to capture it in the log. The engine ran rougher on each single mag as opposed to both, but I noticed the miss in all 3 configurations. Ceilings were low and I wasn't comfortable performing the test under 2k AGL.

I have added two screen captures of the last flight. One is during the run up and the other during cruise and initial arrival. I placed comments on the screen captures. You can see the dips (which I believe represent the miss I am feeling) in all phases of the flight. Please let me know if there is a better way to share these.

I am wondering if this could be an ignition switch going bad (I have the ACS key type Off/L/R/Both/Start type). I am going to remove and tape off the leads to the mags before my next flight to see if I can confirm/disprove the issue.

As always any thoughts are greatly appreciated.

Please describe the miss with a bit more detail. It looks like all 4 cylinders are either not combusting or receiving an equal amount of power reduction for a 5-10 second interval.

Given that symptoms are identical with the ignition tests, I think you can rule out ignition; at least excluding the switch. Can't be electrical, as it happens on the mag also. If both ignitions are run through the ignition switch (i.e. the switch has the ability to stop the sparking on both via grounding a lead), that is a likely culprit. An intermittent grounding fault in the switch could cause these symptoms. I would bypass it completely and test.

A reasonable possibility is intermittent interrupted fuel flow. It is about the only thing that can account for the data, excluding instrumentation issues (wouldn't sense the miss) or a faulty ign switch. Do you have a purge valve? Could it be opening and closing on it's own? Do you see dips in the fuel pressure when the miss happens?

I will say that the ign switch seems somewhat improbable, as there is great consistency in the failure interval length and that is suspicious for a vibration induced electrical issue.

Can't realistically be compression or valve train, given the equal treatment to all cylinders.

Going intermittently over rich could also be a possibility and cause all four cylinders to stop firing. Failing check ball in the accel pump could cause issues like this, as well as could needle/seat issues, but less likely given the occurrence when LOP.

The rapid drop in CHT, along with the EGT implies that the cylinders instantly stop combusting vs going rich or lean (would see EGT move before CHT). Fuel pump issues should also show varying EGTs as the volume in the bowl drops. Because the EGT CHT drop in tandem, a fuel pump issue seems unlikely with a carb.

Larry
 
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That's more than what I would call a momentary "miss". From the data, that looks like a several-second loss of power.

You can post a link to the Savvy data itself, and others can take a look at all the parameters like RPM, FF, FP, etc. Might be able to help better. Just cut and paste the URL.
 
Switch? Maybe..

Run with a hot switch on the next flight -- disconnect the GRD wire from the center terminal of the ignition switch and re-run the flight. Don't bother with a mag test, it won't work....and BE careful around the prop.

If the issue goes away, then it's probably the switch -- great.

Other than that I would look to fuel delivery as the culprit -- I don't know whether or not you mentioned it before, are you fuel injected or carbed? Which brand?

B
 
An interesting note is on the second graph at the 00:30:00 minute mark the decrease in EGT and CHT didn’t follow all cylinders; cylinder #4 temp remained without a drop off.
 
Update #2

Answering a few questions.

* The "Miss" - It is extremely short, not even 1 sec. My wife completely missed it was happening until I pointed it out to her. I tried to record it with a go-pro but it is not picked up audibly. You can feel it in your feet/butt when it happens, but it is more of a "did you feel that" as opposed to "oh $hit"

*The engine is a carbed (Stock MA-4SPA) O-320-E2C (150Hp) mainly 90 Oct Mogas (Eth Free)

*Here are the two links to the Savvy Site. I only have CHT/EGT, Oil temp and OAT. OAT is way off as the probe is in the exhaust path (on the list to move to the wing on the next annual)

1. https://apps.savvyaviation.com/flights/601539/ea2f08b0-a89a-48a4-af24-b6bbef0d4c083

2. https://apps.savvyaviation.com/flights/601537/ae5yd5ab-cefb-41e6-9a61-d8b454ae4be9

*The engine monitor only logs every 5 seconds (UBG-16). I wish I could be more granular.

* I can not run at LOP as it is too rough. My ROP testing is at Lean until rough and enriched 1 fill turn of mixture (~50-60 degrees)

The one item I noticed in the logging, specifically #1 is that the Oil Temp drops out at the same time as the CHT. The Oil Temp gauge is a lagging gauge, I cant imagine it could show that amount of temp difference in such a short window. Could I have a larger grounding issue somewhere? Would that cause the miss....
 
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The inflight mag check along with a good analysis of recorded EGT and CHT like above is a good method to determine if it is caused by ignition. My IO360M1B will have a tiny, almost imperceptible, miss nearing the lean limit and will wander in and out in flight. An increase in FF of only 0.1 will eliminate it. It is not specific to one cylinder/plug.

Misfires caused by weak ignition are amplified with lean mixtures and if specific to one cylinder/plug, then it can be identified this way, but if not . . . then it is likely mixture related.

I get this barely perceptible miss on my IO-320 with dual PMags if I lean the engine too aggressively. In every case, enriching the mixture by about 20 degrees EGT makes it go away.
 
Interesting data!

I'd start looking at Engine/Airframe grounding issues next. In addition to the switch.

edit: The multi-second drop out shown the data would elicit a very strong response from my spouse (read: "WHAT THE @!#$% WAS THAT?!?!?! ARE WE GONNA CRASH?!?!"), I don't think I would need to point that event out... :)
 
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wow, this looks exactly like what I went through for 3 years on my RV7. O-360, Carb, no electronic ign. I started with replacing all my plugs and did Ohm tests even on new ones. Then the A&P thought it might be the Mags so we sent both to be checked and rebuilt (both came back with no issues found but at least my 500 hr was done on it). Then a few months later we took the carb out and I decided that I didn't want to mess around with re-jetting and bought a brand new one ( not rebuilt). About a year and half in to it by now. That didn't fix the problem (oh well that was a waste of money ) so we thought maybe the engine driven fuel pump. I was desperate to get this fixed ,but I also knew throwing new parts at a problem I didn't understand isn't going to magically fix it, but I changed the pump anyways because it was cheap insurance.

We did everything that has been listed above... wobble test, checked pretty much everything everyone said - - - problem was it would do it on some flights and not others so it may go away for a few months and I would think it was fixed and then when it got really hot it would do it all the time for a month, then only once in a while. No consistency at all other than heat. Heat was the common denominator ... It would never do it when it was cold or during the winter. 2 years went by and I also replaced the wires and thought that was it but nope, it came back like a sneaky rat. Then one of my friend A&Ps said it may be the ignition switch. Spruce sells a rebuild kit so we rebuilt the ignition switch to make sure the contacts were good. We couldn't really do the test where I took the ground lead off because I never knew if it was going to do it on that flight or not and didn't want to leave the plane sitting with a hot mag. After the rebuilding the switch, it stopped doing it for months (in middle of the summer). I thought finally I found the issue. But then one day the following summer, I felt it again but only once. The following Winter I sold the plane to someone at my airport and he sold it to someone else. I still talk to the person who bought it last and told him all about it but he has not experience it. So, maybe it was the ignition switch. The rebuild kit is cheap so may be worth it.
 
Instrumentation?

CHTs and Oil Temp drop 20F in 5 seconds - can that really happen?

If not, is it possible that all nine thermocouples share something jittery (eg, a single cold-junction compensator)?

Bo
 
CHTs and Oil Temp drop 20F in 5 seconds - can that really happen?

If not, is it possible that all nine thermocouples share something jittery (eg, a single cold-junction compensator)?

Bo

I see no OT data, so not sure where you are getting that. Yes, CHTs can drop 20* in 5 seconds if flying at 170 MPH and the cylinder stops combusting. They are aluminum and shed heat rapidly.
 
I see no OT data, so not sure where you are getting that. Yes, CHTs can drop 20* in 5 seconds if flying at 170 MPH and the cylinder stops combusting. They are aluminum and shed heat rapidly.

Yeah, but *oil temperature*?
 
Update

I wish I could say I found the smoking gun, but unfortunately still troubleshooting. I have to say I have learned a ton on this journey and have fixed many little issues that were caught while I have been looking for the cause of the miss.

One of the items that is constant in all of my engine log captures is spikes throughout the flight. One theory is something is causing the mag(s) to ground causing the blip in the logs and the engine. The other is I have a bad ground between the engine probes and the monitor which is not related to the actual miss on the engine. Either way I need to do some work testing the grounding of the engine and other systems.

I have read a bunch from Bob Nuckolls, but I am still at a loss. What is the best way to test for good grounds between the engine and the forrest of tabs for example? Is there a limit for how many ohms I should see? Also is there a good process for cleaning the connections chemically, like deoxit or another product? Should I pull every connection or test first?

Thanks in advance
 
I wish I could say I found the smoking gun, ...

I have read a bunch from Bob Nuckolls, but I am still at a loss. What is the best way to test for good grounds between the engine and the forrest of tabs for example? Is there a limit for how many ohms I should see? Also is there a good process for cleaning the connections chemically, like deoxit or another product? Should I pull every connection or test first?

Thanks in advance

If you are reading pretty much ANY ohms between engine ground and airframe, that's too much. 1.0 ohm is TOO MUCH. That said, it is hard to accurately measure as any little bit of grease or oxidation can make it look like a bad ground.

A very peculiar problem. The ignition switch is a fair guess. Also check the p-leads where they pass through the firewall. Rubbed through the insulation?

You might spend the $$ and get Savvy to look at your problem. Sure wish you had RPM in the log.
 
A very peculiar problem. The ignition switch is a fair guess. Also check the p-leads where they pass through the firewall. Rubbed through the insulYou might spend the $$ and get Savvy to look at your problem. Sure wish you had RPM in the log.

I signed up and waiting to get their read on the situation.
 
..... What is the best way to test for good grounds between the engine and the forrest of tabs for example?....

Check the resistance between devices. Many will test each end of the copper/ground path. That only tells you the resistance of the associated wire/cable. For your example, check resistance between the stud or unused tab on the ground "bus" and an appropriate spot (bare, clean) on the engine proper. Use that mindset going forward.

Sorry it's not specific to your problem but this methodology can avoid some future heartache and wasted time.

Best of luck
 
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