Rare miss in flight

[Dan B]

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.

[Carl Froehlich]

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

[Ed_Wischmeyer]

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.

[n82rb]

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.

[Manchu16]

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.

[stringbender]

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.

[BillL]

Originally Posted by rvanstory

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.

[auj4]

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

[Manchu16]

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.

[lr172]

Originally Posted by Manchu16

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