redbaron1982 Posted October 27 Report Posted October 27 Hi, I have a recurring issue (say 1 out of every 3 flights) when in cruise the engine stumbles (misses a beat). It is noticeable. Maybe in a 1hr flight it happens 1 o 2 times. Always while in cruise. My last flight, yesterday, it did it 3 times. This is my flight data from yesterday: https://apps.savvyaviation.com/flights/shared/flight/10152924/66425963-23ae-401f-8ad9-abe3f3d3e8c0 One of the stumbles happened while I was leaning the mixture to get a new measure of my GAMI spread. I did a mag check while still LOP and the engine run smoothly on each mag. I asked Savvy a couple of times, and nothing conclusive yet. I opened a new ticket today, but I'd like to see if you guys have any thought.
PT20J Posted October 27 Report Posted October 27 Can you see an EGT drop when this happens to figure out which cylinder? Could be ignition, fuel injector nozzle, or sticking valve. I’d eliminate the easy stuff first: clean the nozzles and spark plugs. 1
GeeBee Posted October 27 Report Posted October 27 Usually when that happens my experience is the magneto is in need of service. The only thing I can figure out is oxidation between the rotor and distributor. Usually after servicing the magneto with a 500 hour inspection it ceases. 1
Yetti Posted October 27 Report Posted October 27 Had 2 friends that had a stumble. Oil control ring went away. Had to use blasting media to clean up the cylinder and get the coking off.
redbaron1982 Posted October 27 Author Report Posted October 27 4 hours ago, PT20J said: Can you see an EGT drop when this happens to figure out which cylinder? Could be ignition, fuel injector nozzle, or sticking valve. I’d eliminate the easy stuff first: clean the nozzles and spark plugs. I don't see anything really telling by looking at the EGTs. Around on of the misfires there is some change in the #3 EGT, but I'm not sure. Can a nozzle act up like this? The engine runs fine just except this stumbles every now and then. The GAMI spread is 0.4GPH. Would there be other symptons for a sticking valve? 2 hours ago, GeeBee said: Usually when that happens my experience is the magneto is in need of service. The only thing I can figure out is oxidation between the rotor and distributor. Usually after servicing the magneto with a 500 hour inspection it ceases. The magneto was IRAN 100 hours or so ago, and 20 hours ago it was taken to Aero Accessories at Van Nuys, they bench tested and said it was perfect and was not worth tearing it appart.
ArtVandelay Posted October 27 Report Posted October 27 Does pulling back on throttle cause backfiring? If yes, could be an induction leak.
redbaron1982 Posted October 27 Author Report Posted October 27 28 minutes ago, ArtVandelay said: Does pulling back on throttle cause backfiring? If yes, could be an induction leak. It does not. I haven't noticed any backfire ever.
DC_Brasil Posted October 27 Report Posted October 27 I've been looking at the data and I can't see anything that would indicate a problem. No noticeable drop in any parameter, even during the lean check (which starts at 37min, right?). Can you try to describe exactly how you perceived the "stumble"? Was a rough vibration? Does it feels like it stopped working for a second? How hard did it shake? Think of an engine starting flooded when it catches but almost dies before catching for good... was it like that? If you can give more descriptions of how it behaved we could help assisting. A clogged injector would only affect one cylinder and could lead to a very rough engine especially when operating lean. Fly with the mixture richer than ROP and see if this issue manifests itself. A bad magneto would be noticeable if when you do the in flight mag check. If it keeps running smooth in either mag and CHTs seem normal, you can pretty much rule out the mag. If it were bad mag timing, or broken gear teeth inside would be getting consistently very rough engine and perhaps detonation, pre ignition, etc. Induction leak could lead to a significan decrease in manifold pressure and could hiccup the engine if it were bad enough, but would it be random and repeating only a few times? I'm not sure. How's your fuel? Any signs of recent contamination? Have you resealed your tanks (or done any other work) recently? Have you been draining the gascolator? A sudden hiccup (albeit not noticeable in the engine data) may be related to fuel contamination (affecting all cylinders at once).
PT20J Posted October 28 Report Posted October 28 Usually sticking valves show up when cold on start up. But I've had them stick momentarily in flight. I've also had an IO-360 miss a beat when running LOP. There is a lot of cycle-to-cycle variation in combustion and this is exacerbated with lean (slow burning) mixtures. In fact cycle-to-cycle variation is cause of the roughness you feel when you lean to the limit. So, everything being a bell shaped curve, if running very LOP, there will be an occasional combustion event out on the tail of the curve that will reduce power from that cylinder momentarily. The fact that it doesn't show up on the EGT trace indicates that it was likely just a random transient event. If it keeps doing it on rich mixtures, a wobble test is the definitive test of valve stem-guide clearance.
redbaron1982 Posted October 28 Author Report Posted October 28 3 hours ago, DC_Brasil said: I've been looking at the data and I can't see anything that would indicate a problem. No noticeable drop in any parameter, even during the lean check (which starts at 37min, right?). Can you try to describe exactly how you perceived the "stumble"? Was a rough vibration? Does it feels like it stopped working for a second? How hard did it shake? Think of an engine starting flooded when it catches but almost dies before catching for good... was it like that? If you can give more descriptions of how it behaved we could help assisting. A clogged injector would only affect one cylinder and could lead to a very rough engine especially when operating lean. Fly with the mixture richer than ROP and see if this issue manifests itself. A bad magneto would be noticeable if when you do the in flight mag check. If it keeps running smooth in either mag and CHTs seem normal, you can pretty much rule out the mag. If it were bad mag timing, or broken gear teeth inside would be getting consistently very rough engine and perhaps detonation, pre ignition, etc. Induction leak could lead to a significan decrease in manifold pressure and could hiccup the engine if it were bad enough, but would it be random and repeating only a few times? I'm not sure. How's your fuel? Any signs of recent contamination? Have you resealed your tanks (or done any other work) recently? Have you been draining the gascolator? A sudden hiccup (albeit not noticeable in the engine data) may be related to fuel contamination (affecting all cylinders at once). What I feel is like a sudden, short, desceleration. The airplane doesn't shake much. It something that last 1 second. If I have to describe is like if the engine would shutdown for 1 second and then relight again. I've never seen water in the fuel, the airport where I fuel has a pretty good record of not having water in the fuel. The airplane is hangared all the time. 3 hours ago, PT20J said: Usually sticking valves show up when cold on start up. But I've had them stick momentarily in flight. I've also had an IO-360 miss a beat when running LOP. There is a lot of cycle-to-cycle variation in combustion and this is exacerbated with lean (slow burning) mixtures. In fact cycle-to-cycle variation is cause of the roughness you feel when you lean to the limit. So, everything being a bell shaped curve, if running very LOP, there will be an occasional combustion event out on the tail of the curve that will reduce power from that cylinder momentarily. The fact that it doesn't show up on the EGT trace indicates that it was likely just a random transient event. If it keeps doing it on rich mixtures, a wobble test is the definitive test of valve stem-guide clearance. As far as I can remember, it never missed a beat running ROP. This time for instance, the leanest cylinder was 75F LOP, while the rest were all around were between 30F and 50F LOP. If it is a random thing result of running LOP I would be ok, my main concern is if there is something else going on that might end up with a loss of power in flight. One additional peace of information, not sure if it is relevant, I'm a relatively "new" Mooney driver, the mixture control "lags", what I mean is if lean the mixture, say 1/2 turn, it would take maybe 10s for the fuel flow to stabilize in the new value. This happens either leaning or enriching the mixture. Not sure if this is normal or not. Some times I would make a full turn without seeing any change in FF, and then after several seconds the flow would start moving to the new setting.
PT20J Posted October 28 Report Posted October 28 75 deg F LOP is pretty lean. Keep in mind that we are dealing with last century mechanical technology here. I sometimes notice a lag in FF with mixture changes. I have a spot around 9.9 gph where it seems to hang with no change in FF even though I can see the EGTs rising as I lean the mixture. Try running the leanest cylinder no leaner than 50 deg F LOP and see if that solves it. 3
DC_Brasil Posted October 28 Report Posted October 28 6 hours ago, redbaron1982 said: This time for instance, the leanest cylinder was 75F LOP, while the rest were all around were between 30F and 50F LOP. I believe we can safely say what you're experiencing is an over-lean engine misfire. My plane has the exact same engine (IO-360-A3B6D) and at around 75 LOP it will start behaving badly a will lose a lot of power. The Lycoming operation manual says best economy for these engines is exactly at peak EGT. My G3X EIS says it is slightly below (around 25LOP gets more miles.per galon). Overly lean mixtures require a stronger spark to ignite. You'd need to have 100% sure your magneto, ignition harnesses and spark plugs are all in pristine shape to operate confidently at 50 LOP. Tiny variations in induction airflow or fuel injected at these settings will likely cause the hiccups you've seen. Different engines (even the same model) will have slightly different behaviors: my engine will run well at peak EGT up to around 50 LOP. The equates (depending on altitude and temperature) to 8.6 all the way to 7.4 Gal/h fuel flow. At peak I will see between 139 and 144 kts TAS. At 50 LOP I get around 122 to 127 kts TAS. I use these settings if ATC instructs me to hold or I know I'll have a lot of traffic in front of me. 3
Bartman Posted October 28 Report Posted October 28 I’ve had that before and 75 LOP is too lean. I typically run 20 LOP at lower altitudes and the higher I fly, the less LOP. By the time I’m at 8-9K I’ll be at or near peak. If over about 10K I’ll lean to the first cylinder to peak or run slightly ROP. GAMI spread of 0.4 sounds good, but the deeper LOP that spread likely increases and as Skip stated, the cycle-to-cycle variations can amplify the misfire. The closer to 0.0 you can get, the deeper she will run LOP without a stumble or running rough. Cleaning the injectors and swapping the first to peak with the last can improve the GAMI spread. That worked for me. Try a magneto check in cruise around 8,000ft and 50LOP. You may discover which cylinder, and find something that points you toward a bad plug, wire, or magneto. 2
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