Diagnose this brief engine anomaly encountered in flight

[DXB]

Recently I was cruising in smooth air in my M20C (Lycoming O-360-A1D) at 8500 ft on a cool clear day, enjoying unusually good performance (>150kt TAS) during a ~1 hour flight.  OAT was 5 degrees C. Carb temp was 17F. The same engine roughness event happened twice, spaced 15 min apart.  See below.  I've provided traces of the only features that changed on the EDM900 monitor (1 sec sampling rate).  Nothing else happened.

Event 1: There was an instantaneous dip in RPM, then brief spike in RPM (no more than 50 either direction). The engine ran rough for ~10 seconds during this period before everything went back to normal.  I reflexively enriched the mixture at the start of the roughness.  There was a recorded very SMALL dip in oil pressure when it it started (1-2psi) but it's literally the only other thing that changed on the monitor during this time, so I show it here.

Event 2: Same thing except it was even more brief in duration, and I did not touch the mixture knob or anything else during the ~6 sec or so of roughness. It resolved on its own.  The rest of the flight was normal, as was my next flight. 

The monitor traces show no changes in EGTs during this time, so I doubt it's an ignition issue. The event almost felt like the timing slipped on one mag and then went back to normal quickly, but I don't know how that's possible.  I have a Surefly left mag and a  bendix right mag that is 150 hrs since overhaul . I did do a quick in flight mag check after the second event resolved, and that was unremarkable. 

I also wondered if there might have been a smidge of water in the fuel that ran through the engine.  I did refuel before this flight, but the plane sat for at least an hour after fueling, and I did sump the tanks before getting going. 

I even wondered if it was a weird little windshear event that made the prop governor hunt for a few seconds- maybe the roughness I perceived in the engine was really in the air interacting with the prop; I didn't really notice any turbulence but I was hyper-focused on the engine at that moment.

My only other thought was a prop governor / governor oil line issue.  Can these lines have brief intermittent leaks near the prop hub? I have a ~4 year old pcu-5000.  I have been noticing more oil in the bottom cowl for the last 10-20 hours, enough to put a couple of drops on the nose wheel after the plane sits for a week.  There's more oil than previously on the air box, front of the carb, and the floor of the cowl.  The rest of the visible engine including the accessory case and prop governor look bone dry.  My A&P has been telling to consider changing the front crankshaft seal for the last couple of years given that it leaks a bit, and I am now thinking of getting to this in a couple of months at annual 

EVENT 1

EVENT 2

 

 

 

 

[N201MKTurbo]

I think your water theory, holds water (pun intended). 
 

You might want to drain your carb to see if there is any water left in it. Although the main jet pickup is near the bottom of the float bowl, there is some space for water to collect that won’t get sucked up by the main jet.

[A64Pilot]

Carb ice?

[N201MKTurbo]

28 minutes ago, A64Pilot said:

Carb ice?

Could be, but probably wouldn’t go away that fast.

[A64Pilot]

28 minutes ago, N201MKTurbo said:

Could be, but probably wouldn’t go away that fast.

It will on a C-85, but they are known as “ice machines” a C-85 will make ice when nothing else does. A brief stumble, almost a cough if you will brings on the carb heat for someone that flies those things.

I theorize with no actual knowledge that a little ice builds up and breaks off and this ingestion of that little bit is the stumble. I have had that happen on initial application of carb heat

Oh, the other thing that can do it according to Lycoming is a valve sticking. I wouldn’t react to just two very rare instances but if it started happening with no smoking gun, it may be worth a look.

From here https://www.lycoming.com/content/operational-and-maintenance-procedures-avoid-sticking-valves

A logical question after this long series of things to do and things not to do might be this, “Is there any way to tell if a valve is sticking before serious damage occurs?” There are sometimes warning signs that should be investigated. Although there may be other causes, an intermittent hesitation or miss in the engine may be an indication that carbon or other similar contaminants have built up inside the valve guide causing the valve stem to drag instead of moving freely. These contaminants should be removed by reaming the guide to the size specified in the Lycoming Table of Limits (SSP 1776). The procedure to be used when reaming to remove valve guide deposit buildup is found in Lycoming Service Instruction 1425. Known as “the old rope trick” to many A&P mechanics, this valve guide reaming procedure restores valve stem to guide running clearance and can be accomplished without removing the engine from the aircraft.

[PT20J]

The interesting thing is the dip in fuel flow. Fuel flow is going to be dependent on throttle setting, mixture setting and rpm. Water would cause a power loss, but the rpm doesn't change much. I wouldn't expect the small rpm drop to cause that much drop in fuel flow, but this would be easy to test by noting the fuel flow reduction when reducing rpm with everything else set as it was during the anomoly.

If something got between the float needle valve and seat, the float bowl will overfill which should cause a drop in fuel flow (because there is no where for the pump output to go) and it will increase the head pressure which will increase fuel metered through the main jet perhaps enough to make the engine run rough until the level drops. If it were mine I would check whatever fuel filters are installed.

Skip

[DXB]

13 minutes ago, PT20J said:

The interesting thing is the dip in fuel flow. Fuel flow is going to be dependent on throttle setting, mixture setting and rpm. Water would cause a power loss, but the rpm doesn't change much. I wouldn't expect the small rpm drop to cause that much drop in fuel flow, but this would be easy to test by noting the fuel flow reduction when reducing rpm with everything else set as it was during the anomoly.

If something got between the float needle valve and seat, the float bowl will overfill which should cause a drop in fuel flow (because there is no where for the pump output to go) and it will increase the head pressure which will increase fuel metered through the main jet perhaps enough to make the engine run rough until the level drops. If it were mine I would check whatever fuel filters are installed.

Skip

I don't really think there's a dip in fuel flow here (light blue trace).   The 0.5gph fluctuation may just be within the range of second to second variability I see in the carb'd setting (the entire trace is like this).  Note fuel flow goes up during the first event simply because I enriched the mixture.

53 minutes ago, A64Pilot said:

It will on a C-85, but they are known as “ice machines” a C-85 will make ice when nothing else does. A brief stumble, almost a cough if you will brings on the carb heat for someone that flies those things.

I theorize with no actual knowledge that a little ice builds up and breaks off and this ingestion of that little bit is the stumble. I have had that happen on initial application of carb heat

Oh, the other thing that can do it according to Lycoming is a valve sticking. I wouldn’t react to just two very rare instances but if it started happening with no smoking gun, it may be worth a look.

From here https://www.lycoming.com/content/operational-and-maintenance-procedures-avoid-sticking-valves

A logical question after this long series of things to do and things not to do might be this, “Is there any way to tell if a valve is sticking before serious damage occurs?” There are sometimes warning signs that should be investigated. Although there may be other causes, an intermittent hesitation or miss in the engine may be an indication that carbon or other similar contaminants have built up inside the valve guide causing the valve stem to drag instead of moving freely. These contaminants should be removed by reaming the guide to the size specified in the Lycoming Table of Limits (SSP 1776). The procedure to be used when reaming to remove valve guide deposit buildup is found in Lycoming Service Instruction 1425. Known as “the old rope trick” to many A&P mechanics, this valve guide reaming procedure restores valve stem to guide running clearance and can be accomplished without removing the engine from the aircraft.

I also wondered if it could be a small chunk of ice might have formed on the nozzle and broken off inside the carb.  Of note there was no preceding roughness or drop in MAP before the two very brief events.  The air was cold and dry that day and the carb temp was at the bottom of the caution range (18F).  I wasn't using partial carb heat to keep it above 40F (which I do in cruise when I fly in precip, IMC, or humid conditions).  I did briefly pull full carb heat on a while after the second event resolved but I got no roughness.

Regarding a stuck valve, I've had no morning sickness symptoms (roughness on startup, cold EGT on one cylinder). I would have expected to see some kind of effect on EGT if a valve briefly stuck open in flight, but that was not the case on the engine monitor. 

[PT20J]

Maybe I'm mixing up the fuel flow and oil pressure traces -- it's hard to differentiate the colors on my screen. But if so, is it normal for your fuel flow to vary this much?

[DXB]

39 minutes ago, PT20J said:

Maybe I'm mixing up the fuel flow and oil pressure traces -- it's hard to differentiate the colors on my screen. But if so, is it normal for your fuel flow to vary this much?

Yes - I think possibly because of periodicity in the position of the float in the bowl?  Here's a much wider snapshot of RPM vs. FF. The one big spike in FF is me enriching the mixture at onset of the first roughness event.  

 

[phxcobraz]

For what it's worth I have had a similar roughness on my M20C a handful of times in the 300hrs I've owned it. Enough that my compass and other gauges start to vibrate more than normal. It always clears up within a minute or so and I can't ever see any immediate changes in RPM/MP/FF/CHT/EGT. If they are there they are so minor it doesn't register outside of normal in-flight fluctuations. I have kind of chocked it up to mountain windshear as well, as the times it has happened have almost all been while climbing out or through a set of mountains in CA/AZ. Whatever validity that possibly has. I never have had any morning sickness or really any trouble starting, but I like the idea of checking for evidence of a stuck valve.

Like you, I never found any in-flight lean mag check makes any difference or pinpoint. Of which when I did have a mag needing overhaul last year, that absolutely showed the 1 mag running rough.

How do the intake gaskets look? A couple of mine were blue stained earlier this year and replaced, it helped stabilize my EGTs some, which could also cause roughness if air is getting in.

Edited December 4, 2023 by phxcobraz