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 Post subject: Engine roughness
PostPosted: Wed Aug 09, 2017 9:17 pm 
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Location: Typically hanging around N07, 12N, KLDJ, KCDW
So I was out flying today, out of and back into my airport within the NJ Trump TFR. I was descending to land at my home airport when my engine started to vibrate and a second or two later the
RPMs started to drop off.

The adrenaline was now flowing.

I was only 10 miles from the airport and had my hand on the throttle, so I immediately gave it a little push and the engine sprang back to life and ran smoothly. I decided to keep the throttle right where it was. I knew I had plenty of fuel as I had measured it with a dipstick and departed with 30 gallons for a 30 minute flight. A quick scan of the G1000 MFD engine instruments showed everything in the green and where it should be. The mixture knob, fuel selector, mags, everything was where it should be set.

Who needs noise cancelling headsets? It was just me in there, but it is always very noisy in a Skyhawk while cruising along. Now the cockpit felt extremely calm and quiet. My ears and body were hyper sensitized to feel the engine sound and vibrations, but the sound of the wind rushing over the airframe was suddenly erased from my perception.

My eyes were scanning the ground for landing spot and it was beginning to get dark. I'm glad I left OXC a little earlier than I could have on this evening. I could see a few ball fields nearby and then I realized that I was flying right over my town. I could see my son's school and the adjacent football field. I also had the airport beacon in sight up ahead. I kept watching the ground as I progressed to the airport to make sure I had a spot to land if I suddenly lost the engine.

I advised the tower that I momentarily experienced engine roughness and asked if I had any traffic between me and the airport. They confirmed that there was none and that I was clear to land straight in. I decided to keep the power and my altitude until I was certain I could reach the airport by gliding if I lost the engine when I had to reduce power.

I had some miles to go, so I flipped to the engine page on the G1000 and looked at the detailed data for EGT, CHT, fuel flow, oil pressure and temp to see if there was something going on in there. Everything looked ok at first glance and then I noticed one cylinder had an EGT that kept going up and then down about 20 degrees every 30 seconds or so. That looked odd so I made a note of it to tell the mechanic tomorrow.

At 2 miles out, I made my GUMPS check and started to reduce power and slow down for a normal landing, everything worked well from that point on.

Not a panic or chaotic, but it was tense for a few minutes. I'm very glad I didn't have to conduct a practical test of my academic skills of stuffing an airplane into a tiny patch of grass in fading daylight.

I suspect that the engine is losing an exhaust valve. :(


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 Post subject: Re: Engine roughness
PostPosted: Thu Aug 10, 2017 5:33 am 
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Thanks for sharing, good story! :)

Could be an injector (as it went away by adding some throttle), or maybe in the ignition (but these often worse-up with more throttle, albeit with fixed-pitch are somewhat RPM-specific, so may briefly smoothen as well).

Let us know if they found something! :)

-Esa


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 Post subject: Re: Engine roughness
PostPosted: Thu Aug 10, 2017 7:32 am 
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Location: Typically hanging around N07, 12N, KLDJ, KCDW
I should add a few other details. I did have a clue before this flight that there was a problem, but it was misdiagnosed.

The flight immediately before mine had a rough running engine on startup. It was shaking and idling poorly. The pilots in the plane assumed that it was a fouled plug so they performed a static run up to high power and leaned the mixture to burn off deposits. The engine performed well during the mag check with a rise in EGT across all cylinders and all systems showing normal on their gauges.

The plane flew just fine for them.

My start and run-up were normal, so it made me feel comfortable that the issue was a fouled plug. Clearly it was not.

The reason I think it might be a bad valve is because of the cycling EGT on one cylinder and the bout of roughness on the engines first cold start. The valve might be sticking in its guide until the head warms up.

The cycling EGT may be an indication of damage to the valve face and/or seat that causes poor sealing as the valve rotates around every minute. If the valve is sticking, combustion products might be making their way up the guide and fouling up that area.

I'm curious to learn what the problem is. I understand Lycomings are notorious for sticky valves.


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 Post subject: Re: Engine roughness
PostPosted: Thu Aug 10, 2017 9:12 am 
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Yea, that's almost a telltale. It is not that the valve 'face' itself was necessarily broken, which evidents itself more or less regardless of how out run the engine, but just failing to close completely, which may remain a transient problem.

"Plug problems" are somewhat overrated.

-Esa


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 Post subject: Re: Engine roughness
PostPosted: Thu Aug 10, 2017 9:29 am 
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Location: Typically hanging around N07, 12N, KLDJ, KCDW
Well it just so happens that this same engine had a plug insulator fracture about 2 oil changes ago. I've never experienced plug fouling on the engine, but the broken plug had a pronounced 200 RPM loss, vibration and clearly would not go away with changes to throttle or mixture.

Maybe it would have been smoother by isolating the mags, but they were right over an airport and just put it down there without wasting much time for troubleshooting.


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 Post subject: Re: Engine roughness
PostPosted: Thu Aug 10, 2017 9:42 am 
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Oracle427 wrote:
Well it just so happens that this same engine had a plug insulator fracture about 2 oil changes ago. I've never experienced plug fouling on the engine, but the broken plug had a pronounced 200 RPM loss, vibration and clearly would not go away with changes to throttle or mixture.

Maybe it would have been smoother by isolating the mags, but they were right over an airport and just put it down there without wasting much time for troubleshooting.
Yes, that would be a clear ignition issue. These are certainly not "over-rated"! And IMO, with that certain symptoms, I wouldn't care them troubleshooting too much. These would be easily isolated on a run-up safely on the ground. Supposed plug fouling issues are what I mean being "over-rated". They do naturally clear up, and if you have them regularly, then your engine is mis-adjusted or even having the wrong plugs. In general, as my punch line goes. :)

A single specific thing re. ignition system that pilot should be aware of if he wishes to help the maintenance would be any high-altitude symptoms. Such as roughness, or abnormal EGTs associated with high altitudes but non-evident down low. In particular, if he can isolate this to one mag. Some developing magneto faults first show themselves at altitude, as the air looses its dielectric strength with increasing altitude, introducing off-time internal sparking, and these would be impossible to test at ground.

Your case would indeed put my bets towards a mechanical issue, with this much info at hand. :)

-Esa


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 Post subject: Re: Engine roughness
PostPosted: Fri Aug 11, 2017 10:04 am 
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As suspected, the exhaust valve in the cylinder with the cyclic EGT was found to be hanging open. The engine barely has 400 hours on it and worse yet, this same cylinder was replaced at 300 hours due to scoring and ring damage. It's a factory overhauled engine on top of that. :(

Now I wonder if there are other factors contributing to this cylinder's troubles...


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 Post subject: Re: Engine roughness
PostPosted: Fri Aug 11, 2017 11:51 am 
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IMO, these just roll out with the dices when back luck comes in. GA tech is inherently unreliable, mainly due to dubious-at-best quality assurance all the way from the materials.

You're still on warranty? Even if not, I'd find the most efficient way to disassemble/reassemble the cylinder and the valve top with skilled people. Sometimes they are like Legos: a bad one completely rebuilt may behave better.

-Esa


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 Post subject: Re: Engine roughness
PostPosted: Fri Aug 11, 2017 12:30 pm 
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We are under warranty. I have a feeling that it's going to be a matter of fixing what it is in there if it isn't damaged in some way. I trust this shop, but it also depends on what Lycoming wants to do.

Edit: It's actually worse, two cylinders have 0 compression hot and cold. :(


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 Post subject: Re: Engine roughness
PostPosted: Sat Aug 12, 2017 2:51 am 
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Oracle427 wrote:
Edit: It's actually worse, two cylinders have 0 compression hot and cold. :(
Valves hanging also?

One of the...funniest stories on how things can go wrong (and right!) with GA engines is from years before I was even born. It involved a GTSIO engine from a big twin Cessna, or some of its siblings, which were used to run mail and everything that time. The engines were notoriously unreliable, basically with anything possibly breaking with no warning, and basically anytime.

Now, during a routine oil change, the greaser noted something while draining the old oil: as he was draining it through a strainer (as one is supposed to!), the net caught a small retaining ring, or the remains of one. Of course, all the movement stopped for a moment, because there are no retaining rings in an engine that should drain out with the oil. Eyebrows remained high, and people shrugged their shoulders. The gurus were shaken awake, and started loitering around with book on the table and the part's remains on the palm.

After some time, someone was able to identify the retaining ring: it had to be a one of those that keep the crankshaft balance weights in place! Apparently there was some issue in the material or its treatment, making the ring too hard and brittle. Of course, the engine was off-wing then, because a total disassembly was required, and of course, they decided to change all the balance weight retaining rings while at it.

Reportedly, after all that was done - the engine basically disassembled and re-assembled, and back in service, after time it turned out to the one of the most reliable individual engines of the model. It was never determined if it was just coincidence, or if the complete dis-/re-assembly with time and care actually played a part in that.

-Esa


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 Post subject: Re: Engine roughness
PostPosted: Sat Aug 12, 2017 8:27 pm 
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I'm thinking of following a similar course of action. Instead of trying new parts for the sake of trying new parts, it may be a safer course of action to spend a little more time to tune-up the problem parts in-situ. Whether or not other problems will manifest remains to be seen.

How does one perform a compression test to evaluate the condition of the ring/cylinder interface if the valves aren't sealing? :(


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 Post subject: Re: Engine roughness
PostPosted: Sun Aug 13, 2017 4:42 am 
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Oracle427 wrote:
How does one perform a compression test to evaluate the condition of the ring/cylinder interface if the valves aren't sealing? :(
You don't, but why would you want to? Do you expect issues there as well, or having a history of bad compression?

Edit: Stuck valves alone do not warrant for all the pessimism I wrote about on GA engines. :) On Lycomings, this is a fairly well-known issue. On the short time I spent in GA world, I ran into none, but there are lots and lots written about these, and some Lycoming procedures for reaming the guides, checking the clearances and so on and so forth, many to be scheduled on regular basis. See this one [pdf] for starters, though you likely are familiar with it already. The issue is apparently a design... flaw of sorts in Lycomings, in which the valve stems run somewhat hotter than in Continentals for instance. Further, it is said that lowering the operating temps some tend to reduce the issue. All this makes sense, but can't really comment personally.

-Esa


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 Post subject: Re: Engine roughness
PostPosted: Sun Aug 13, 2017 9:00 am 
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Oh don't get me wrong, aside from the fact that it briefly tried to kill me, I don't fear it. I understand what is happening in there and like working on my car engines as a hobby.

I look at all this as a teachable moment in that I now better understand the warning signs and won't confuse then with the much maligned plug fouling. :)

This engine did have one of these same cylinders replaced just a little over 100 hours ago and it had scoring. We are confused because it isn't a pattern that we have experienced in the past. Normally we run rather smoothly up to and past TBO with an occasional hiccup along the way.

I guess this new unit still has some teeting problems to work out and we still have things to learn about its personality.


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 Post subject: Re: Engine roughness
PostPosted: Sun Aug 13, 2017 9:46 am 
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Oracle427 wrote:
Oh don't get me wrong, aside from the fact that it briefly tried to kill me, I don't fear it. I understand what is happening in there and like working on my car engines as a hobby.

I look at all this as a teachable moment in that I now better understand the warning signs and won't confuse then with the much maligned plug fouling. :)
Indeed, always good to read from first-hand experiences. :) They seldom are obvious when one first runs into one, regardless of what is the actual issue.

Oracle427 wrote:
This engine did have one of these same cylinders replaced just a little over 100 hours ago and it had scoring. We are confused because it isn't a pattern that we have experienced in the past. Normally we run rather smoothly up to and past TBO with an occasional hiccup along the way.
New cylinders, or OVH's, btw? Not that it had any significance most certainly, but just curious.

-Esa


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 Post subject: Re: Engine roughness
PostPosted: Sun Aug 13, 2017 9:52 am 
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An exchange engine from the factory for an overhauled unit. We contemplated rebuilding ours that served us well for 2400 hours (SNEW) The "new" one came to us with 4400TT.


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