One cylinder coming in low...

ArrowFlyer86

ArrowFlyer86

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The Little Arrow That Could
Note: I know there are other threads on low compressions, which I've scanned through. In the likely event I have follow-up Q's I wanted to post them here rather than necro'ing threads from 5-10y ago :)

Dropped aircraft off for annual yesterday.
Ran it up to normal operational temps beforehand (a few trips in the pattern) so they could do the compression test.

Compressions:
Cylinder 1: 62 (A&P comment: "it's a bit low, and there was blow by through the piston rings")
Cylinders 2 through 4: 70+ (didn't get exact #'s on 2-4, just that they were in 70s and good)

Last year c#1 was at 64, and the year before 65.

Specs:
Engine: Lycoming IO360-C1C6
Chrome cylinders
Last OH: 30y ago, Last IRAN: 2020
No metal observed during oil changes.
I'm consistently flying it >= ~20h/month, usually 2-3x per week.

Any suggestions, courses of action or additional questions I might want to provide to the shop? Or equally valuable - anything to avoid doing?
 
Timbeck2 said:
If it isn’t making metal and oil consumption isn’t terrible, fly it. Your next compression test may be different.
Click to expand...
Thanks.

Oil consumption is pretty constant.
About 1 - 1.25qts per 10 hours if I keep it at 6qts.
How much oil it burns seems more correlated to how long since the last oil change.
I usually go 50h between changes (~every 6-9 wks). And no, no metal observed when I cut open the filter and inspect.
 
Yeah it’s passable, so I agree with the above, but would begin thinking about pulling the jug and putting a new or overhauled one on. Could also just get by with swapping the ring sets out, but you’d get more bang for the buck doing the whole cyl.
 
Lycoming service instruction SI1191A is recommended reading both for the OP and repliers :)

"Unless the pressure difference exceeds 15 psi the investigation should not necessarily mean removal of the cylinder; often a valve will reseat itself and result in acceptable compression during a later check which should be made within the next 10 hours of operation.

b. If the pressure reading for all cylinders is equal and above 70 psi; the engine is satisfactory; less then 65 psi indicates wear has occurred and subsequent compression checks should be made at 100 hour intervals to determine rate and amount of wear. If the pressure reading is below 60 psi or if the wear rate increases rapidly, as indicated by appreciable decrease in cylinder pressure, removal and overhaul of the cylinders should be considered."

Sounds to me like you might fall under the first paragraph of the quoted bit but the consistent decrease might be an indicator.

What did your mechanic recommend?

Nauga,
and troubleshooting by proxy
 
Fly, and monitor is what I would do (and have done). If borescoping revealed significant step wear or other anomalies in the cylinder walls, or if oil fouling starts to become a problem, especially if compression continues to decline (see post #5) I would consider cylinder repair or replacement.
 
nauga said:
Lycoming service instruction SI1191A is recommended reading both for the OP and repliers :)

"Unless the pressure difference exceeds 15 psi the investigation should not necessarily mean removal of the cylinder; often a valve will reseat itself and result in acceptable compression during a later check which should be made within the next 10 hours of operation.

b. If the pressure reading for all cylinders is equal and above 70 psi; the engine is satisfactory; less then 65 psi indicates wear has occurred and subsequent compression checks should be made at 100 hour intervals to determine rate and amount of wear. If the pressure reading is below 60 psi or if the wear rate increases rapidly, as indicated by appreciable decrease in cylinder pressure, removal and overhaul of the cylinders should be considered."

Sounds to me like you might fall under the first paragraph of the quoted bit but the consistent decrease might be an indicator.

What did your mechanic recommend?

Nauga,
and troubleshooting by proxy
Click to expand...
Danke.

Currently no recommendation from the A&P. He just texted me to let me know about it and said he'll be in touch with suggested solutions soon.
 
www.savvyaviation.com

Ending the War on Jugs - SavvyAviation

Weak compression doesn’t always mean that the cylinder has to come off. For most of my nearly six decades as an aircraft owner and three decades as an A&P, the rule about cylinders was simple: If the compression reading was less than 60/80, the cylinder had to come off for repair or replacement...
www.savvyaviation.com www.savvyaviation.com
Have you considered a solvent ring flush?
 
odie451 said:
Have you considered a solvent ring flush?
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This is 100% what I would do in your situation. I would also borescope the cylinder (well, all of them, but especially #1) to make sure the valves and seats are okay. I know he said it was going through the rings...trust but verify.

The solvent flush is even something you could do on your own if you can do oil changes. It may not unstick your ring, but it can't hurt, and I would definitely exhaust all options before pulling a jug.

If it makes you feel any better, in one of those webinars, Mike relates the following story:
Incidentally, there is no correlation whatsoever between low compression caused by leakage past the rings and engine power. To demonstrate this, Continental ran a new 300-hp IO-550 engine in its engineering test cell in Mobile, Alabama, and measured its horsepower output on a dynamometer. They then filed the compression ring gaps oversize to reduce the compression of all six cylinders to 40/80 and repeated the dyno test. There was no measurable decrease in horsepower.
Click to expand...
From this article, which is a good lesson on compression tests:
www.aopa.org

Opinion: Unbelievable compression

Each annual inspection begins with a moment of terror when the IA removes the top spark plugs and takes a compression reading of each cylinder. We hold our breath awaiting the verdict.
www.aopa.org

Here's his directions on the ring flush: https://www.google.com/url?sa=t&sou...oQFnoECCMQAQ&usg=AOvVaw0EQi94XxsZH7l1uE1uWog5
 
Compressions can vary though, perhaps just monitor it and check compressions again at your next oil change if that’s an option?

At my last annual, my A&P changed what I believe is cylinder ring, maybe you need this? Or use a dye to see if it’s leaking and then swap one out if so (that’s what we did with my prebuy). You’ll have to break in the new cylinder in that case, not a biggie.
 
Oil wise I think I’m burning 1qt every 30 hours but if I overfill it then it blows out pretty quickly. She likes just under 6 qt, nothing more. Next oil change I’ll probably do 6.5 qt plus the camguard. And hopefully one day I can take out the oil filter without spilling oil everywhere.
 
Got an update...
They checked out the valves and said no issue there, so that's good.
They're doing a solvent flush now with a concoction of Xylene, mineral spirits and Aeroshell W100 then they'll test it again.
 
Solvent wash did the trick! Appreciate that recommendation folks!
Compression increased from 62 -> 68. Others in the 70s. All good for now.

And it sounds like AQI is going to honor the warranty on an OH turn coordinator I got last year which stopped working (warranty is only 1y and I'm a few weeks outside of that).
Mags going to G&N for 500h inspection.

So far so good on the annual now :)
 
ArrowFlyer86 said:
Solvent wash did the trick! Appreciate that recommendation folks!
Compression increased from 62 -> 68. Others in the 70s. All good for now.

And it sounds like AQI is going to honor the warranty on an OH turn coordinator I got last year which stopped working (warranty is only 1y and I'm a few weeks outside of that).
Mags going to G&N for 500h inspection.

So far so good on the annual now :)
Click to expand...
Awesome!!
 
Glad that helped. Wasn't going to in my case. Burned up a valve. 30/80. Engine monitor would have probably caught things.
 
Oomph a bit of thread drift but got a surprise on the mags inspection. Turned into an OH for both with G&N, work totalling about $3k (Bendix).

Glad I sent them in though cause they clearly needed some work.
 
RyanB said:
Running too lean?
Click to expand...
Most likely the opposite. Only had it for about 60 hours, so who knows how it was actually run before me. But from what I've read, not ground leaning and not leaning enough tends to cause more problems.
 
MRC01 said:
Running in the "red box" kills valves. If you lean it enough, you're out of the box. If you enrich enough, you're out of the box. The worst case is to lean it, but not enough to get out of the red box.
Click to expand...
"red box"? How can you tell what that region is?
 
MRC01 said:
Running in the "red box" kills valves. If you lean it enough, you're out of the box. If you enrich enough, you're out of the box. The worst case is to lean it, but not enough to get out of the red box.
Click to expand...
That’s a bunch of hogwash. Savvy Aviation has so much wrong information that will end up LEADING toward engine damage, especially the LOP operation they speak about. Lycoming’s own literature doesn’t even mention the “red box”. About every four cylinder engine in a flight training environment doesn’t get leaned so precisely to avoid the phenomenon and still go well beyond TBO. What kills valves is temperature, specifically CHT’s beyond 400f and CHT’s above 1450f which is basically what the so-called red box is based around.
 
RyanB said:
That’s a bunch of hogwash. Savvy Aviation has so much wrong information that will end up LEADING toward engine damage, especially the LOP operation they speak about. Lycoming’s own literature doesn’t even mention the “red box”. About every four cylinder engine in a flight training environment doesn’t get leaned so precisely to avoid the phenomenon and still go well beyond TBO. What kills valves is temperature, specifically CHT’s beyond 400f and CHT’s above 1450f which is basically what the so-called red box is based around.
Click to expand...
Not just Savvy, but also GAMI. They have only been doing this with many engines since the 90s. Based on how the airlines ran the big radials back in the day to over 3000 hours between overhauls.
 
Pinecone said:
REALLY?
Click to expand...
Yes, really.
Pinecone said:
Not just Savvy, but also GAMI. They have only been doing this with many engines since the 90s. Based on how the airlines ran the big radials back in the day to over 3000 hours between overhauls.
Click to expand...
Let’s see the data on it. Lycoming says nothing about a “red box”, what they do tell us are operating limits based on temperature, which is primarily what their self imposed red box is created from, it’s nothing magical. Fuel is cooling. Running too lean without the proper equipment will do far more harm than good. If CHT’s and/or EGT’s are above the prescribed limits, that’s where valve and cylinder damage will occur. Being rich, is better than being too lean, even though neither are ideal.
 
RyanB said:
Let’s see the data on it. Lycoming says nothing about a “red box”, what they do tell us are operating limits based on temperature, which is primarily what their self imposed red box is created from, it’s nothing magical. Fuel is cooling.
Click to expand...
Yes, excessive fuel does cool the engine. And it's better for power because the mixture vs power curve is asymmetric: being X% lean loses more power than being X% rich. Yet it is also less efficient (obviously) and over time it gunks up the valves.

At the risk of stating the obvious, for the benefit of others who are asking:

If you lean far enough, it also cools the engine, mainly because you're making less power. And the engine runs more efficiently and cleaner, compared to getting that same cooling by running rich. But you lose more power, due to the asymmetry of the mixture vs. power curve. This is called LOP "lean of peak", where "peak" refers to temperatures, not power. Of course it is on the lean side of both temperature and power. But the key to LOP is that you lean so far that CHTs drop (not just EGT).

The "red box" is the range between these, where the mixture is close to optimal for peak power. This is where temperatures are hottest, and should be avoided. How you avoid them, by running rich, or by running lean, perhaps combined with reducing power, is up to you as the engine operator.

Some engines can't get lean enough to run LOP. As you lean, the engine gets rough before it gets lean enough for CHTs to drop. In this case, LOP operation is not an option. You either slow down to < 70% power and run lean, or run rich of peak at higher power levels.
 
RyanB said:
Yes, really.

Let’s see the data on it. Lycoming says nothing about a “red box”, what they do tell us are operating limits based on temperature, which is primarily what their self imposed red box is created from, it’s nothing magical. Fuel is cooling. Running too lean without the proper equipment will do far more harm than good. If CHT’s and/or EGT’s are above the prescribed limits, that’s where valve and cylinder damage will occur. Being rich, is better than being too lean, even though neither are ideal.
Click to expand...
What is a "prescribed limit" on EGT?
 
I got comfortable with leaning my IO-470 based on this video from Continental. I still run rich of peak, but below certain power settings, it doesn’t really matter where you run, rich, lean, or in-between.

It’s about a 10 minute watch but will clear up confusion or concerns about leaning, without any tales from elderly wives or washing of hogs…unless you believe in conspiracy theories that TCM wants you to burn up your engine so they can stay in business. I can’t help with that.

 
Hang 4 said:
What is a "prescribed limit" on EGT?
Click to expand...
Less than 1450f per the manufacturer.
MRC01 said:
Yes, excessive fuel does cool the engine. And it's better for power because the mixture vs power curve is asymmetric: being X% lean loses more power than being X% rich. Yet it is also less efficient (obviously) and over time it gunks up the valves.

At the risk of stating the obvious, for the benefit of others who are asking:

If you lean far enough, it also cools the engine, mainly because you're making less power. And the engine runs more efficiently and cleaner, compared to getting that same cooling by running rich. But you lose more power, due to the asymmetry of the mixture vs. power curve. This is called LOP "lean of peak", where "peak" refers to temperatures, not power. Of course it is on the lean side of both temperature and power. But the key to LOP is that you lean so far that CHTs drop (not just EGT).

The "red box" is the range between these, where the mixture is close to optimal for peak power. This is where temperatures are hottest, and should be avoided. How you avoid them, by running rich, or by running lean, perhaps combined with reducing power, is up to you as the engine operator.

Some engines can't get lean enough to run LOP. As you lean, the engine gets rough before it gets lean enough for CHTs to drop. In this case, LOP operation is not an option. You either slow down to < 70% power and run lean, or run rich of peak at higher power levels.
Click to expand...
That is the theory behind it, yes. Problem is, I’ve seen too many folks come into the shop and have low compression and have to change out jugs far too early and each of them ran LOP, or at least attempted too. The problem is that most people don’t understand how to do it and Savvy is bad about claiming it can be done on nearly any engine, even carbureted, which sends people down a path of premature engine damage. The fact of the matter is, running lean, if not done properly WILL contribute to valve and cylinder damage. You’re right in the fact that running too rich will lead to excessive deposits on valve stems, cylinder heads and plugs, but that’s if you don’t lean at all. The majority of the lead will get scavenged on its own by running at operating temperatures and running near a best power mixture setting. I am not a believer in lean of peak operation over the long haul and especially if you don’t have the equipment to it correctly. Fuel is cheap, engines are not.
 
Some of the strategies to follow with a low jug are good to use

PRIOR to an Annual Inspection.

If the Inspection has run out then you really have no options if

the Tech will not sign as Airworthy.

There are a lot of folks with opinions that have no skin in the game.

The Inspecting Agency DOES.

If it does not meet at least minimum the owner is in a bind.

Ferry Permits still require a signature.

Flying out of Annual can bring FAA in and Insurance Co. out.
 
RyanB said:
Yes, really.

Let’s see the data on it. Lycoming says nothing about a “red box”, what they do tell us are operating limits based on temperature, which is primarily what their self imposed red box is created from, it’s nothing magical. Fuel is cooling. Running too lean without the proper equipment will do far more harm than good. If CHT’s and/or EGT’s are above the prescribed limits, that’s where valve and cylinder damage will occur. Being rich, is better than being too lean, even though neither are ideal.
Click to expand...
So, running LOP with lower temps than ROP is bad?

You contradict yourself.

Running TOO lean is not a problem. Running NOT LEAN ENOUGH is.
 
Always thought a burnt exhaust valve is caused by a valve that is not seated well when closed. Nil to do with mixture.
EGT numbers LOP and ROP can be exactly the same. ex 1300 dF.
Which 1300 dF is burning your valve? The ROP one or the LOP one?
 
Pinecone said:
... Running TOO lean is not a problem. Running NOT LEAN ENOUGH is.
Click to expand...
That's true. However, "not lean enough" is a common problem. Many engines can't get lean enough to reduce temperatures. The engine gets rough before it gets that lean. So the pilot leans as much as he can, just rich of rough, which puts the engine in the red box. He gets good fuel economy and the valves are clean, but a few hundred hours later he's got to replace a jug due to a burnt exhaust valve. Oops.

Running LOP is great if you can do it properly, but not everyone can. If you can't, your option is to run ROP unless power is below 70%.
 
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