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.
 
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.
 
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