First annual with plane, extremely low compression Lycoming IO-360

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Jamie Kirk

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JamieK
Had s prebuy done in August 2018, all cylinders were mid 70s. Annual from 2018 and all cylinders were mid 70s.

Today was IFR so couldn’t fly and warm it up properly just did a ground run up. Cylinder 4 tested in the 30s, cylinder 2 tested in the 50s and the other 2 were in the 70s.

IA says he is going to borescope it and see if he sees anything. If not then he says when we are done and before we do anything we will put it all back together and go fly it and test again.

This something that happens or is it the start of signs I need to dig deeper? I just finished my PPL training and got my ticket Friday so the last 3-4 flights have been doing maneuvers and not really leaning correctly and keeping it full rich. IA doesn’t seem concerned but I am sweating it.
 
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70's isn't "extremely low"
 
What plane?
Does it make expected static RPM?
Does it shake significantly?
Does the Prenup cover low pressure?
 
My vote is you burned a couple valves.
 
FORANE said:
What plane?
Does it make expected static RPM?
Does it shake significantly?
Does the Prenup cover low pressure?
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Commander 112.

When I purchased it the plane had the JPI 730 with no prop or manifold pressure sensors but RPMs got up to almost 2,700 on the gauge. I had the RPM and manifold sensors installed and the tach has shown 2,670.

No shake whatsoever. It runs smooth.

Prenup? I’m confused.
 
Find out where it's leaking. Valves? Rings? One compression reading it not all, but those are low enough to warrant further investigation. Engines lose virtually no power from low compression, but the loss of compression, if sustained over time, is a signal of something amiss.
 
chemgeek said:
Find out where it's leaking. Valves? Rings? One compression reading it not all, but those are low enough to warrant further investigation. Engines lose virtually no power from low compression, but the loss of compression, if sustained over time, is a signal of something amiss.
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IA is going to borescope it.

I’m assuming he knows how to tell where the loss is coming from. But for my knowledge how can I ask if it was tested?
 
If your mechanic isn’t concerned, why are you? I’d ask him why he isn’t concerned about it. You’ll learn more from that conversation than you will by posting a few numbers with no information and expecting a bunch of pilots, many of whom aren’t mechanics, to tell you what’s wrong and what to do.
 
mondtster said:
If your mechanic isn’t concerned, why are you? I’d ask him why he isn’t concerned about it. You’ll learn more from that conversation than you will by posting a few numbers with no information and expecting a bunch of pilots, many of whom aren’t mechanics, to tell you what’s wrong and what to do.
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He said it could just be carbon and wished we could have flown it before doing the compression test.

I’ll know more after a borescope
 
Jamie Kirk said:
He said it could just be carbon and wished we could have flown it before doing the compression test.

I’ll know more after a borescope
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What is the borescope going to tell you?

You haven’t told us where the air is leaking from.
 
Jamie Kirk said:
IA is going to borescope it.

I’m assuming he knows how to tell where the loss is coming from. But for my knowledge how can I ask if it was tested?
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You listen while the cylinder in undergoing the leak test. Hiss at oil filler = rings; hiss at exhaust = exhaust valve; bit harder to hear the intake hiss, IMO, but worth trying.
 
Jamie Kirk said:
Commander 112.

When I purchased it the plane had the JPI 730 with no prop or manifold pressure sensors but RPMs got up to almost 2,700 on the gauge. I had the RPM and manifold sensors installed and the tach has shown 2,670.

No shake whatsoever. It runs smooth...
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Anything is possible, but I put odds in favor of no real problem to sweat about at this point.

Unless there is some other indication of a problem, beyond just a single set of compression readings, if it was my plane I would go fly it on a decent cross country (yes, before doing a borescope). Get it up to some reasonable altitude for your geography, lean if out properly, run it at a somewhat higher power setting than usual and fly it for an hour or more.

If everything is normal and there is no indication of a problem I would just keep flying it.

If you go ahead with the borescope, let us know if your mechanic finds anything.
 
Oil filler hiss could be caused by all of the rings lining up (lucky) or a broken/worn ring or worn cylinder. Running it and testing again will tell which. If still low, add a small amount of oil into the plug hole, If broken ring, nothing will change much but if worn rings or cylinder, then compression will go up. Either way would mean pulling the cylinder.
Given your name, shouldn't you have Scotty look at it?
 
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Borescoped it anyways and the valves were great. I’ll upload the videos.
 
Jamie Kirk said:
Borescoped it anyways and the valves were great. I’ll upload the videos.
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And if it was borescoped and the bore looks good, I’d have a tougher time believing a broken ring (which would tend to cause scores in the cylinder).

How many hours on the top end? Maybe just worn.
 
455 Bravo Uniform said:
And if it was borescoped and the bore looks good, I’d have a tougher time believing a broken ring (which would tend to cause scores in the cylinder).

How many hours on the top end? Maybe just worn.
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Entire motor is just over 700 hours
 
Tell us on a leakdown test whether the sucker is leaking from the rings (hiss in the case vent), from the exhaust valve (hiss in the muffler) or in the intake valve (hiss [muffled}) in the carburetor.

And stop wasting our time guessing until you do this. I don't give a hairy rat's @$$ what your borescope showed you.

Jim
 
weirdjim said:
Tell us on a leakdown test whether the sucker is leaking from the rings (hiss in the case vent), from the exhaust valve (hiss in the muffler) or in the intake valve (hiss [muffled}) in the carburetor.

And stop wasting our time guessing until you do this. I don't give a hairy rat's @$$ what your borescope showed you.

Jim
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You’re free to just continue on, no one forces you to read or participate.
 
Jamie Kirk said:
Borescoped it anyways and the valves were great. I’ll upload the videos.
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Go fly the airplane as soon as the annual is done and stop worrying about it. I am quite certain the biggest single contributor to poor compression checks is airplane engines not being used frequently enough.
 
Putting the new plugs in today the IA was having issues on the cylinder that was reading low. The heli-coil was coming out with the plug. He removed the help-coil now we are going to replace it and get the plugs in.

He said that could have caused the low reading also.
 
Jamie Kirk said:
He said that could have caused the low reading also.
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It’s quite possible, but the air wouldn’t have been coming out of the oil filler tube then.

Also, why is the helicoil messed up? Is the cylinder head cracked?
 
mondtster said:
It’s quite possible, but the air wouldn’t have been coming out of the oil filler tube then.

Also, why is the helicoil messed up? Is the cylinder head cracked?
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Threads are also chewed up. Doesn’t think it’s savable. New cylinder is the suggestion.
 
Jamie Kirk said:
Threads are also chewed up. Doesn’t think it’s savable. New cylinder is the suggestion.
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The threads are messed up in the cylinder AFTER he removed the helicoil? Don't know if they are used in aviation but there are oversized helicoils for that situation.
 
alfadog said:
The threads are messed up in the cylinder AFTER he removed the helicoil? Don't know if they are used in aviation but there are oversized helicoils for that situation.
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The helicoil came right out, you could see how chewed up the cylinder threads were. Also closely looking at the spark plug that came out of there it’s threads are also chewed up.
 
Jamie Kirk said:
The helicoil came right out, you could see how chewed up the cylinder threads were. Also closely looking at the spark plug that came out of there it’s threads are also chewed up.
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Google oversized helicoil (heli-coil, thread insert). Again, don't know if these are used in aviation but might could save the cylinder. Also, call a cylinder shop for their advice.
 
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