Suddenly low compression on one cylinder - time to pull it?

[walkman]

Background: 2004 Superior XP-360 approx 950 SNEW. Bendix RSA-5 FI.

Purchased aircraft in Feb, 78+/80 all around. My inexpert borescoping showed nice shiny valve edges, "bullseye" look to exhaust valve faces, no scoring in cylinder walls, honing pattern visible.

All oil analysis since new look good, but high aluminum constant since new, but not vastly high and no spikes, not increasing.

Put a little over 100 hours on it in 7 months, lots of formation, some LOP cross country, a little acro. 2 oil changes. oil seems to go black quite quickly. always lean aggressively on ground, usu. lean to 10 gph for formation work. aeroshell w100 with ASL camguard

CHTs rarely over 380, mostly 340's except even lower when LOP cruising. Even in hot, high power climb out CHT's 380 or below.

Very rough running has slowly developed when the engine was hot, has gotten worse slowly. engine fine when cold, but 10 minute taxi and starts idling roughly. makes good power when airborne. all plugs, intake gaskets, hoses are new. plugs all tested.

Compression test 3 weeks ago showed 79 on 2,3,4, 68 on #1. Ran another 10 hours. comp test yesterday 2,3,4 all the same, #1 below 60.

Another borescope valve edges and faces look great. Can't see the seats. both smooth and no sticking in their travel. cyl walls on #1 MAY show some vertical scoring in bore up at top in choke area, this may or may not be normal, my lack of experience here. sorry no pics.

Hard to tell where air is leaking during compression test (full inverted oil system) but I don't think its valves, so where else but rings?

I'm thinking its time to pull this jug and look see. broken rings maybe? before I do, does anyone have another opinion?

[jrs14855]

You should read the Mike Busch articles in Sport Aviation on this subject. Mike is a strong advocate of NOT pulling cylinders for low compression.
Is your compression check hot or cold. If cold, repeat the test hot.
If you had serious ring problems you would be using a lot of oil and fouling plugs.
The hot idle issue is to me typical FI Lycoming. Are you leaning the mixture aggressively for taxi, both before takeoff and after landing.

[rocketbob]

Originally Posted by jrs14855

You should read the Mike Busch articles in Sport Aviation on this subject. Mike is a strong advocate of NOT pulling cylinders for low compression.

Contrary to Mr. Busch's poorly reasoned opinion I find that its ok to have a look because its better to fix a cylinder problem before it becomes too scored or too damaged to require replacement. In other words, if there's a problem its easier to fix now than later. Why wait? There is no reason to if you suspect something is wrong.

[DanBaier]

... Mike Busch articles in Sport Aviation on this subject. Mike is a strong advocate of NOT pulling cylinders for low compression....

First, plus 1 to Bob's advice.

The quote about Mike is okay as far as it goes. I think Mike's complete position is not to pull a cylinder SOLELY on the basis of compression. He often refers to the Continental SB that basically says to inspect a suspect cylinder with a borescope in conjunction with a compression test. Looking for evidence of a broken ring, valve problems, etc.

That this occurred suddenly means something has happened, and if you can't draw a bead on the problem without pulling the cylinder, then off it should come.

Look at the plus side - the airplane got you home. If it were me, I'd be reluctant to stretch that into 'normal Lycoming.'

FWIW.

Dan

[walkman]

Originally Posted by jrs14855

You should read the Mike Busch articles in Sport Aviation on this subject. Mike is a strong advocate of NOT pulling cylinders for low compression.
Is your compression check hot or cold. If cold, repeat the test hot.
If you had serious ring problems you would be using a lot of oil and fouling plugs.
The hot idle issue is to me typical FI Lycoming. Are you leaning the mixture aggressively for taxi, both before takeoff and after landing.

Same # hot or cold. Also, that's the reason I flew another 10 hours before re-doing the compression test.

I'm not sure if I have one or two issues. I did check the injectors, all pumping exactly the same amount of fuel. Cleaned them anyway.

re: Dan's comment above. Yes, its the "sudden" change. "Sudden" meaning it has happened in 7 months. Maybe it happened in the last 2 months, but my most recent measurement prior to three weeks ago was Feb. But to go from 78 to 58 in a few months is not good.

I'm going to change the oil and do another oil analysis this week.

What other evidence for broken rings is there?

[rocketbob]

Its pretty rare to get broken rings. I suspect your rings have carboned up and suggest putting in some Marvel Mystery Oil. The stuff people say doesn't work but does.

[walkman]

Originally Posted by rocketbob

Its pretty rare to get broken rings. I suspect your rings have carboned up and suggest putting in some Marvel Mystery Oil. The stuff people say doesn't work but does.

Thanks Bob. I forgot to mention, I did put in a pint of MM a month or two ago. I put it in, then flew a few touch and goes to get it hot and circulated, then I let it sit for a week.

It did seem to have a brief but minor improvement on the rough running issue but it didn't last. At the time I was thinking it was a sticky valve, but since have checked all the valves on all cylinders.

Perhaps I didn't use enough MM, or use it properly?

What is the proper application of MM and in what quantity?

[PerfTech]

... Before pulling the cylinder, you may want to check for a tight valve as this will act much as you described and generally get worse with heat at idle. Thanks, Allan...

[walkman]

Originally Posted by PerfTech

... Before pulling the cylinder, you may want to check for a tight valve as this will act much as you described and generally get worse with heat at idle. Thanks, Allan...

Checked all intake and exhaust valves for broken springs, and checked for stickiness. All smooth as silk with no evidence of sticking. All turn smoothly while seated. All exhaust valves have a tiny wobble in the guide. Didn't use a dial gauge, but feels about right.

Again, its the significant change in the results of the compression/leakdown test that is concerning.

[Mike H]

You need to determine the leak path. Disconnect some of the oil breather hoses and listen for air escaping there as well as through the exhaust pipe or intake. If the leak is from the valves the cylinder has to come off. If leaking past the rings you can try to run it some more and hope it gets better but it sounds like a substantial leak down and that indicates a problem that will most likely get worse if not corrected soon. Have you checked the cylinder head casting between the fins adjacent to the head/cylinder junction for cracks?