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

walkman

Well Known Member
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?
 
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Compression

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.
 
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.
 
... 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
 
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?
 
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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. :)
 
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?
 
... 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...:confused:
 
... 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...:confused:

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.
 
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?
 
Did you check the suction screen during the recent oil changes ... Agree with the above that broken rings would likely result in noticeably increased oil consumption and dirty plugs ... But the screen would have caught any larger pieces of broken ring or other metal. Also, how have the filter inspections looked, any more than your previously normal amount of debris in the paper pleats?

Reggie
 
68/80 is a noticeable leak. Put the compression tester back on that cylinder, remove the oil dipstick and listen at the dipstick (oil fill) tube, then the carb air filter, then at the exhaust pipe.
It's coming from one or all of those, and no where else.
If you have an intake valve leak, well that's rare, and the end of the line, the cylinder should probably come off. Loss of an intake valve, or even being stuck open is instant loss of power for a carburated engine, and nearly as bad with fuel injection.
Piston rings rarely align their end gaps, but would cause a big leak. Rings eventually lose their seal and hot gas blows by, causing an increase in oil temperature, and maybe you would notice that one cylinder head temp has been increasing? (if you have individual cylinder temp gauges) This might be visible thru the spark plug hole with the piston at Bottom Dead Center. The cylinder wall would be brown from exhaust / heat staining. Or you might see some scoring. Sometimes a bit of sand or broken piece or ring, or a micro weld can start scoring. Scoring never stops, so you take the cylinder off and fix it if caught early.
Has the exhaust temp on that cylinder been changing? That and a hiss at the exhaust pipe.. well you get the idea.
 
... 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...:confused:

Just re-read this.

By "tight valve" do you mean one where the clearances in the valve train are too small, that is, pushrod effectively too long?

There's enough clearance cold for the rocker to move when the valve is closed, but I have not done a proper test of that. Is it possible that the valve stem could have stretched or something?
 
Did you check the suction screen during the recent oil changes ... Agree with the above that broken rings would likely result in noticeably increased oil consumption and dirty plugs ... But the screen would have caught any larger pieces of broken ring or other metal. Also, how have the filter inspections looked, any more than your previously normal amount of debris in the paper pleats?

Reggie

Have not checked the screen, will do when I change the oil Cut open filters are clean. Nothing but a tiny amount of carbon, nothing sticks to a magnet.
 
The valves stretch in length and the stem gets smaller in diameter. This is normal and expected. The clearances are way over the amount that this would affect the valvetrain unless the engine was assembled at the tight end of the dry tappet clearance.
 
68/80 is a noticeable leak. Put the compression tester back on that cylinder, remove the oil dipstick and listen at the dipstick (oil fill) tube, then the carb air filter, then at the exhaust pipe.
It's coming from one or all of those, and no where else.
If you have an intake valve leak, well that's rare, and the end of the line, the cylinder should probably come off. Loss of an intake valve, or even being stuck open is instant loss of power for a carburated engine, and nearly as bad with fuel injection.
Piston rings rarely align their end gaps, but would cause a big leak. Rings eventually lose their seal and hot gas blows by, causing an increase in oil temperature, and maybe you would notice that one cylinder head temp has been increasing? (if you have individual cylinder temp gauges) This might be visible thru the spark plug hole with the piston at Bottom Dead Center. The cylinder wall would be brown from exhaust / heat staining. Or you might see some scoring. Sometimes a bit of sand or broken piece or ring, or a micro weld can start scoring. Scoring never stops, so you take the cylinder off and fix it if caught early.
Has the exhaust temp on that cylinder been changing? That and a hiss at the exhaust pipe.. well you get the idea.

I really appreciate all the insight and input from everyone on this thread and via pm. While I've built a few 911 engines, and got 25 years of Lycoming ownership and hands on, I still consider myself a backyard mechanic.

wrt to the comment above, listening to the engine is a bit of a black art that requires experience and a tuned ear. Back in the day I could put a screwdriver up against a 911 block, the other end in my ear, and tell you if it was rich or lean. No longer I'm afraid :-(

At this point, it doesn't make any difference if its a leaking intake valve, leaking exhaust valve, or broken rings. They all mean the cylinder's gotta come off. :-(

Cylinder walls are really greyish, perhaps with a tiny tinge of brown. Some appearance of vertical scuffing or scoring up near the choke, I don't know how much is normal. I have individual CHT and EGT, but it doesn't record. Nothing appears abnormal with those temps, but I haven't been writing them down and comparing over time.

I think my plan of attack is to do a quart or so of MMO again and dee what happens thursday night. Perhaps I'll find a local source for the lard and make my own :)

If that doesn't help, I'm pulling the jug on Saturday.

If the rings are indeed coked up and not broken, whats the best way to clean them? I'm assuming oven cleaner is not the best direction.
 
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I have put removed carboned pistons in MMO (think marinade) and the carbon will definitely loosen up. Wouldn't hurt to try putting the cylinder on TDC then fill it with MMO and let it sit for a day or two, then remove the bottom plug and drain it. Worth a try perhaps...I've never done this so YMMV.
 
I have put removed carboned pistons in MMO (think marinade) and the carbon will definitely loosen up. Wouldn't hurt to try putting the cylinder on TDC then fill it with MMO and let it sit for a day or two, then remove the bottom plug and drain it. Worth a try perhaps...I've never done this so YMMV.

If you do this use old plugs. If you saturate the plugs with penetrating oil or fuel for long enough they will develop an internal short and they may spark outside of a cylinder when tested, but they will not fire across the gap under compression.
 
I have put removed carboned pistons in MMO (think marinade) and the carbon will definitely loosen up. Wouldn't hurt to try putting the cylinder on TDC then fill it with MMO and let it sit for a day or two, then remove the bottom plug and drain it. Worth a try perhaps...I've never done this so YMMV.

If you do this use old plugs. If you saturate the plugs with penetrating oil or fuel for long enough they will develop an internal short and they may spark outside of a cylinder when tested, but they will not fire across the gap under compression.

I had considered doing that. I may run the engine on the ground, get it hot, put an old plug in the bottom, get it to TDC on the compression stroke, and pour MMO in the top plug. I will work the prop back and forth a bit to get some MMO worked around the rings as much as possible, then fill it up and let it sit till the weekend.

Excellent ideas I will advise on results.
 
At this point, it doesn't make any difference if its a leaking intake valve, leaking exhaust valve, or broken rings. They all mean the cylinder's gotta come off. :-(

Not Necessarily,
You have to identify the source of the leak first and it doesent sound like you have done that yet. If the source is a valve there are tricks for removing the valve springs and checking first to make sure your valve stems arent sticking (they should move freely by hand in and out) and second to make sure you dont have a carbon build up on the valve face or seat. The valve can be lapped with lapping compound to remove carbon if that be the case. If the guides have build up causing a sticking valve the guide can have a reamer run through it in place as well. Of course if you have a burned valve or seat you are dead in the water.
If you find out it is a valve I will be happy to go into more detail about how to accomplish some of these tricks (and it isnt hard)
I have no problem pulling a cylinder if it needs pulled but if you can fix it in a coulple hours with out having to do so thats the way to go.


Ryan
 
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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.

Mike Busch is the Dr. Oz of aircraft maintenance. See this thread, of one the many examples of bad advice given: http://www.vansairforce.com/community/showthread.php?t=115438

Last year I pulled two cylinders on 55BC because of low compression. It was on Aeroshell and Camguard. What I saw inside the engine completely unimpressed me. Here is a photo of one of the pistons removed:

IMAGE_12.jpg


Nothing but Phillips XC from there on forward.
 
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At this point, it doesn't make any difference if its a leaking intake valve, leaking exhaust valve, or broken rings. They all mean the cylinder's gotta come off. :-(

Not Necessarily, You have to identify the source of the leak first and it doesent sound like you have done that yet. If the source is a valve there are tricks for removing the valve springs and checking first to make sure your valve stems arent sticking (they should move freely by hand in and out) and second to make sure you dont have a carbon build up on the valve face or seat. The valve can be lapped with lapping compound to remove carbon if that be the case. If the guides have build up causing a sticking valve the guide can have a reamer run through it in place as well. Of course if you have a burned valve or seat you are dead in the water. If you find out it is a valve I will be happy to go into more detail about how to accomplish some of these tricks (and it isnt hard)
I have no problem pulling a cylinder if it needs pulled but if you can fix it in a coulple hours with out having to do so thats the way to go.

Ryan

Mike Busch is the Dr. Oz of aircraft maintenance. See this thread, of one the many examples of bad advice given: http://www.vansairforce.com/community/showthread.php?t=115438

Last year I pulled two cylinders on 55BC because of low compression. It was on Aeroshell and Camguard. What I saw inside the engine completely unimpressed me. Here is a photo of one of the pistons removed:

IMAGE_12.jpg


Nothing but Phillips XC from there on forward.

Ryan thanks, checked the valves already.

Big recommendation for the Jolly Time Tools valve spring compressor BTW

I've lapped 'em on the engine on previous motors. I'm inclined to remove and do it in the future. Pulling a jug isn't THAT difficult.

Mike Busch seems to recommend against HAVING A SHOP PULL A JUG mostly because of the number of errors made by mechanics in reassembling afterwards. Or at least that's what a cursory read seems to me
 
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Mike Busch seems to recommend against HAVING A SHOP PULL A JUG mostly because of the number of errors made by mechanics in reassembling afterwards.

Yep mechanics are dumb and we should avoid fixing things because of our inherent stupidity.:p
 
saavy

Just looked at the website, they sure are impressed with themselves.
 
Ryan thanks, checked the valves already.

Big recommendation for the Jolly Time Tools valve spring compressor BTW

I've lapped 'em on the engine on previous motors. I'm inclined to remove and do it in the future. Pulling a jug isn't THAT difficult.

Mike Busch seems to recommend against HAVING A SHOP PULL A JUG mostly because of the number of errors made by mechanics in reassembling afterwards. Or at least that's what a cursory read seems to me

I think Mike Busch is a pretty smart guy but like Bob I certainly don't agree with all of his ideas. If it's time to pull a cylinder it's time! Like I said before I certainly have no problem pulling one when I'm sure that's what needs to be done and I certainly don't understand the fear of doing it. Good luck and let us know what you find.
Ryan
 
You guys might want to actually take the time to read the Busch articles and listen to his webinars. You are misrepresenting his views. His point is that many cylinders have been unnecessarily removed. Correct diagnosis of the cause of the problem and use of a borescope can prevent this. No one is arguing that a cylinder should never be removed.
 
I have read his articles and I disagree with some them. Some I agree with but certainly not the recent one a few months ago where he discusses cylinder fastener fatigue, and to avoid cylinder removal because of it. Neither Lycoming nor Continental spell out and fatigue limits for thru bolts or studs. They would specify torque cycles if that were the case, unlike rod bolts that have stretch limits. If fatigue limits were a concern on those fasteners they would certainly generate a lot of income for attorneys.
 
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