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Cylinder compression lower when hot

david.perl

Well Known Member
Rapid drop in Cylinder compression -exhaust valve culprit

I have a Lycoming IO 360 M1B engine with 350 hours of air- time on it over six years. The number one cylinder was giving a slightly low compression reading of 68 psi. We could hear that the air was escaping from the exhaust valve during the test.

The engineer suggested I run it hard for an hour or so in-flight which I did and we then retested within 10 minutes of landing whilst the engine was still hot. The compression on the cylinder dropped to 61.

We left it for a couple of hours and the compression dropped further to 54
 
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350 hrs is in range for a possible valve sticking, but I think (hoping) Lyc has solved that with the M1B. Honestly I would go run it for 30-60 min at high power lean cruise and be careful on the full mix return and landing phase. You could easily have a lead pearl under the valve. Flying is more fun that removing the VC and whacking the rocker with a hammer under pressure anyway. ;) Either would work for a next diagnostic step.
 
Thanks for the prompt reply Bill.

I flew it for 90 mins right before we tested at 25 and 25 and 50° lean of peak, I have EGT On All 4 exhaust so I can monitor carefully the LOP settings

I also only went to full rich mixture only a couple of minutes before landing.

We also have taken off the valve cover and wacked the valve rocker arm with a rubber hammer a few times which didn’t make any difference. The engineer thought it might be a little lead pearl that was stuck underneath like you suggested However that was before we retested and saw that it dropped
 
Pure conjecture

Pure conjecture on my part. Differential expansion rates, related geometries, etc. always come into play for such a design. The primary sealing force for valves are the related springs. If valve seat interface geometry and condition is good, I would not expect temperature delta to measurably affect it.

On the other hand, cylinders have choke and the "seating" force is supplied by the rings themselves. I had always assumed, correctly or otherwise, testing closer to operating conditions would (only?) show benefit via increased ring seating force. Hoping to hear more on this.

On another note, I'd fly some more and retest before any additional inspection efforts.
 
More information, we retested a couple of hours later with the engine cool and the compression has now dropped to 54! I’ve asked the engineers to take over to for further investigation.
 
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Definitely not a transient issue.

More information, we retested a couple of hours later with the engine cool and the compression has now dropped to 54! I’ve asked the engineers to take over to for further investigation.
Assuming you have definitely verified it is exhaust leaking- tube up exhaust and listening to tube. . . .

Looks like you are moving to the next stage(s). Visual inspection of open and closed valve. Removal of rocker arms, valve spring, and (possibly) exhaust pipe for this cylinder. More inspection of valve face as it is rotated. Wobble (even if by hand) test. Verify that lifter is not extended/locked. Take appropriate actions.
 
have agreed with engineer to borescope next as im concerned by the significant drop with the engine hot.

out of interest, when told to fly it hard for a few hours, what power settings are we looking at? i was flying around at 25/25 - is that hard enough?
 
KISS approach

Did you by chance rock the prop slightly when you were checking the leak down? I have seen the numbers go from low 60s to high 70s by carefully rocking the prop back and forth a few degrees to help the rings set after a good bore scope revealed nothing abnormal about the valves.
 
^^^
Yes, it is helpful to seat the rings by slightly rocking them but the leak was heard from the exhaust pipe versus the breather; so ,doubting it will be helpful this time

@OP , yes 25 squared probably would have done it if it were a more "typical" leaky jug. The info you've provided since the initial post makes it look like something more than the usual contaminated valve seat/ring gap alignment/etc. Hoping it's something simple for you and your wallet. Keep us informed.
 
Thanks for all the input.

Im Still awaiting the engineer to do the borescope and will report back.

In the interim because im curious (I suppose one has to have a degree of curiosity to want to build an RV) Icame across an interesting video by Mike Busch on Borescope Ascendancy.

https://www.youtube.com/watch?v=7R-dAmv9VZk

At 9.40 he talks about how continental took a new engine and run it for a few hundred hours in controlled settings and did the compression readings regularly. The reading were all over the place and showed how readings are non-reproducible.

He asks the question, are compressions pointless now we have cheap borescopes?

So makes sense for me to get a borescope done before i fly given its better accuracy than compression testing.
 
Thanks for all the input.

Im Still awaiting the engineer to do the borescope and will report back.

In the interim because im curious (I suppose one has to have a degree of curiosity to want to build an RV) Icame across an interesting video by Mike Busch on Borescope Ascendancy.

https://www.youtube.com/watch?v=7R-dAmv9VZk

At 9.40 he talks about how continental took a new engine and run it for a few hundred hours in controlled settings and did the compression readings regularly. The reading were all over the place and showed how readings are non-reproducible.

He asks the question, are compressions pointless now we have cheap borescopes?

So makes sense for me to get a borescope done before i fly given its better accuracy than compression testing.

Having been a mechanic and engine design&test engineer I would say the precision of the test repeatability may be in question but not the usefulness. It does take a measure of perspective and understanding. When combined with a good resolution borescope it is quite valuable IMO. People want absolutes - this is like economics or medicine, it is not an absolute number, but can be effectively used for decision making. Basic interpretation can be learned without too much difficulty.

The most important thing is learning practical safety measures when testing so the prop does not whack someone.
 
Borescope and fly

Are you uploading your engine monitor data to savvyaviation? If not, I would recommend that. Mike Busch says that compression tests that show low numbers are not in of themselves necessarily a bad thing. As others have wisely suggested, I'd get your borescope in there and have a look around. If you see nothing wrong, and the engine runs ok, I would keep flying. Many A&Ps are too quick to pull a cylinder due to a low compression reading.
 
Borscope video

In my impatience i managed to find a fellow RV flyer with a borescope - i thought it can't be that hard as i had conducted a few endoscopies using steerable and very expensive Olympus endoscopes about 30 years ago - I was very wrong. Its not as easy as it looks!

In our complete ignorance we spent most the time looking at now what i realize was the intake valve and not the exhaust valve. Anyhow, there are a few images of the exhaust valve that ive posted to youtube if any one wishes to comment.

I'm still waiting on the maintenance engineers to get the plane into the shop to do a proper inspection

here's the link to the video https://www.youtube.com/watch?v=DtoCuSBBLJk
 
Kind of hard to see, and I don't think I saw the whole valve. What I saw looked ok, but I could see some deposits or something on the valve seat.

Lapping the valve in place is one option. Another is to try to run unleaded for a while, assuming you are not already. I'd also lean aggressively.

There are several videos and podcasts that describe lapping in place. Might work for you.
 
In my impatience i managed to find a fellow RV flyer with a borescope - i thought it can't be that hard as i had conducted a few endoscopies using steerable and very expensive Olympus endoscopes about 30 years ago - I was very wrong. Its not as easy as it looks!

In our complete ignorance we spent most the time looking at now what i realize was the intake valve and not the exhaust valve. Anyhow, there are a few images of the exhaust valve that ive posted to youtube if any one wishes to comment.

I'm still waiting on the maintenance engineers to get the plane into the shop to do a proper inspection

here's the link to the video https://www.youtube.com/watch?v=DtoCuSBBLJk

Ah, been there. Note the black shiny patches on the exhaust valve seat area. You should have a nice constant ring, not a bunch of non-interconnected patches. That is lead build up on the exhaust valve seat. Assuming you don't have a burned valve (yet), then the next step is to lap the valve seat in place. Your engineer may never have done this. It isn't hard. I just did this on one of my cylinders that was getting a bit leaky.

https://www.youtube.com/watch?v=71cMqxZtgAs

Rotocoils are a Continental thing, but there is a similar part for the Lycoming.
 
better HD images

ive taken the liberty of posting the link to the HD images on the main forum as not sure how many see this.

Anyhow, got an ablescope 400 latest model with HD camera - much better images and also of the correct valve this time!

any help appreciated

https://www.youtube.com/watch?v=YkNg6kTYvRY
 
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