Cylinder woes?

[bdash]

During my last flight in my M20J, I noticed a slight roughness that corresponded to a drop in the EGT of cylinder #4 by around 60º. This lasted for 10-15 minutes, went away for a few minutes, then came back for the remaining 5 minutes until I landed. I downloaded the engine monitor data after flight, and it showed the change in EGT very clearly. Looking back over data for previous flights I haven't seen any similar change, or anything else out of the ordinary.

So I took the airplane over to my local shop. They did some investigation, and reported back that the underlying problem is an intermittently sticking exhaust valve. That seems to line up with engine monitor data. From some research, addressing this typically involve cleaning the exhaust valve guides.

However, the shop reported that as part of their diagnostic process they performed a compression test and cylinder #4 came back with a very low compression reading (< 20/80) due to leakage past the piston rings, and because of this they're saying the cylinder needs to be removed and either overhauled or replaced.

The engine was developing full power during the last flight and, with the exception of the EGT drop in the most recent flight, all other data related to the cylinder looks normal and hasn't changed since the annual inspection (at a different shop) around 3 months ago when the compression was in the 70s.

One confounding factor here is that the local airport currently has both runway and taxiway closed due to resurfacing, so any engine warm-up prior to the compression test they performed would have been done via a static run-up. Advice I've seen about low compression numbers has been to fly the airplane until everything is up to operating temperatures and re-test, but that won't be possible until the airport reopens in mid-November.

Does this warrant pulling the cylinder and overhauling / replacing it, or is there further investigation that should be performed before going down that path?

My inclination would be to clean the exhaust valve guides, fly the airplane for an hour or so, and perform a compression test immediately afterwards, on the assumption that the low reading is an artifact of testing rather than an actual problem. But if the likely outcome of this is that the cylinder comes off anyway, that's just extra cost and time wasted.

Edited November 1, 2019 by bdash

[irishpilot]

My vote is retest compressions and borescope if it comes back low. Just went through this a few months ago.

Fly Safe,
Safety Forum Mod

[jaylw314]

IIRC, there's some guidance in either the Lycoming or Continental SB's to NOT pull the cylinder until it has been flown again and rechecked.   I can't recall the specifics though.

Since your plane is grounded by the airport for a couple weeks anyway, I'd say it makes little sense to proceed with any invasive repairs until you've had a chance to do that.

[bdash]

Contintental's SB03-3 has very detailed guidance about compression tests on their engines. If the compression test reads below their leakage threshold, boroscope the cylinder, and if all looks normal then fly at 75% power for at least 45 minutes before testing compression again. Lycoming's guidance is much less detailed.

[Marauder]

I had a cylinder replaced in September for low compression due to leaks past the exhaust valve and the rings. Since the shop doing the annual was an MSC, I trusted they tried to check it several times. When I got the plane back, it was clear they ran it up several times to get it to pass, it didn't.

In my case the compression was 52/80 consistently. I have the failing cylinder and will have it rebuilt and kept as a spare in case any of the other 3 decide to follow suite. 

As for the failure mode. The last flight before the annual, I flew back from upstate New York and noticed a sluggish climb. Cylinder head temps looked fine as did the EGT temps. It was just anemic in the climb.

[ArtVandelay]

How many hours do you have on the engine?


Tom

[bdash]

Major overhaul was about 750 hours ago. All four cylinders were replaced by the previous owner around 200 hours ago (3.5 years).

[takair]

20/80 is very low and should have a fairly obvious cause.  I suggest going to the shop and having them talk through what they did and what they see and hear.  Don’t think I’d fly it with compression that low, but a ground run is easy enough.  Keep in mine that if they do the valve and compression does not come up, you end up paying extra labor, so it is good to factor that in.

As someone with 4 jugs off the plane right now, I feel your pain.  In my case, compressions, bore scope, and oil analysis were all good, but all the rings were shot and I had major blow by and high oil burn.  You may have a broken ring...

[bdash]

10 minutes ago, takair said:

Keep in mine that if they do the valve and compression does not come up, you end up paying extra labor, so it is good to factor that in.

The potential extra cost, along with the extra time waiting for the airport to re-open, is part of my dilemma with taking that approach.

I'd expect something like a broken piston ring, or any high amount of leakage past the rings, to have resulted in increased oil consumption and blow-by. I've not observed any of those things. That's one reason the reported low compression is so puzzling to me.

[cferr59]

The engine doesn't need much compression to make full power, but that is pretty low.  Can you try running it up on the ground and retesting? I have heard that the rings rotate during operation of the engine and if the gaps in the compression rings become aligned, it will significantly reduce the compression of the cylinder.  That is why the cylinder can be 60/80 and then magically be 74/80 on the next check.  Also, do you hear the air coming out of the breather?

Edited November 2, 2019 by cferr59

[takair]

35 minutes ago, bdash said:

The potential extra cost, along with the extra time waiting for the airport to re-open, is part of my dilemma with taking that approach.

I'd expect something like a broken piston ring, or any high amount of leakage past the rings, to have resulted in increased oil consumption and blow-by. I've not observed any of those things. That's one reason the reported low compression is so puzzling to me.

Are they sure your issue was a sticking valve?  Often hard to diagnose unless they did the wobble test.  I really would visit them and talk face to face. Have them show you the leak check and where the air is going.  In my experience 20/80 is hard to correct if it’s not a stuck valve.  Even broken ring usually won’t account for that low a reading.

[Yetti]

You should be able to hear the air leaking out the muffler with that low.   Like stated above.  Wobble test.   ream the valve guide.   sticking valve after warm up does not make much sense since things get bigger as the warm up.  Plugs tell a story.   camera the valves.

Edited November 2, 2019 by Yetti

[carusoam]

Want to share the engine data?

Download it, post it to savvy drop the link back here... we can all take a look at it in any depth we are capable of...

If the valve is sticking... there are signature signs when that happens...

Its really cool when the signature signs are clear...

Sticking valves often show a sawtooth EGT pattern...

Then there are tests for valve health...

One visual test can be done with a dental camera through a spark plug hole...

sticking valves don’t have the nice pizza image... 

how does the oil look? If you get blow by from warn rings...the oil turns black and smells like exhaust in a few hours of flight, after a flight, do you see foamy oil when you check the oil level..?

While scoping the cylinder... look for deep scratches, smooth surfaces or nicely defined cross-hatch patterns...

Rusty, pitted, scratched, and shiny smooth are typical signs of wear and age...

Lots can be done to identify the challenge before spending the big bucks...

A step change in EGT is typical of something breaking, sticking, failing.... it would be great to see the data... extra cool to have some really good run-up data... lots of time on each mag to collect enough data to show... not the quicky to demonstrate you own two mags type...

PP thoughts only, not a mechanic...

Best regards,

-a-

[larryb]

This is my #2 exhaust valve. New cylinder goes on next week.

[carusoam]

Ouch... great pic Larry.

Great example of what @bdash may be looking for...

Best regards,

-a-

[larryb]

My plane was running great. Good performance. No issues on the JPI. But I got out the scope during an oil change and found this. 1207 TTSN for that jug. The other 5 are fine. We are replacing it based on this pic alone.

[bdash]

My understanding is that the sawtooth EGT pattern, as well as the asymmetric pattern on the exhaust valve when looking via a borescope, is due to an exhaust valve that's leaking due to erosion after uneven heating, rather than one that's sticking. I haven't seen any pattern in the EGT data to suggest that uneven heating is the issue.

That said, borescoping is what I've requested as the next step to see if there's anything wonky with the valve or if it's just a matter of reaming the valve guide, and to see if the cylinder walls show any signs of ring problems.

[carusoam]

The oddity of a drop in EGT.... is... well, odd...

half burned fuel usually shows up still burning in the exhaust... with an increase in EGT...

To get a decrease, something hast to, at least, intermittently stop the ignition process...

Since the mag test indicates the mags are working... fuel and air are both getting to the cylinder...

It pretty much sounds like compression is not occurring... and a lot of fuel is getting dumped to the exhaust... This should feel like a significant problem is occurring as one cylinder isn’t producing full power with the low EGT... but, without sharing the graph... this could be completely mis-leading...

There are two types of stuck valves...

• the one that causes the sawtooth EGT pattern is a failure to rotate... this is kind of minor as valve failures go...
• The nasty one is a failure to go up and down following the cam... sticking open the fuel charge escapes probably burning slowly past the EGT sensor....

Wear of the cylinder seem pretty unlikely. It’s relatively new. It was working well until it stopped working...

intake valves don’t usually misbehave. When they do, they may light the intake on fire...

The picture supplied by Larry above indicates an exhaust valve that has stopped rotating... cooling of the valve becomes uneven over time... both the valve and the valve seat are getting damaged by this...

So... doing the compression test shows a low compression... something isn’t sealing... the sound of air escaping gives a hint of where it is going...

• Case vent... probably a ring issue... dirty, foamy, smelly oil would confirm the observation...
• intake... probably a stuck or broken valve... highly unlikely as intake fires can be pretty noticeable...
• exhaust... probably a stuck valve... the kind where the valve fails to close, not the mild rotation kind...

If an exhaust valve fails to close... it can strike the piston top resulting in a bent valve rod... and a matching dent in the piston...

It would be better to pre-emptively clean the valve guides to make sure the guide is not inhibited in motion...

Some engines have a tendency to fill the valve guide with carbon that inhibits valve cooling and causes serious sticking...

This type of carbon deposit is typical of engines that don’t get monitors... they are run very hot in the climb... while the pilot is mostly unaware of what is happening with a single CHT instrument and a POH that is more of a POS....

Use caution, because it sounds like the exhaust valve may not be closing properly...

PP thoughts only... not a mechanic... and I didn’t see the graphs...

Best regards,

-a-

 

[N201MKTurbo]

It surprises me that nobody repairs cylinders anymore. An exhaust valve and guide is less than 1/2 AMU. If you talk to them real nice, the local cylinder shop can change it in less than a day. I can do it myself for that matter. 

If you have low compression and the cylinder is not scored, just replace the piston and rings. 

[pwhicks]

Where are you located? I just had Sal in DFW rebuild my four cylinders. One was the first to show major signs and 2 others also followed soon afterwards.

Oil consumption was up. Mag checks were different. Oil traces on bottom plugs and oil on the exhaust side of plane. It still flew Mooney fast out of climb.

Have you looked at the plugs on that cylinder?

Stuck and shot ring for me. Also when they were previously rebuilt 500 hours prior by a previous owner, Sal was not impressed with several things as he inspected on tear down and the bore was worn to tolerance on the 3 that showed blow by. I saved the piston that the ring was digging into that needed replacement. Glad we found it before it let go in IFR with my family, which would be normal Murphy law.


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[larryb]

1 hour ago, N201MKTurbo said:

It surprises me that nobody repairs cylinders anymore. An exhaust valve and guide is less than 1/2 AMU. If you talk to them real nice, the local cylinder shop can change it in less than a day. I can do it myself for that matter. 

If you have low compression and the cylinder is not scored, just replace the piston and rings. 

I am replacing my cylinder with new to minimize downtime. Two-way shipping + time in the cylinder shop would probably add near two weeks to the job. Once the job is complete I will look into having the removed cylinder overhauled and put on the shelf as a ready spare. I only consider the repair since I know the cylinder has 1207 TTSN and was not a previous overhaul-exchange with no known history. 

[kortopates]

12 hours ago, larryb said:

My plane was running great. Good performance. No issues on the JPI. But I got out the scope during an oil change and found this. 1207 TTSN for that jug. The other 5 are fine. We are replacing it based on this pic alone.

You won't be able to see anything on your engine monitor in flight, but if you look at the downloaded data you can often see symmetric EGT pattern. Have you looked at it? doesn't change anything here but perhaps it may have been detectable before your oil change.

[larryb]

3 minutes ago, kortopates said:

You won't be able to see anything on your engine monitor in flight, but if you look at the downloaded data you can often see symmetric EGT pattern. Have you looked at it? doesn't change anything here but perhaps it may have been detectable before your oil change.

I do check the engine monitor graphs regularly, did not see anything unusual in the #2 graph. 

[kortopates]

21 hours ago, bdash said:

They did some investigation, and reported back that the underlying problem is an intermittently sticking exhaust valve.

A sticky exhaust valve is usually very clear in the engine data, especially if you have a TSO'd primary that on at startup - i should be visible the. But a poor compression test, especially <20/80 with leakage past the rings has nothing to do with a sticky valve. These are very different things.

Regardless though, the weak compression is real. Since its so low, I would just run it up slowly to normal CHT temperatures on the ramp and re-do the compression test. Being that low, its very unlikely it will come back up. Secondly, you want to see corroborating evidence from the borescope if possible, but if a ring just broke, you will not necessarily see evidence of greater oil consumption or cylinder wall scoring. 

[bdash]

Just now, kortopates said:

But a poor compression test, especially <20/80 with leakage past the rings has nothing to do with a sticky valve. These are very different things.

The sticky exhaust valve was said to be the cause of the EGT drop that led to me taking the aircraft into the shop. The low compression was something additional that the shop reported seeing while investigating the issue.

3 minutes ago, kortopates said:

Secondly, you want to see corroborating evidence from the borescope if possible, but if a ring just broke, you will not necessarily see evidence of greater oil consumption or cylinder wall scoring. 

Right, if something went awry very recently I'd have not had time to notice a change in oil consumption.

Would something like a broken ring be visible in any form in the engine monitor data?