Lower compressions in low time engine

[goalstop]

Bought my M20E a year ago with 200hrs SMOH on the engine, and great compressions (need to check but I think between 72/80 and 78/80). This year they ranged from 66 to 74. Unfortunately, the plane only flew 40-50 hours between annuals, since it sat from July - December in a maintenance hangar (thankfully indoors). CHT’s are normally 350 or below in flight, and never get near 450, lean of peak. All regular oil changes, no metal in the filter. Hangared in the inland southeast (ATL)

1) how worried should I be about this big of a fluctuation?

2) can sitting for 5 months be this destructive to the engine?

3) is there anything I should be doing to preserve the engine while flying

4) any way to improve the compressions or any chance they come back up as I fly more? Could they be lower if the mechanic didn’t run the plane long to read them?

 

[goalstop]

Oil was changed in July/early Aug before it sat if that matters

[Marc_B]

https://resources.savvyaviation.com/wp-content/uploads/articles_eaa/EAA_2013-07_compression-in-context.pdf

https://resources.savvyaviation.com//wp-content/uploads/articles_aopa/AOPA_2017-05_borescope-ascendancy.pdf

Best suggestion is fly it till next oil change and recheck compressions.  Sometimes the rings rotate and compressions improve.  At that time would be a good time to scope the cylinders and take a look at the valve faces.  If signs of asymmetric burning on the valves, then lap the valve.  If all looks well and compressions fine, then fly and don't worry about it.

I've had  a cylinder that compression dropped more than others and at next oil change was right back to baseline.  It can be a somewhat inaccurate test of condition.

Happy New Year!

[Pinecone]

Mechanic should have determined where the air was going.  Which pressure is applied to leaking cylinder, listen at the intake, exhaust, and oil filler.

That will help determine the path forward.

[Marc_B]

23 minutes ago, goalstop said:

1) how worried should I be about this big of a fluctuation?

2) can sitting for 5 months be this destructive to the engine?

3) is there anything I should be doing to preserve the engine while flying

4) any way to improve the compressions or any chance they come back up as I fly more? Could they be lower if the mechanic didn’t run the plane long to read them?

1) not so much; but worthwhile to repeat at interval and scope the cylinders.

2) sure it may invite corrosion, but in a dry environment that may not be a huge issue if it's flown regularly outside of that down time

3) read up on LOP and avoiding the "red box" / "red fin".  Proper engine temps and routine flight are the best way to care for your engine.

4) make sure compressions are done with a warm engine (standard practice) and perhaps start with the questionable cylinder compression first next time.

[EricJ]

+1 on the above, i.e., not a big deal yet but recheck when appropriate (maybe at 100 hours or the next annual).

Even the Lycoming guidance says it's not really an issue until it gets down to 65/80, and then the guidance is to fly it another 100 hours and check it again.  See Lycoming Service Instruction 1119A. 

Compression readings can vary from test to test.   If you test it twice in the same day you likely won't get exactly the same results.   Sometimes if a cylinder reads low, just moving the prop a little will seat the rings and the reading will come up.   It's not an exact science, so you're really paying attention to trends more than the results of a single test unless the results are really bad, which yours aren't.

Also +1 that the next time you get low compression on a cylinder the obvious thing to do is to sort out where the air is going.   It can make a big difference in what to do next, if anything.

[A64Pilot]

41 minutes ago, Pinecone said:

Mechanic should have determined where the air was going.  Which pressure is applied to leaking cylinder, listen at the intake, exhaust, and oil filler.

That will help determine the path forward.

This is your answer, because if it’s a valve, you want to get on that.

‘Lycomings usually have great compressions if there isn’t an issue, I mean like 78/80 or so, Continentals, not so much, they can be really low and be healthy,I believe Conti’s are low tension rings is why, low tension rings rely on compression to blow them out for a good seal, and the 80 PSI that we do compression checks with just isn’t enough.

[Utah20Gflyer]

1 hour ago, goalstop said:

Bought my M20E a year ago with 200hrs SMOH on the engine, and great compressions (need to check but I think between 72/80 and 78/80). This year they ranged from 66 to 74. Unfortunately, the plane only flew 40-50 hours between annuals, since it sat from July - December in a maintenance hangar (thankfully indoors). CHT’s are normally 350 or below in flight, and never get near 450, lean of peak. All regular oil changes, no metal in the filter. Hangared in the inland southeast (ATL)

1) how worried should I be about this big of a fluctuation?

2) can sitting for 5 months be this destructive to the engine?

3) is there anything I should be doing to preserve the engine while flying

4) any way to improve the compressions or any chance they come back up as I fly more? Could they be lower if the mechanic didn’t run the plane long to read them?

 

66 is actually just fine as compression readings go.  Provided your oil consumption hasn’t increased dramatically and the borescope inspections look fine I wouldn’t give it a second thought.  By themselves compression readings don’t mean much.    Don’t get spooked from a metric that is more noise than anything else.  

[Guest]

Lycoming SI 1191A covers cylinder compression testing.  Note that they only suggest removing cylinders when the test value is below 60/80.  Interestingly Continental uses a different set of values, with lows down in the 40’s before doing work.

https://www.lycoming.com/sites/default/files/Cylinder Compression.pdf

[toto]

7 hours ago, Marc_B said:

4) make sure compressions are done with a warm engine (standard practice)

This.

I have used a shop that insists on doing compression checks on a cold engine, and it can make a huge difference. The first thing I would do if the compression numbers look unusually low is to insist on having the check done with the engine warm - ideally right after a flight. 

[goalstop]

Thank you all! This gave me a lot of peace of mind. I’ll ask the mechanic where he heard it coming from, and he’s going to compare apples to apples with last year’s compressions. T-minus one week until she’s back in the air!!! :knock on wood:

Gonna be a whole new plane after all the repairs, upgrades, and replacements 

[goalstop]

On 1/1/2023 at 3:14 PM, Pinecone said:

Mechanic should have determined where the air was going.  Which pressure is applied to leaking cylinder, listen at the intake, exhaust, and oil filler.

That will help determine the path forward.

Mechanic said it’s coming out of the exhaust. Cams and valves are fine. Which is worse?

[Greg Ellis]

13 minutes ago, goalstop said:

Mechanic said it’s coming out of the exhaust. Cams and valves are fine. Which is worse?

I'm no mechanic but if it is coming out of the exhaust then the only place that can come from is the exhaust valve, no?  So you may have an issue with an exhaust valve.  This just happened to me.  Compression test on #1 cylinder had air coming out the exhaust.  Other three cylinders were silent with no air leaks.  Borescope of #1 cylinder showed a problem with the exhaust valve.  Discussed lapping it in place but it failed the wobble test miserably.  Cylinder was removed and sent out for work.

[Will.iam]

33 minutes ago, goalstop said:

Mechanic said it’s coming out of the exhaust. Cams and valves are fine. Which is worse?

Cams are worse, you have to open up the  the engine to replace the cam and might as well do a full overhaul while you got it off and open up. The exhaust valve can sometimes be lapped in place but if that doesn’t work removal of 1 cylinder for overhaul is much cheaper than overhauling the cam. 

[dzeleski]

54 minutes ago, goalstop said:

Mechanic said it’s coming out of the exhaust. Cams and valves are fine. Which is worse?

Ask to have the valves lapped if it doesnt improve after running it again. Youll also want to scope the exhaust valves and look for burnt valves or starting to burn valves. I just went through this a year ago with my #2, unfortunately the valve seat ate itself and I needed a replacement.

You absolutely want to get ahead of this before something fails and costs big $$$$.

On a cold start how does the engine run? If it runs like this you need to ream the valve guide out ASAP and do not fly it until its resolved:

 

[goalstop]

8 minutes ago, dzeleski said:

Ask to have the valves lapped if it doesnt improve after running it again. Youll also want to scope the exhaust valves and look for burnt valves or starting to burn valves. I just went through this a year ago with my #2, unfortunately the valve seat ate itself and I needed a replacement.

You absolutely want to get ahead of this before something fails and costs big $$$$.

On a cold start how does the engine run? If it runs like this you need to ream the valve guide out ASAP and do not fly it until its resolved:

 

Wooooah no that would give me a heart attack.  All cylinders stay within 50* EGT of each other pretty consistently.  CHT's stay around 325-350*, no more, during LOP cruise and peak.  I also haven't started her up in 5 months.  But things were running perfectly with zero fluctuations when I dropped her off at the shop in July

[dzeleski]

20 minutes ago, goalstop said:

Wooooah no that would give me a heart attack.  All cylinders stay within 50* EGT of each other pretty consistently.  CHT's stay around 325-350*, no more, during LOP cruise and peak.  I also haven't started her up in 5 months.  But things were running perfectly with zero fluctuations when I dropped her off at the shop in July

Thats good news. Hows climb performance and takeoff roll? Normal? Making max RPM?

Personally I would scope it today and look at the valves, if they look ok I would run it for 10-20 hours and then re check compressions as well as scope the valves again. If the valve looks bad then lapping and inspection/replacement of the rotator cup might be required.

After having to replace a cylinder I now check my valves every oil change (50hrs).

[A64Pilot]

Pull another compression check if it’s still leaking at the valve, try staking the valve if that doesn’t fix it then you can try lapping, but in my opinion that’s another band aid, will work for awhile though, if you lap it be sure to do the wobble check.

This is no big deal, sure nobody likes to pull a cylinder, but it’s a pretty normal aircraft thing.

Bad cam, that’s a big deal.

[PT20J]

Severely worn cam will reduce valve lift and power but won’t affect compression. Spalling lifters will make metal but the engine runs fine.

I would run it for a couple of hours and recheck compression and borescope. That will tell you what needs to be done, if anything. 

[EricJ]

8 hours ago, goalstop said:

Mechanic said it’s coming out of the exhaust. Cams and valves are fine. Which is worse?

How does he know the valves are fine?   +1 that a borescope check is a good idea if they haven't done one and there is consistently low compression and it's coming out the exhaust.   If the valves look good and the compression stays low after flying for a while (at least an oil change, imho), then another borescope check and if it still looks good maybe lap the exhaust valve or just keep flying it and keep an eye on the trends.   If your engine monitor allows data download, watch the exhaust temps for that cyl closely to make sure it doesn't have the periodic rise-fall associated with a burning valve.    A burning valve is one you want to get ahead of, but other issues like spalling lifters and eroding cams progress very slowly and just slowly degrade performance.

[Andy95W]

66 to 74 compressions on a COLD Lycoming are fairly normal. They are also 100% legal which is why a lot of mechanics take the shortcut of checking them cold.

If you’re really concerned, have your mechanic run the engine for 10 minutes and recheck them right away. 99% of the time they will all be nicely in the mid 70s or higher.

Or just go fly and have them checked at your next oil change while the engine is still hot. 

Edited January 4, 2023 by Andy95W

[Pinecone]

If a valve is leaking, it is NOT OK.

[N201MKTurbo]

I had an exhaust valve on my new Continental cylinders start leaking. I was going to try to lap it in place, but couldn't get anything on the valve stem to spin it, so I just spun it with my fingers for about 1/2 hour (when my fingers got sore). It stopped the leak and it has been fine ever since.

[Andy95W]

7 hours ago, Pinecone said:

If a valve is leaking, it is NOT OK.

While this is true, having a little air go past an exhaust valve on a cold Lycoming is not uncommon.  Warmed up, the valves will nearly always seat just fine because there is nothing wrong with them, they just needed to be at operating temperature.  Or there was a bit of carbon on the valve seat that needs running in order to clean off.  Again, this engine needs to be run up and re-checked hot and if a cylinder is low, borescoped.

The original poster never answered the basic question that was asked of him: did his mechanic perform the compression check hot or cold.  Until that question gets answered, none of us can even tell if there’s an actual issue or not.

I’ve been an A&P since 1998 and I’ve done more Lycoming compression checks than I can remember.  And of course, if an exhaust valve is truly leaking due to damage, it’s not okay.  

[Pinecone]

That is fine.  BUT you do need to figure out why the valve is leaking.

It may not be a big thing, but it could be.

Of, if you did the compression test and found a valve leaking, would you ignore it?  Or determine why?  Which may be simple or not.