Low compression on number 4 cylinder

[GMBrown]

Hey all.  Just wondering if anyone has suggestion on how to proceed with this situation.  The TSIO-360LB engine in the M20K (new to me) has been flawless (or so I thought) for the 40 hours I've put on it, but it is now in annual and my mechanic said that during the compression check, the number 4 cylinder indicated about about 63 when the other 5 cylinders were all upper 70s.  The engine was rebuilt (factory reman I believe-mechanic has the books) about 300 hours ago, so there shouldn't be a problem at this point.  I normally run rich of peak and I haven't seen any issues with #4 on the JPI, but I'll be eyeing it more closely now.  Mechanic wants me to keep running and we'll recheck in 10-15 hours.

Any suggestions about why this may be happening so soon after rebuild or how to address going forward?  Thanks. 

Garry

[smwash02]

Compressions will fluctuate year to year even on ones that are good. A bit of carbon can cause a lower compression. Staking it or running it hard can bring it up sometimes as it allows the valve to potentially seat better.

What does it LOOK like from the scope pictures? Do the valves have good patterning?

I had one that had fine compressions but the pattern wasn't good. I pulled the rotocoil and it was bad despite only having 400 hours on it. With the new the pattern improved greatly.

Edited January 20, 2022 by smwash02

[DonMuncy]

Continental has a SB on this. Paraphrasing, it says to go fly the plane for 45 minutes to an hour and check the compression again. The odds are very high that there is no problem at all. There have been many cylinders replaced on 231s for this "reason", that is really no reason at all. I would have zero concern at this point. 

 

[KB4]

Done Cold? Warm? What was differential?

[201Steve]

I'm betting that he did not scope the cylinder after discovering this news, and if there is anything to do next, that's the only thing to do- scope it. It's probably completely fine, but you'll know for sure if you scope it. 

Not a mechanic, just an enthusiast who's studied the scenario and seen this question 400 times.

[GMBrown]

I don't he scoped it (yet).   I recall the SB and I think you're right.  We need to run it a while then retest. 

Not sure if the engine was cold or not when he checked it but I think not, and that would make a difference too.

 

[carusoam]

Welcome to Continental ownership 101…

Compression readings carry too much weight….

They are too easily skewed by random ring placement/rotation…

See the SB and proper tools and procedures for that…

Sometimes a mechanic only has the Lycoming compression tools…. Or forgot there was a difference…

Good luck with the next reading…

Best regards,

-a-

[A64Pilot]

15 hours ago, DonMuncy said:

Continental has a SB on this. Paraphrasing, it says to go fly the plane for 45 minutes to an hour and check the compression again. The odds are very high that there is no problem at all. There have been many cylinders replaced on 231s for this "reason", that is really no reason at all. I would have zero concern at this point. 

 

This, Conti’s have low tension rings, this means they rely on cylinder pressure to push them against the cylinder and create a seal, newer cars are also the same way.

So low pressure compression tests like we do on aircraft often just isn’t enough pressure to create a good seal. I’ve seen perfectly good running Conti’s have compressions  in the 40’s occasionally, then later be in the 60’s.

The SB does a better job explaining than I can and of course is the official word

https://www.aircraftspruce.com/catalog/pdf/servicebulletin.pdf

[anthonydesmet]

Calling @kortopatesto help point you towards the SB and the library of SAVVY volumes on this subject.  Bottom line:  don’t lose any sleep over a compression in the 60’s for a TCM.  Make sure your mech did the compression properly, do the borescope to get a baseline of how things look and keep doing your engine oil analysis.  With a turbo I am a 25 hour oil change kind of a guy…..also plenty of posts on the oil interval subject.

 

ohhhh, also welcome to ownership and things that keep you up at night.   

[kortopates]

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

TCM SB03-3 is readily available on the internet, just google it, but its no longer the official guidance since most all TCM SB/SL/SI's have been incorporated into one large maintenance manual, TCM's M-0. (and its not free). That said the guidance has not appreciably changed from what in SB03-03. But Mike article, and countles others, explain it better.

Edited January 23, 2022 by kortopates