Lean or not to lean - the old question; again

[NicoN]

I joined a Ownership for a 1980 M20K in 2018; right after all 6 cylinders had been replaced.

Analyzing the old logs from our EDM830 clearly showed, that the aircraft was flown often hot if not too hot with CHT over 400°F for a longer period of time.

We came along to fly LOP to save the engine's life forever.

Since then we are donig this; flying LOP with 58-65% power and CHTs never over 390; in cruis we have TITs of ~1535 and FF 8.8-9.2 gals. GAMI  spread is also fine

Now, again 2 cyl are with low compression. Even though the exxhaust valves are looking fine. The cylsa are loosing pressure via the valves and also into the crankcase.

In-place-lapping improved one cyl and had no impact on the second.

Now, the shop says, our problmes are caused by flying LOP!! and he sees those problems more and more often as people flying LOP and especially in TC Mooneys and Cirrus.

Okay, what to do?

If Mike Busch is right propagating LOP with lower CHTs then we made everything accordingly. But if not?

 

[ArtVandelay]

Makes no sense unless you ran the engine at low power and cylinder temperatures were less than 300°. If you do this the lead scavenging chemical reactions won’t remove the lead and you will valves coated with lead. I believe Busch explains this.
There seems to be a lot of mythology in piston aircraft maintenance that gets passed down from one generation to another.

Did you use a borescope to visually check the valves condition? Chances are your old school mechanic didn’t. Doing so might help figure out what is causing the leaking valves? Lead, burnt, broken…..

And did the mechanic do the test with a cold or warm engine?

[Schllc]

I completely agree with the architect, at those power settings there isn’t a danger zone, and I am skeptical of mechanics diagnosis of these things.  I have seen many cylinders changed that didn’t need to be.  If you are down around 20-30psi maybe it’s an issue but if it’s 50’s and making full power, I would wait and see before I grounded my plane  

im not familiar with that engine monitor, but how far back can you really go with the data?  
What are they considering low compression?  Is the engine still making full power?

 

[Pinecone]

First thing is to make sure they are going the compressions correctly for a Continental.  And Continental has very low compression limits.

Do a borescope to see if there is uneven heating of the valves.  

 

[Shadrach]

Have you personally verified leak into exhaust? It would require an unusual scenario for a healthy looking exhaust valve to leak after being lapped. No evidence from what you’ve said that mixture setting has anything to do with this. How long have the cylinders been in service?

[skykrawler]

I have friends with C182s and Barons. and others.....seems like Continentals and even Superior cylinders do this - rich or lean of peak, matters not.

The last one I saw was valve stem/guide wear.

 

[A64Pilot]

Lapping is a band aid, it can make a difference if there is carbon build up etc.that causes the leak as it will remove carbon, but that’s something that comes and goes from one day to the next. It’s like ring flushes, you’re clearing the fault maybe but haven’t corrected the condition that caused it. However for people that fly infrequently getting another 50 hours before the problem re-occurs may give you another year, or if your selling may get it past the pre-buy.

Often “staking” a valve can remove the carbon and raise compression, but be careful staking done incorrectly it can cause damage, but done incorrectly so can lapping.

If it’s carbon you aren’t really fixing anything just getting the numbers up to pass a test, the carbon may or may not reoccur, carbon formation is more likely ROP than LOP but often it’s from oil so mixture may not have always have a great effect

Lapping won’t remove any significant metal and recondition seats, if you lap aggressively attempting to recondition seats you will wear a ridge into the valve / seat.

Then as was already stated other things like clearance between the valve and guide can be causing the leak. 

Compression testers, actually leak down testers are so cheap and easy to use it might be worth purchasing one for yourself, a lot can be learned by listening for where the leak noise is heard, if heard in the intake, it’s the intake valve, same for exhausts and of course if heard / felt from crankcase breather it’s the rings.

LOP or ROP isn’t really the issue, avoiding detonation and keeping temps in check are. It is however easier to avoid detonation and keep temps down ROP than LOP, so for new pilots especially it’s likely better to start ROP and not worrying about it and once everything about operating the aircraft is down pat add LOP to your technique.

It may also not be a bad idea if things get complex requiring more attention than normal like bad Wx etc to just default back to ROP and remove that from your task loading.

[jlunseth]

Take Pinecone’s advice. Get TCM’s instructions on compression testing and make sure your mechanic, or preferably another mechanic, actually follows them. I am not an A&P, I am an owner of a K, so I track this whole issue just enough to understand what my A&P is telling me. As I understand it, TCM issued SB03-3 about 20 years ago. To my knowledge, it is still TCM’s current advice. Paul Kortopates or another A&P could tell you. TCM’s methodology is different from other engine manufacturers and places less reliance on straight-up compression testing to determine whether a cylinder needs to be replaced. Among other things, the SB says that a borescope should be performed. It also says that compression testing should not be done on a cold engine, the engine must be operated first, preferably flown at 65-75 percent power for about an hour before testing, so the compression test is done on an engine with full lubrication. If compressions are low, the SB says to recheck. There have been many instances where the first compression test is low, the engine is then flown and retested, and the readings are completely different. Last but not least, TCM’s minimum compressions for the K engines are much lower than other manufacturers’.  Compression in the low to mid 40’s are acceptable. Testing has born this out, engines with intentionally low compressions (rings filed) put out the same horsepower as those with higher compressions. 

TCM’s advice on compression testing in the early 2000’s was controversial, you will actually find many articles online about it, and Mike Busch requested and got a legal opinion from the FAA about it. Lot’s of mechanics still don’t follow it.

I have a new engine (TSIO360-LB). I replaced my old one when it approached 2300 hrs, which is nearly 500 over TBO. LOP operations were very good for that engine, and most of the time I was at 71% LOP, not 65 or in the 50’s. I replaced the engine because it was 22 years old and I thought getting a factory engine would result in the least down time for the aircraft. That did not turn out, but that is another story. In the last five years of the engine’s life (about 500 or 600 hrs) my number 2 was down in the 50s and eventually in the 40s. Borescoping made clear, however, that there was nothing fundamentally wrong with the cylinder. I never had any trouble with that cylinder. Bear in mind that at that point the engine was a few hundred hours past TBO.

At about 1200 hrs on the engine, I had the engine IRAN’d because it had obviously been run very hot by the prior owner, and because I had to make an emergency descent to landing that caused some ring slap in the cylinders. I never got a statement that said what was done, whether cylinders were replaced or just rebuilt. It was quite awhile ago. So I can’t tell you if cylinders were replaced now 14 years ago on that engine. I can tell you that the shop did a great job, the engine was far better than before the IRAN. Before the IRAN the engine would use as much as a quart every two hours. After the IRAN, and until the engine was replaced, I could generally go from one oil change to another at 25 hrs and not have to put in oil. If I went to 35 or more hours between changes then I would need to add a quart. I sometimes went as much as 40 or 50 because I was flying frequently and there was no point in changing oil every two weeks in my view. Oil use increased only very slightly toward the end, sometimes I would need to put in one quart between 25 hr. oil changes on an engine way past TBO. Also, I have to say that from my own experience I do not agree with mechanics that try to fault LOP operations for problems in an engine. Just was not my experience at all.

Honestly, I wouldn’t worry about the compressions unless borescoping says there is something wrong with the cylinder. Just my opinion.