Cessna 150 with good compression but oil in #4
 

Helping my AP do the annual on my 1976 Cessna 150M. Continental 0-200 engine about 100 hours beyond TBO. Purchased about a year ago.
Plane has always run strong and steady with no issues. Run-ups always with identical 50RPM drop. No hesitation, no sputtering. Oil pressure is always right on the money.

Compression was all in the high 70's....76 - 78 (out of 80) and above. Same as when the plugs were changed in October of 2019.

When we pulled the top plugs yesterday, I looked inside and saw some oil in #4 and that one plug was a bit blacker than the others, but not by much. Pulling the bottom plugs and #4 was wet with oil. The others were dry. No oil was detected back at the October 2019 plug change. Only around 25 hours on the engine since then. Also, the oil analysis from October found nothing unusual. Cutting the filter open back then and spreading it out found nothing.

AP thinks it's just the ring gaps lining up. Can I have good compression and still have oil leaking into the cylinder?

Your help and guidance is most appreciated.

Thanks!

I'm no expert on aircraft piston engines, but I've rebuilt plenty of car and bike engines over the years.
If you're getting good and near identical compression on all cylinders I doubt it's anything serious.
I would guess it's probably just oil filtering down from the head and entering the cylinder via an open valve.
If you wanted to be sure run the engine again and put the #4 in a position with both valves closed, leave it for a couple of hours and have another look.

Yes. One trick when troubleshooting a low compression cylinder is to spray oil in the cylinder and recheck the compression. This helps determine if the leak is at the valves or the rings as the oil will "seal" the rings. However, since you have physical evidence of oil bypass you're now at the beginning of the end on the wear side. Since you're over the TBO, which for an O-200 is pretty good, you'll probably start having other issues as well. And considering you've probably been operating the engine differently than its previous owner, these issues may start to accelerate. Perhaps start checking things more often to stay ahead of the curve, however, you also may want to start looking into overhauling/replacing the engine in case one day something further pops up. And as an FYI, you may also look to post these questions on a US based website as aviation rules are much different in other parts of the world which Pprune is more specialized in. Good luck!

TBO for GA aircraft has largely been discredited as an arbitrary manufacturer's figure. No longer used by the US military or the airline industry. A better determination is oil analysis and the general upkeep and health of the engine in question. Other than this recent discovery, no other issues have presented themselves. Continental O-200's are relatively low compression, low temperature running engines and can easily go for 3,000 hours or more if taken care of... as this one has... based on the engine logs from the prior owners.

If anyone has overhaul experience with the 0-200 and can provide some guidance as to where the oil is leaking from, that information will be helpful to me.

Thanks

We have a similar problem with an 0-200 with 400 hours since TBO and the engine it replaced did the same. It doesn't seem to affect starting or mag drop. I am not an engineer and there are a couple of issues over the behaviour of oil that puzzle me. The second one is that, when we check the oil level when the engine has been standing for a day or so, the level on the dip stick is always a good half inch above the limit. When you wipe the dipstick and recheck, the indicated level is normal. This suggests to me that, as the engine cools, the oil draining back into the sump gathers into odd places such as No 4 cylinder head and the dipstick tube. We have found that, if we stop the engine at the end of the day by slowly winding the mixture back, we get less problems with oil in No 4.

Thanks for this reply. Losing lots of sleep over this and good to know I'm not alone with this issue and even with a low time engine, to boot. Hopefully, we can both get some help on this forum to sort this all out.

I always wipe the stick first, then check the level. Oil can run up the stick just sitting there. Wicking, I believe it's called.

I also was wondering if how the engine came to rest at the end of the flight makes any difference. That is, does it tend to stop with the same piston valve open and can that lead to an oil leaking/weeping issue. The #4 is the furthest out, front-most piston in the 0-200. So, one can assume it gets the most air and might be the coolest of the four. I have a single-channel CHT-EGT temperature gauge and it's on #1* since, being the back-most cylinder, it's generally considered to be the hottest of the four.

Can this potential uneven heat issue also tell us something?

BTW: I get around 225 degrees F on cylinder #1 so this is not a very hot running engine by any standard.

The cylinders on an O-200 have always been a weak link. This has been known for years. One reason is they were/are designed for 80/87 avgas and not 100LL avgas or Mogas. Usually the cause for the oil is excessive wear on the cylinder wall at the combustion area. And depending on what cylinder P/N you have installed will normally determine if the that wear happens at 500 hrs or 1000-1200 hrs. It is not unusual to see 1 to 3 sets of cylinders replaced on an O-200 before the entire engine needs an overhaul. Now if you can get a set of original cylinders to last 3000 hrs on a O-200, more power to you.
Not quite. While I don't understand what your use of "US military or airline industry" has to do within the context of an O-200, certain aircraft can legally operate beyond engine OEM TBO limits. But only those under Part 91 ops. An O-200 or any engine for that matter operated under Part 135 or Part 121 is required to follow the OEM TBO limits per the FARs. The military as their own TBO schedule dependent on theater of operations. So hardly a "discredited" figure.

I never said these were OEM.

I meant that in the context of how setting manufacturers TBO to a particular time based on hours operated and say that covers all engines in all planes in all situations is not generally followed today by most GA AP/AI's and plane owners who have studied the matter, as I have. There are more scientific methods for determining when to overhaul an engine based on fact.

I'm fairly certain my issue and that of Pulse 1, above is not cylinder/ring related. That the oil is coming from elsewhere.

And I never asked either. So how much time on your current installed cylinders?
Scientific methods? Care to share a few that are so exacting? Have used a number of methods to determine if an engine needs overhauling, however, never considered any of them to be "scientific."
Curious. How many disassembled aircraft engines have you been around? Considering you're losing sleep over this perhaps have your mechanic pull that cylinder and take a look?

If you were getting some blowback past the rings I would expect the oil to be slightly darker than the norm, did you reinstall the plugs run it again and recheck?

Although a lycoming have a read

https://www.cessnaflyer.org/maintena...intenance.html

https://www.cessnaflyer.org/maintena...intenance.html

https://generalaviationnews.com/2012...ug-oil-fouled/

I'd suspect worn valve guides allowing oil into the cylinder.
Your compressions will be good but eventually the valve/s may not seat properly or begin sticking when the stem/guide clearances increase.

I was thinking of that as well, but I would expect to see some blue smoke on start up as the oil burns off.

Usually, oily spark plugs in these engines is coming from piston rings(s). You may have good compression/blow-by values but problem propably lies on oil ring(s), not in compression rings only. Valve guides may be worn, but especially on O-200 you will start having another problems very soon. (sticky valves) You may also have rotated rings, and gaps are parked on lower side of the piston circumference but this also greates bigger compression lost if all of them are in bad position.

Need to remember that Continental valve guides are located higher than oil supply, oil is coming via pushrods and directed to valve head and rocker axles. Guides are hiding behind valve spring and its retainer, so oil is not flowing over them directly. There is oil mist and oil everywhere under rocker covers but guides are located in fairly "dry"-ish area.

It's disappointing when a thread originator doesn't respond to advice and suggestions one way or the other. I've had this several times but this one will be the last. I've got plenty of other things to do, just like the others.