Cessna 150 with good compression but oil in #4
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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!
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!
Last edited by chesterspal; 11th May 2020 at 18:29. Reason: clarification
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.
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.
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Originally Posted by wrench1;10779736...
Since you're over the TBO, which for an O-200 is pretty good, you'll probably start having other issues as well....
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.
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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.
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.
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.
Last edited by chesterspal; 12th May 2020 at 14:12.
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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.
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?
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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/
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.
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.
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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.
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.