Hi,
During annual, I noticed that the spark plugs on cylinder 4 were noticably darker than the other 3. I decided to use a borescope and I saw oil on the cylinder bore as the piston goes down.
Engine is a Lycoming IO360 with 500h SOH with new cylinders. The other 3 cylinders look clean.
It’s not clear to me if it’s a stuck or broken ring.
Would you do/try anything else before pulling that cylinder ?
A broken ring will usually show a deep scratch on the cylinder wall .
That said, you will probably be pulling the cylinder for a ring job in both cases.
I have always seen some oil on the cylinder wall, immediately after the piston comes down, looking through the spark plug hole. I thought that was normal 
This may be worth a read. My engine went to 1 quart every hour, and after the “remedial action” is now back to 1 quart every 5 hours, and no cylinders were pulled.
How much oil are you losing?
Peter wrote:
have always seen some oil on the cylinder wall, immediately after the piston comes down, looking through the spark plug hole. I thought that was normal
Yep, some residue oil is 100% normal.
Michael wrote:
A broken ring will usually show a deep scratch on the cylinder wall .
Michael wrote:
Yep, some residue oil is 100% normal.
Peter wrote:
How much oil are you losing?
I may try your “remedial action” before the next 50h check if that doesn’t self resolve.
What are your oil change intervals, what oil are you using ? How hot are you flying ?
EuroFlyer wrote:
What are your oil change intervals, what oil are you using ? How hot are you flying ?
Until 2012 (before the EDM700 was installed) we used to run the engine ~25° ROP (Now, I know it’s not the best place to run an engine…).
CHT were usually 360-380°F
Now we run the engine 25° LOP (GAMI spread is 0.0 if we slightly retard the throttle). CHT are 300-320°F.
In some cold winter days we can’t even set the engine LOP otherwise it would run too cool !
This CHT is very low and AIUI it can cause the oil to make residues.
My version of this problem was caused by several consecutive flights at FL200-210 i.e. very low power and low temps like yours.
There is nothing wrong with 25F ROP. The CHT curve is very flat around there. You are just burning a bit more fuel than necessary, and the CHTs are higher in the ROP region, which is normally a problem already, but not in your case.
However I am surprised you went from 360-380 down to 300-320 just by moving from 25 ROP to 25 LOP. That is a huge drop. Have you checked the EDM700 calibration? For example IIRC there is a config for the thermocouple type (J or K). At low temps, the two are similar (1.019mV versus 0.798mV at +20C) so this may not be obvious on the ground.
TOTAL oil is reportedly Aeroshell with different printing on the bottle.
You don’t say whether you’ve done a differential pressure test. Although not definitive it may give you a clue if there is a ring issue; listen for leakage at the oil filler. Removing and replacing a cylinder should take less than a day by any competent maintenance organisation and maybe honing and fitting new piston rings will resolve your problem.
Peter wrote:
However I am surprised you went from 360-380 down to 300-320 just by moving from 25 ROP to 25 LOP.
What I observe LOP is that it takes very little change in mixture to make a huge difference in CHT. Enriching by less than 0.5 gph can make the CHT rise by 30-40°F
Guillaume, how’s the temperature of your oily cylinder compared to the others?
