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IO390EXP119 at 100h

glider_rider

Active Member
I'm somewhat surprised by what I saw inside the cylinders of my IO390 EXP 119 during the 100 h inspection. Some background: stock RV14 EXP configuration, one magneto, one Pmag. In cruise CHTs in the range 350-370 F. On longer flights I operate slightly LOP, short flights are ROP. Aeroshell 100, 100W, 15W50 and recently W80plus. All in all 4 oil changes during those 100 hours.
Coming back to horoscope picture:
1. all piston heads are covered with black residue
2. "something", maybe cooked oil builds up on intake valve guide on cyl 2 - the hottest one
Is this normal / expected ?
How do I get rid of the black stuff ?
How do I prevent it from re-appearing?
 

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Piston tops usually have a coating that gets thicker over time. It is usually more dark grey than black and is from a collection of carbon lead and additives from the gasoline and cannot be avoided. I see liquid oil on the piston crown, indicating that you have a ring / wall issue (likely glazing as I can see what looks like it in the pic, unless that is a reflection of the liquid oil) causing oil to be present where it shouldn't be and that black stuff is coked oil.

Not common to see coked oil on an intake valve. It is probably due to too loose of a clearance between guide and stem in addition to something causing that valve head to retain more heat than normal. What does the valve face look like? Any signs of hot spots or burning? Due to the constant stream of cool air flowing along with latent heat evaporation of the gas being squirted into the intake chamber, intake valves do not typically get hot enough to cause these types of problems and there shouldn't be enough oil flowing onto the tulip to coke, especially in a horizontally opposed engine. I suppose it is possible that valve was inadvertently coated with grease during assembly and then burned during the first hour.

I would check to be sure #2 rocker box is draining properly.

Larry
 
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here is the cylinder wall and the valve face, sorry for the low quality
The outside face of intake cylinder looked fine
 

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here is the cylinder wall and the valve face, sorry for the low quality
The outside face of intake cylinder looked fine

Very clean and NO signs of glazing in that pic. However, the first post pic clearly show pooling oil on the piston crown. When all is well, there should be NO liquid oil on the piston crown. Several possible reasons for that and more investigation is warranted.

Suggest a borescope that examines the entire cyl wall area on #2. Glazing is often not full coverage. Shinny metal color like this pic is good. A tan, opaque coating on the wall is glazing. Partial glazing is enough to burn oil.

Larry
 
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