Weird 0-470 problem...

[SoCalPlaneDoc]

Having some difficulty getting my brain around a problem I'm having with an O-470.. maybe I'm just tired.. but I thought I throw it out to the braintrust here and see what happens..

in-flight; engine ocassionally has a tiny, almost un-noticable burble (best word I have to describe it)
engine monitor shows it's the #5 cylinder acting up... here's the part I cant wrestle into understanding.

CHT goes up.. kind of fast
EGT goes down.. just as fast.

the whole transient lasts only 5-10 seconds, then everything returns to normal.

I'm not thinking ignition since the CHT goes up, although the #5 plugs usually have the greatest deposits when I check them. Compression is usually between 68-72...

OK engine forensics experts.. what do you think?

 

[lwford]

Hum, sounds like preignition to me. Simple test is two new plugs in #5 then see if it comes back.
Lynn the crate :shock:

 

[danschultz]

could be valve sticking, or valve float caused by weak springs, just a couple more ideas.

Dan

 

[SoCalPlaneDoc]

could be valve sticking, or valve float caused by weak springs, just a couple more ideas.

Dan

I dont think that would cause the CHT to climb.

 

[SoCalPlaneDoc]

Pulled and opened the oil filter - full of non-magnetic flakes.
Pulled the #5 cylinder and found the #3 ring shattered. The broken pieces shredded and eventually took out the piston area between the #3 and the #2 ring grooves.
The cylinder looked beautiful and measured out within service limits, both valves looked perfect.
Anyway, it's got a fresh overhauled chrome cylinder and piston now
It went 1750 hours and was the last original jug on the engine.