Lycoming IO-54O Cyl #6 puzzle

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lucius

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Hello, my #6 cyl exhibits the following problem:
It has gone cold on two recent flights during descent. Engine is running rough when it is going cold. See two attached engine monitor plots.
What has been done and excluded by the mechanic are the following items:
[1] Check if valve is stuck. Negative.
[2] Clogged fuel injector. It's a GAMI. Was filthy but not clogged. Was cleaned.
[3] Check plug. Bottom was dirty, but could be due to rich run ups. Should not lead to cylinder going dead.
[4] Air blow spider and fuel lines.
[5] Swap injector. Negative.
[6] Swap temperature probe. Negative.
[7] Check valve guide and spring. Somewhat worn, but should not lead to cyl going cold.
[8] Compression 65/80. But may change after 45 min flight. Should not lead to cyl going cold.

Cyl #6 has 2100 hours on it, and has worked great every single time for the last 500 hours. Never any work performed. Plane regularly flown.

Can't be magneto or ignition harness, since only one single cylinder affected.

After all these [8] points, still during run up at 1500 RPM cylinder still goes cold, but after engine warms up it stabilizes and is good to go according to engine monitor.

What else to check?
Safe to ferry flight? (Plane is not at my home airport with limited repair facilities)

Thank you!
 

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Leaky intake runner? Any blue staining at the tube connections? Clamps tight?
 
lucius said:
Hello, my #6 cyl exhibits the following problem:
It has gone cold on two recent flights during descent. Engine is running rough when it is going cold. See two attached engine monitor plots.
[6] Swap temperature probe. Negative.
Click to expand...
Does it run rough? Vibrate? Lose power? If not, the most likely problem is a bad temp probe lead. The connectors in many systems are notorious for failing. After all, they're in a hot, vibrating, sometimes oily, rather nasty environment, and they're transmitting tiny voltages and currents that are easily defeated by miniscule resistances. Cessna's Garmin connectors are yellow plastic things that get baked, their contact tangs get oxidized and/or oily, and the wire clamp screws in them can get oily or loose. Other instrument makers use a variety of connectors that have had a history of troubles. It can make for some fun finding the bad connection, which might be in the engine compartment or under the panel. Look at the wire bundle where it leaves the engine and is clamped to the engine mount; if there isn't six or eight inches of slack wiring between the engine's clamp and the mount's clamp, the internal wire might have failed through fatigue. I have found several airplanes with cable bundles clamped way too securely and not allowing for the rather large engine movement you can get during start or stop.

The mechanic might try switching the probe connections between #6 and #4 and see if #4 now shows cold.

If a bad connection is found, fix it and apply Stabilant 22 to the connectors before assembly. It helps a lot. And it ain't cheap.

https://www.amazon.com/Stabilant-5ml-Kit-Makes-30ml/dp/B001E50GQS
 
Dan Thomas said:
Does it run rough? Vibrate? Lose power? If not, the most likely problem is a bad temp probe lead.
Click to expand...

Yes, it runs rough and vibrates. We did swap temp probes and injector to eliminate them as a reason. See point [5] and [6] above.
 
but, oh....it could be a magneto. The distributor gear spins and distributes voltage (spark) to each plug lead. Could be a bad terminal inside the cap. Seen that.

Does the plug look cold?
 
Checkout_my_Six said:
but, oh....it could be a magneto. The distributor gear spins and distributes voltage (spark) to each plug lead. Could be a bad terminal inside the cap. Seen that.

Does the plug look cold?
Click to expand...

If it was a mag, wouldn't that result in a temp increase because only one plug is firing? Unless both plugs are dead in that jug, I don't see how it goes cold with an ignition issue. Isn't that why we have two?
 
Chip Sylverne said:
If it was a mag, wouldn't that result in a temp increase because only one plug is firing? Unless both plugs are dead in that jug, I don't see how it goes cold with an ignition issue. Isn't that why we have two?
Click to expand...
Hmmm....yup. Good point there.
 
Since it happens on descent, (don't know from what altitude) when the engine is leaned out anyway, I'm thinking some sort of vacuum leak, fuel flow restriction or other reason that jug starves for fuel. Does it make redline power at WOT? Does it get overly warm before it goes cold?
 
I was able to re-position my plane to my home airport. Same issue again during descent from about 2000 feet #6 goes cold at minute 00:48. However, the engine monitor data now show something it didn't show before: A zick-zack pattern with regular 27 second periodicity. See blue line of EGT plot attached. One possibility I was told is hydraulic lifter collapse, due to sludge. But not sure how probable this is, and it would not cause the small EGT oscillations every 27 seconds.

The zick-zack may suggest a burned exhaust valve, but in this case, periodicity would be 60 seconds, instead of 27 seconds. Also, a burned valve would not lead to a cyl going cold and then coming back, unless there is an edge broken off and only high fuel flow is able to sustain ignition.

flight #3.jpg
 
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At least the lifter is a quick, easy look-see.

I noticed your #6 has the hottest CHT in cruise by a pretty good margin, and a coolish EGT before it goes cold. Has that always been the case? Usually it's number 5 that runs hot, because of the design of the valve bosses, cooling fins and baffles. Again suggests to me a vacuum leak.

Have you sent the trace to Mike Busch?
 
lucius said:
The zick-zack may suggest a burned exhaust valve, but in this case, periodicity would be 60 seconds, instead of 27 seconds.
Click to expand...

I'm pretty sure in the FEVA algorithm the periodicity is dependent on RPM.
Have you borescoped the cylinder?
As someone mentioned, send the logs to Savvy Analysis also.

Luis.
 
Is number 6 the rear cylinder on the left side? Does that one have a primer line on it? I've seen issues with those primer lines on the four cylinder engines...
 
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