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temporary high EGT on one cylinder

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During my last long cross country in my RV9A with Lycoming O-320, I got an alert on the G3X that one EGT was above 1500 and steadily climbing.
There was no change in CHT for that cylinder and no change in engine performance. I was already running at a low power setting and peak EGT which was about 1325 so my initial thought was how is it possible for EGT to get even higher? I was about to head for a nearby airport to investigate when it suddenly went back to normal, and stayed that way the rest of the flight which was over an hour. The total time of the EGT meltdown was a few minutes.

My guess is this might be the first indication of a loose thermocouple wire connection or possibly the probe coming loose and getting closer to the exhaust stream? I am not the original builder so not sure what to look for. I will pop the cowl and follow that probe back and look for anything loose. I have a tip-up so might be able to get to the connections behind the panel as well. Any thoughts appreciated.
 
If there was no other indication, like CHT dropping, rough running, etc, my money is on the thermocouple failing. The G3X will put a big red X over the probe if it becomes disconnected.
 
I had one do the same. Climbing like a blowtorch then suddenly dropped to normal. I replaced it just in case, but I suspect a connection. The CHT on the same cylinder was also wacky. Reading 60 below the others even stone cold. It was reading -10F when I changed it.
 
Check your plugs in that cylinder. When one plug stops firing, the Egt will rise about 150-200 degrees from where it was when running mid power or higher. Impact is much less at low power. 1325 -> 1500 sounds like that to me. The key here is that you went well past peak. That can’t be from fuel and reduces the possible suspect’s. One plug not firing is at the top of that list of remaining candidates.
 
If you don’t notice any other symptoms—such as changes in engine performance, vibration, or changes in any other indication, like CHT—you’ve experienced an electrical problem (physical damage) on indication system only, usually caused by poor connections, corrosion, or damage to the electrical wiring near the connections, or in some cases internal cracks in the thermocouple structure.
 
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crisrochasales said:
If you don’t notice any other symptoms—such as changes in engine performance, vibration, or changes in any other indication—you’ve experienced an electrical problem (physical damage), usually caused by poor connections, corrosion, or damage to the electrical wiring near the connections.
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That is far from a universal conclusion. Lossing one plug on one cylinder would create those symptoms without any perciptible change in performance. Other things could do the same. Not saying it can't be an electrical, issue, but that cannot be determined as you suggest.
 
Agreed that an electrical issue would be the first and easiest theory to investigate. I had this happen during Phase 1, and traced it to a loose terminal screw.
 
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lr172 said:
That is far from a universal conclusion. Lossing one plug on one cylinder would create those symptoms without any perciptible change in performance. Other things could do the same. Not saying it can't be an electrical, issue, but that cannot be determined as you suggest.
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Very easy to test. Do a run up and mag check. If one of the plugs is not firing, one cylinders EGT will plummit when one of the mags is turned off. Roughness will be obvious. Can also do this while flying if the symptoms reappear.
 
lr172 said:
That is far from a universal conclusion. Lossing one plug on one cylinder would create those symptoms without any perciptible change in performance. Other things could do the same. Not saying it can't be an electrical, issue, but that cannot be determined as you suggest.
Click to expand...
Hi Everyone
Thank you for the observation and encouragement to be more specific.
When I mentioned an electrical problem, I meant only in the indication system, but that information was missing.
When you have a serious problem in a engine cylinder, such as a misfire or inefficient combustion, you can immediately observe a change in CHT of that specific cylinder
(even if it's small) that persists and perceive the change in engine vibration, especially at low RPMs, it's if you have a real sensitivity to the machine. Also, a real mechanical problem does not recover on its own after some time; this is a common characteristic of indication only.
In over 20 years of experience in aviation, with installations, retrofits, and especially troubleshooting, I have learned not to state exactly where the problem is without analyzing it first.
I am just trying to help.
And for information, I am not a Chat Bot; I am an Engineer with 30 years of experience in the field of electrical systems and over 20 years of experience in avionics systems. I have performed troubleshootings you wouldn't even imagine. If anyone is interested, they can contact me at [email protected].
I hope the troubleshooting is successful.
 
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