I got this yesterday on a 3hr flight, after about an hour.
Cylinder #3 went to about 1750F, which I would regard as impossible in terms of physics, short of pre-ignition. The engine is running OK, delivering right power, and the CHTs are perfect.
I’ve had a few dud probes but usually they are eaten by the hot gas and go open-circuit fairly quickly. The indicated temp increases and eventually at about 2000F the EDM flags it as a bad probe. This one sat at 1600F (I enriched the mixture a bit to bring down this value, just in case it is real) for 2 hours, with some variation within a ~40F band.
If the probe worked itself loose, that could also perhaps do it. I know the holes must be drilled very equidistant from the cylinder head outlet. The probe actually reads an average of the exhaust pulses which are much hotter than this.
When I get back I will change the probe, and if it doesn’t go away then I will swap two probes to check if it is the instrument. I will also borescope the cylinder, obviously. Unfortunately one cannot easily test the probe because the high reading is not there at lower EGTs e.g. during taxi. That itself is weird.
Besides probe, would simply one spark plug not firing not explain it ?
Fuel injector clogged? Making it too lean?
What happened if you tried switching to a single mag?
I am no expert, so asking this for my own education.
Note there is no CHT reflection.
Indeed could be one plug blocked. I will test on the way back. There is a procedure whereby you have to close the throttle before switching back, to avoid exhaust damage.
But it wasn’t blocked upon landing or on the ILS.
You have to cut the mixture before switching back to both and then enrich slowly again if you experience roughness on one magneto to get rid of the unburned fuel. However, I doubt that it is/was a fouled plug. We once lost ignition on one plug in cruise and only noticed during mag check the next day. Even when analyzing the engine data you have to look very closely to spot the temperature rise.
The mags were fine on the post landing mag check.
Update :
Flew 2.5 hrs from Glasgow back to Shoreham this morning.
On preflight, cylinder 3 no EGT when ignition switch is on L position. Who can work out which plug is it? Usually the bottom one 🤣
Then same for the 2.5hrs airborne. I flew a bit rich to keep the EGT reading below 1600.
After landing I could not reproduce it. Looks like my +10G landing cleared it.
EDIT: I taxied to a test area and there I could again reproduce it, by applying a fair but if power. It is intermittent, but was very stable during the flights.
How is that possible?
Spark plug.
Magneto (D3000).
Ignition Harness.
The L3 plug / lead might be partially working but ignition form the right plug prevents that cylinder from flooding. If it is barely able to work, a few cycles of no ignition when L mag only will flood it. Surprised you are not getting some good backfires. Also, you mention no CHT reflection – is it potentially actually slightly lower?
Had similar symptoms recently.
EGT 6 was higher than usual and others. I thought, “well, another prob to change”.
I flew like that a few hours.
Then on a ground test, I lost cylinder 6 on one magneto, EGT going to zero.
I thought a polluted plug, and it was that actually.
In flight, all was perfect, including CHT reading; except the higher EGT reading.
Lower plug, cylinder. 6
Engine ground test when it failed:
