High EGT one cylinder

Is the sensor location the same as the others, could be a difference.

You need to troubleshoot a bit.

Sam is correct that the distance from the head can be a factor,  but I'd try this. You can either swap another EGT wire or swap the EGT sensor with another one and see if that EGT reads the same or follows the swap. Many times EGT sensors can go bad and they just need replacement. I've replaced many and just keep them in stock because of that. Dynon instrument folks told me once that they are good for 200 hrs., but after that there is no guarantee. That said many last for 1K - 2K hrs. You just never know. Some parts for some people seem to last forever and some give up the ghost earlier than others.

The positions are as near the same as I could manage. However every pipe curves differently so interpreting Rotax's 75mm (or whatever the distance they specify from the flange is) is not straightforward.

I have considered sensor failure but as the reading is pretty much the same as the other 3 at both idle and full throttle, it would require gross non-linearity which i would consider an unlikely failure mode. I swapped the wiring to eliminate non-linearity in the electronics but as it's an MGL which scans round the sensors, I presume there is only one correction circuit anyway so it would apply to all 4 sensors.

I can only prove it by swapping two sensors (or replacing one) so I will have to try it.

Hi Mike,

After re-reading your first post and second post it doesn't sound like a sensor location issue or a failure, but #3 is around 200F hotter.

It won't hurt to try and add to your diagnostics with a swap on a wire or sensor to just rule out some things.. 

One other bit of info. Checking comps last week, 3 cyls were 80/80, no 3 was 80/79. It's not the exhaust valve though, you could hear it bubbling through the oil.