High EGT

Swapped probes. High EGT followed probe. Ergo faulty probe. Replace probe.

Think the probe may have a very small hole/crack (could not find it) as aluminium heat insulator had a melted section close to probe.

Replaced EGT probe with new Dynon supplied part - still have problem.

Tried to note exactly what is happening when EGT goes to over temperature:

Seems to happen when I switch from Cruise pitch - to Climb and reduce rpm. Increase rpm and EGT temp reduces. Reduce rpm to say 3000 and EGT temp reduces.

Doesn't make sense to me but could the engine be being driven by the prop causing a lean condition?

In all other respects engine running well. 

Sean,

If we assume your probes are OK, and the high EGT is actually occurring, then the conditions you describe make perfect sense.  EGT represents a measure of energy wasted through the exhaust, and CHT represents energy used within the engine to produce thrust.  When you quickly reduce the prop pitch, this unloads the engine and you would expect to see an increase in EGT and decrease in CHT.  Once you increase RPM to fully load the engine at the reduced prop setting, the energy is once again directed to thrust, and EGT would decrease while CHT increases.  Think of it this way, the combustion heat has to go one place or another.  Unload the engine and the heat goes out the exhaust, load the engine and it goes into the heads and out to the radiator.  

Because we don’t have control of mixture, we tend not to think about these principles as much.  With conventional aircraft that have mixture control you see these effects more clearly.  It’s a common mistake for a pilot to unload the engine to attempt to reduce EGT, only to experience the opposite effect.

Thanks for that excellent reasoning Jeff.

There is one problem - the high readings followed the swapping of the probe from left to right.

Following your logic the low reading (assumed to be correct) probe must be the faulty one ??????

Interesting thought about the low probe being faulty.  It would be interesting to have a second new probe to try out. 

With my 912iS Sport engine, the effect of unloading the engine on the EGT is quite obvious.  If I set my ground adjustable prop for 5200 on takeoff roll instead of my usual 5050, then my cruise EGT will be higher by nearly 100 degrees for the same cruise RPM (engine less loaded).  I think this may be more profound on the 912iS engine because of the ECO cruise mode it has, which I believe runs lean of peak. I can push the engine harder and the EGT goes down, partly because it’s loaded higher and also because it comes out of ECO mode and goes into power mode.  So, I keep my prop adjusted for 5050 on takeoff, which avoids the higher EGTs and just seems to work the best overall. At this setting it will still exceed 5500 RPM in level flight.