Robert, I would guess intermittent partial injector clogging, here's why:

1.  I have never seen a thermocouple fail by 100 deg or so; in my experience they either work or they don't, with max scale readings either way depending on how they're wired.

2.  In 150 hours on my 550 I've had two partial injector clogs that have driven up the associated EGT (and CHT down) each time.  I've been quite thorough with fuel flushing and filter cleaning, and never get any sediment out of the tanks (so the fuel from my airport is clean), but somehow contaminants got through.  In each case, I could see the blockage and I extracted it by blowing air backwards through the injector.  My engine's normally aspirated, yours is turbo but that shouldn't make any diff.  Your problem responded to enrichening and leaning, which seems to indicate a mixture-related issue, not a thermocouple problem.

3.  In a Mooney 252 (TSIO 360) I had EGT jump on one cylinder, it turned out to be a stcking exhaust valve.  Mixture changes did not affect that, so I don't think that's your problem.

Charley Brown
Legacy #299  150 hr

On Aug 25, 2012, at 7:56 AM, Lancair wrote:

Last week I had been cruising LoP and during the power back for landing
Cyl #3 suddenly jumped 125F and sent the Chelton lady shouting into my
ear.  That would not be unusual if I had failed to either further lean
or go full rich but in this case I had leaned and the other 5 cylinders
were OK.  I leaned way back and really lost power and then went full
rich, and it finally settled down.  On the return flight I reduced power
and carefully kept leaning and had no problem.   Yesterday it was deja'
vu all over again.  Powered back on approach and only #3 went nutso
until I leaned even more.  On the return flight there was no recurrence.


Does this sound like a problem with the injector, with the thermocouple,
or ? ? ?

Robert Simon
ES-P N301ES
(TSIO550)