Robert M. Gary wrote :
I had a plug go bad and that cylinder's EGT actually went up. I never
understood that.

If one plug in a two plugged cylinder quits working, then there is
only one flame front. Instead of the fuel/air charge burning from two
ignition sources, it burns from one. This results in the charge taking
longer to completely burn. Since the charge took longer to burn, the
gasses are still hotter when the exhaust valve opens and the gasses
pass by the EGT probe.

This will happen anytime you go from 2-plug to single plug operation.
When in cruise, switch to one mag or the other. You should see a rise
on all EGTs.

John Galban=====N4BQ (PA28-180)