<<Every temperature sensor is affected during comm transmit - CHT, EGT, OAT,
CAT and OIL>>

I don't know too much about this system, but I'm assuming the probes are all
thermocouples.  I have taken apart a Vision fuel probe and was amazed to
find that there was no, and I mean none, EMI protection built into the
circuitry, so that might give one a clue about the protection, or lack
thereof in the rest of the system.  For thermocouple leads one could build a
filter circuit and place it just outside the box and see if it makes a
difference.  A fairly simple one would be a 1K resistor in each lead with a
0.1 mfd capacitor between the leads on the box side of the resistors.  The
resistors might affect the reading a little, but probably won't.  Try doing
that on one and see what happens.  Problem is that if it works you need to
protect all of them, a really messy task.  Easy to design into the box, but
messy to fit after the fact.  Another thought is just to twist the
thermocouple wire pairs along their whole length to make a poor man's
shielded wire.  The RFI might be conducted in through the power leads of the
box, but I would expect then it would affect more than just temperature
measurements.  Or it could be directly radiated through the face of the
display into the box, as that is probably the least well protected part.

The JPI in my certificated aircraft seems to work flawlessly so I plan to
use theirs in my ES.

Gary Casey