[s-cars] Coolant temperature questions

[Kevin Day]

Shortly after I got my S4 last fall I had the thermostat replaced, and
for four months or so I saw what I took to be normal temp. values on the
climate control head (and the gauge, until it stopped working): the CC
indicated 92C at highway speeds, and would peak to 96 or so in traffic.

A few weeks ago it changed and I've been seeing 72C on the highway and
haven't seen more than 75C at any time.  I replaced the thermostat
today with a new one from the dealer, and the behavior is the same --
high temps are in the 70s.  Any ideas why the engine would be running
20C cooler now?  What values are you guys seeing? (Anybody want to buy a
slightly used thermostat?)

Could the ECT sensor be going bad?  Parasitic resistance in the wiring
causing the CC to indicate a lower temperature?

I also replaced the MFTS, but my temp gauge still doesn't work.  I'm
going to try grounding the gauge sender wire.  So it's hard to get an
independent read on the actual engine temp.  Does anyone know what route
the MFTS sender wire takes to the instrument cluster?  I can't find it
in the Bentley.

Incidentally, the new t-stat looks different than the one I got from
TPC, and it has a small ball check valve (for what?)  In case anyone
cares here's a picture of both: http://ultrameta.home.comcast.net/tstat/ 

-Kevin