Bad thermostat, weird idle and MPG
 

Just though I'd post this for those interested.

My '05 ST with 2.3 was purchased with a bad thermostat (stuck open). It warmed up according to the gauge in just a couple of miles from cold. It ran well except for an inconsistently high idle, no CEL. The heater put out warm, not really hot air. MPG was ranging from 27-30.

I plugged in the scangauge to get an idea of the temps the PCM was seeing. Turns out that when the gauge reads normal, it doesn't change from 143* to at least 215* which is as hot as it ever got. Often it would run between 150* and 175*. Again, the in-dash gauge always read the same and indicating "normal".

Changed thermostat.

Now it warms up much faster, heater is toasty hot and maintains at least 195* per the specs. Idle is rock solid and consistent. MPGs now are consistently just over 33 in the last 3 tanks.