I have an 03 Liberty and just had the thermostat replaced a couple weeks ago because it was tripping the CEL for running too cold too long (stuck open). I had already replaced it a couple years ago but evidently these particular MOPAR thermostats are utter garbage and only expected to last a couple years. It runs fine now (no engine codes). But at highway speeds, especially above 70 mph or so, it runs cold, just a bit over the 1/4 mark on the temp gauge. But it doesn't trip the CEL like it used to and the mpg hasn't gone down. Today it was about 39 outside, so not horribly cold, and it got up to full temp before I got on the highway in a reasonable amount of time. And it did warm back up once I got OFF the highway to just a hair below the halfway mark (the optimal temp).
I can go up to about 55 without the temp gauge moving significantly below the optimal range. And if it is 45 or warmer outside it never moves once warmed up.
At this point I am guessing this is normal for a Liberty as I go through this mess every winter. Some winters it trips the code (and I have the thermostat replaced) but other winters it just runs cold on chilly nights.
I wonder if the CEL only trips when the thermostat actually is stuck open. But how does it know if it is actually stuck open or if it is just really cold outside? What I would like to understand is which particular sensor tells the Jeep to set the code in the first place, and how that sensor works. Is it merely temperature? If that is the case, why wouldn't it trip today after the temp went down (probably drove 15 miles with the temp low)? Is there a way the computer knows if the coolant is actually flowing and stopping (i.e., the thermostat is actually working normally) such that it makes an exception?
On a side note, after the repair, I noticed the bleed screw (near the upper hose) was weeping a lot (I noted the smell of evaporating coolant and could see the white tracks of the dripping fluid around the part to which the upper hose connects). I tightened it up with my hex wrench and it still bleeds just a little. Is there something I should do to seal the threads better? Plumber's thread tape perhaps?
Thinking about affixing some plastic-core to the front grille to slow airflow around the radiator for the winter. Should last a lot longer than cardboard and it isn't flammable. Sold at most craft stores. Easy to cut with scissors. I could even cut it into strips and use black wire ties to fit it inside the spaces in the plastic grill fascia (keeping it well clear of the radiator itself). During moderate weather I get close to 20 mpg highway. This winter I am lucky to eek out 18.8 (probably because it isn't running at optimal temp).
