Whats cooler than cool?
 

Ice Cold!

And thats how my engine has been running lately. Before I added my trombone-cooler my motor would predictably warm up to 180, and on hot days pull 225+. When I added the cooler the temps came down quite a bit and faithfully stayed at 180. It was a used cooler that I put in BTW. After about a month of use though, the motor never reaches a decent operating temperature. She will hang out around 120 all day long no matter how hard I drive her.

Just this morning I drove into work, sitting in traffic, etc, and checked the lines for heat when I parked. Gauge says 120 degrees, lines behind the thermo are piping hot, lines in front are piping cold. Rarely does the cooler get hot now.

I've read a few other posts like this but have seen no answer. Do external thermostats go bad (ie get stuck open)? Are they serviceable? What do I do?

I suspect your guage/sending unit are not reporting accurately. No way your car is staying at 120.

I can’t believe that your engine temps really remain below 120 degrees no matter how long you drive. The external thermostat isn’t causing this- if it was stuck open the warmup period would be extended but you say that the lines to the front cooler are cold. I would think its a guage/sender problem. Especially hearing that you’ve only started seeing readings this low in the past month.

snap.
Does Arnie have a moustache in your avatar?

Your external thermostat would not even have opened at 120 so your lines should be cold at that temp. You definitely have a gauge/sender problem. Have you replaced either recently? I see you have a 3.0. If you have the original gauge, you still need the early sender like in a 2.7l.

havent touched either the gauge or the sender, will look at those ASAP. thanks all