I've been running a OEM 180 thermostat and had normal driving temperatures 188-190, rarely much above 194. Late last year I noticed that my temps were a bit higher. Sitting at stop lights where I used to be 194 I would see 199, 201 and 203 even in moderate weather. I recently changed to a severe duty fan clutch and the Dorman fan. At idle I don't see a difference with the new set up. So before I start throwing parts at this, does anyone have any ideas on what could be causing the increase in low speed/idling temperatures? I see 188-190 at 35+mph and the temperatures fall quickly once I begin moving after sitting at a light. I've bleed the system several times. The radiator looks to be in good shape and is 5 years old. Water pump was changed 3 years ago (I only drive 4k miles per year). Thanks for your insights and help.

 

Few possibilities:

1. Clogged condenser/radiator fins - exterior - dirt, oil, mud, bugs, whatever. You could pull them and clean them.
2. Radiator tubes getting plugged/clogged?

I absolutely hate the factory thermostats - they have caused more blown head gaskets than any other component on the Discos. You can pretty easily remove the factory thermostat and run without one for testing purposes if you wanted.

I usually swap parts between good truck and bad truck until I find the problem - you need another Disco.

 

Take a peek in the bleed screw hole when the engine is cold. If it is not overflowing coolant, top it off until it does. Could be a small air bubble developed, which could be a clue to other issues.