My 2003 Dodge 1 ton diesel dually, which I use to haul a small gooseneck horse trailer, overheats slightly when I’m at a stop sign, or when I had to drive up a short (about 1/4 mile) very steep hill to a campground. It’s okay when I’m driving freeway speeds, even with the AC on. But if I have to idle for a long time, such as once when I got stuck in parade traffic, it got almost to the red. I did the turn-the-heat-up-turn-the-fan-on thing and it went down. I had the radiator flushed and filled, and they checked the fan motor. I hoped that would correct the problem, but it still does this. It never goes to the red, and usually just goes a bit over halfway. Could it be the thermostat? I have a friend who is sure that’s what it is. Can a mechanic actually SEE if the thermostat is bad, or is it just a hit-and-miss type of thing. I don’t want to find out it was something else when I’m six hours away from home on a weekend with horses in the trailer!
The thermostat is easily accessible and should be repleced due to age.
The problem is likely with the fan clutch. This engine has a mechanical engine fan that has an electronicaly controled clutch. There are wires going to the radiator fan clutch, controled by the PCM which may have broken. When the engine temperature is above normal, if you elevate the engine speed from idle you should be able to hear a roar from the engine fan, if not the clutch may not be working.
This is almost certainly a cooling fan issue. As far as testing the thermostat, it can be done but is not worth the effort considering the cost of the replacement part. To test the thermostat, you need a hot plate, a pan full of water, and an accurate thermometer. Hold the thermostat in the pan of water suspended by a piece of string to keep it off the bottom of the pan. Hold the thermometer likewise in the water. When the temperature of the water reaches the thermostat’s rated temperature, you should see it open. Not worth the hassle over a <$10 part.