I have gas (8.1 Workhorse), mine (Temp) goes up when I stop or drive slow (under 20mph). The temp sensor is under (behind) the front bumper, (which is not a bumper but a look-a-like), and the sensor picks up heat from the radiator.
I bet even the Journeys have the sensor in the same location. It (the sensor) also picks up extra heat from blacktop parking lots. Works great driving down the highway.
Bruce and Dyanne