Sorry, this isnt about a truck but I'm having trouble with my 99 Caravan overheating. If I turn the heater on it'll cool down some and it also seems to cool down when I slow my speed down to around 55mph. The radiator level is fine and oil level is fine too. Also the fan is kicking on too, so its working. Please help, I have no idea whats going on.

 

First, check for buildup of junk/dirt on the front of the condensor or between the condensor and the radiator. Clean with a garden hose sprayer by spraying from the rear.

Verify "overheating" with an external device such as a non-contact IR thermometer.

Provide more symptoms/observations.....

Steve