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Afternoon, my 2003 Dodge Ram 2500 with Hemi motor is over heating at highway speeds. I’ve replaced thermostat and water pump, have done 2 radiator flushes. While at idle the truck does not over heat. There are no leaks to be found and coolant level does not change. I’m lost on where to go from here.

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There are two things to check.

  • First, ensure there's nothing blocking the radiator. A plastic bag or something in front of the radiator can cause these kinds of issues, especially if it falls back down to where the radiator is clear when the vehicle is sitting still.
  • Your lower (or both) radiator hose may be collapsing. When the engine and coolant system is completely warmed up, if the bottom radiator hose is weak/worn out, it can collapse under the draw of the water pump as it is trying to pull coolant from the radiator. This only happens when the engine is at higher speeds. You may be able to see it do this while the vehicle is sitting still if the engine is being revved at a higher speed. Would most likely take two people to make it happen (one on the throttle, one to observe ... but only if this is a "drive by wire" throttle body). It may be good to just replace both radiator hoses if they've never been changed out before, anyway. This would be considered good preventive maintenance, especially on a vehicle which is over 16 years old. Rubber deteriorates after a while, so replacement every once in a while makes sense.

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