Why is my truck running hot?

Your thermostat could be bad, getting stuck. I had that happen. While driving down the road I noticed the gauge went way over to the hot side then went back. I changed it and all is well. It will run hot if there is something blocking the air flow through your radiator. Good clean oil up to the fill line will help your engine run cooler. If you haven’t changed your oil and filter lately, the oil filter may be clogged (full) and the oil is bypassing it. This will leave gunk and metal parts in the oil that make the oil hotter than it should be. Make sure your coolant level is topped off and you have antifreeze in it. The proper mixture of antifreeze helps keep an engine cool. If there is too much grease and oil buildup on the outside of your engine it will run hotter because the air flow cannot reach the metal and cool it off. Is your radiator clogged. Some people, if they have a leak in the radiator will use stuff to seal it from the inside. These additives can clog the radiator.

Are you sure you have the right thermostat ?