Steps first for the people in a hurry, then the explanation. These are the things I have posted 100 times.

1. Check to make sure it is full of coolant, if not fill.
2. Remove thermostat element from the system to rule out bad thermostat. If you are still running an OEM style you can put a splice from an autozone hose repair kit in its place in the lower rad hose to connect it to the wp hose and then run the upper radiator hose directly to the neck, No tee required.
3. Check if it will idle without overheating
   1. Yes? You likely have a plugged radiator
   2. No? Check if it will drive without overheating on the highway. Start it up cold, head to the highway as quick as possible and drive at highway speeds. If it does, you likely have a bad fan clutch. There are people that stop the fan with their hands while the engine is hot and running but I don't recommend that. You can feel the airflow with your hand and get a sense of it but I can almost guarantee if it won't idle without overheating you have a bad clutch fan unless it is blown HG, see step 5.
4. Check if it will overheat on the highway if it does not overheat idling?
   1. Yes - you likely have a plugged radiator. I have had customers with brand new radiators that were bad. Just because it is new does not mean it is good. You can jump out and check the lower radiator hose and most likely it will be much cooler than the top hose.
   2. No - If it overheats idling but not on the highway, you likely have a bad fan clutch.
5. Both? If you have removed the thermostat and it overheats idling and on the highway, you likely have blown hg or a cracked block. So sorry. Best test is to rent the Advance auto cooling system pressure system and see if it will hold pressure overnight. Test it cold. If it doesn't, you will need to find the leak.

Things that almost never cause overheating:

1. Bad waterpump - they work or they don't. Flowkooler might do better, but stock is good enough for almost all conditions.
2. Plugged heater core - adds minimal cooling, especially in the summer.
3. Bad AC fan - that fan is there to circulate air through the condenser in cool temperatures when the defroster is turned on. Has almost no impact on engine cooling, might help an overheating truck slightly but it is not your root cause.
4. Air in the system. Once you remove the thermostat the any air in the system gets flushed out very fast due to the much higher flow rates and eliminating the jankety factory hose routing.

One final thing, go back and read the sticky on the inline thermostat mod and then read this: https://extinctmotorsports.com/the-e...s-10-min-read/ I have now overheated 3 or 4 trucks pretty badly due to various circumstances without blowing the hg due to running all my trucks unpressurized. If you live at elevation and need the elevated boiling point just run 70% coolant.

Ok, for those that want more of an explanation, here is the rest of it.

You engine needs two things in the right quantity to keep running cool:
1. Waterflow through the engine and radiator
2. Airflow across the radiator.

If those two are present, it is almost impossible for the engine to overheat, even if it does have a blown hg (I only had one that overheated due to blown hg with both of those present, a customer had a cracked block engine and we kept it running over a year as a daily by running no thermostat ).

Lack of waterflow can be due to bad thermostat, low coolant level, or plugged radiator. That's about it.

Lack of airflow can be due to bad clutch fan, plugged up AC condensor (mud, bugs, whatever), or blocked grille (winch, drivinig lamps, etc.)

Hopefully this reduces the number of repetitive "Why is my truck overheating post".