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The fan on your truck has a clutch that is supposed to allow the fan to free spin while moving at highway speeds, but run the fan full force a idle speeds. That fan clutch could have gone out. Does it seem like the vehicle overheats more when idling or when driving at highway speeds?
I'd probably drive it a little and see what it does. It sounds like coolant might not be circulating if the top is hot and the bottom is cold. I've seen a handful of these engine have bad water pumps that will no longer circulate the coolant. If only the top is hot that means it's not flowing like it should and you already eliminated the thermostat as a cause for that.
I'll be here for as long as you need for follow-up.
I come and go- I don't work directly for just answer- I run a mechanic shop- so I'm not always at my computer, but I'll keep checking in to see how the testing goes.
It might be faulty. That's not very common though.
The water pump is below the thermostat housing.
Here is how you replace it:
That's a good plan. Pull the thermostat and see it it improves. If not I would pull the water pump. Usually the way they fail is the turbine blades on the pump disintegrate and no longer move the coolant.
If the water pump looks ok then the only other cause of an overheat would be a headgasket issue. A blown or leaking headgasket can cause an overheat in about 5 minutes.
I just remembered that the water pump is behind the timing belt on this engine. It's a good idea to replace the timing belt and components while you are in there.
Also, with the engine off try spinning the fan by hand. If it spins freely then the fan clutch is probably bad and might be causing the problem.
It's possible to have a head gasket leak or a cracked head and not see and oil in the coolant or coolant in the oil.
It should flow from the top to the bottom. In other words the lower hose feeds coolant into the engine.
Sorry, I was out working on stuff all day.
I'm glad you got it sorted out. What did it end up being?
How weird. It sounds like you got it nailed down.