Water pump?

First, check to make sure that the air flow through the radiator is not blocked by any debris. If that is okay, then I would look about replacing the engine water pump. I would buy the OEM pump.



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Someone here locally just bought an impeller off of Amazon for the factory water pump usually the impeller the plastic peels off the metal and it gets stuck in the bore it spins in. Drop another smooth impeller in there and then it works fine usually You can test it off the car plugged into the car by turning the car on and watching the impeller spin smoothly like the one you just took out couldn't.

 

Third gen's have a couple of possibly relevant issues:

1. The engine coolant pump (aka water pump) are prone to crap out, anywhere between 160K~200K miles. It's got a plastic impeller that becomes difficult to turn, or just comes apart.

2. If the EGR system (including intake manifold EGR passages) has not been cleaned, the head gasket fails, around the aforementioned miles.

Buying a 3rd gen with those miles, there's decent odds it's got either of those, perhaps compounded by head gasket sealer poured into the engine coolant, to mask symptoms.

 

Just eyeball it for obstructions? Dangling a light down on the insde, then looking from the outside, might help.

 

^ +1, what Mendel said above.

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I had this exact problem a few weeks ago. I also replaced the ECT sensor first without success. It's the water pump rotor. It's split, rubbing in its well, and not providing adequate coolant flow at highway speeds. It's a relatively easy and inexpensive fix if you replace rotor only. I detailed my diagnosis and fix in the thread below. While I had the coolant drained, I also replaced the thermostat and cleaned everything EGR related; cooler, pipe, valve, intake manifold.

P0117, intermittent overheat, and water pump rotor | PriusChat