Overheat at interstate speeds after numerous new parts & HG

2014 Prius
236k miles
iCarsoft CR v3.0 diagnostic tool

I have a mild overheat issue at highway speeds and hills. Around town with the A/C on, the temperature stays below 205F with the radiator fans on low speed from the A/C signal. At 60 mph plus, the temperature sometimes hits 205F and the diagnostic tool shows the fan going to high speed. On the interstate at 80 mph at 75F ambient, the temperature is around 210F and spikes to 220+ when the cruise control accelerates going up mild Florida panhandle hills on I-10. I am perplexed.

Work done:

2023, 190k miles – Replaced water pump impeller on original pump when I had intermittent overheat at highway speeds. Also cleaned the EGR cooler and manifold, and replaced the thermostat, coolant temperature sensor, and spark plugs.

Aug, Sept 2025 – 800 mile round-trip on flat, Florida roads with a light trailer, no issues, no spitting coolant, but did not have diagnostic tool attached for real time temperatures.

Nov 2025, ~233k miles – Started seeing slight disappearance of coolant. Suspected head gasket, but no combustion gases in cooling system. Found light coolant seepage on radiator between fin core and side tank. Replaced radiator (Nissens) and radiator hoses (Gates, Dayco).

Feb-Mar, 2026, 235k miles – Continued to have disappearing coolant and worsened to an intermittent stumble on start, progressing to more regular stumbling and misfire codes on #1. Borescope in #1 cylinder showed a glistening, steam-cleaned piston and cruddy, carbon crusted #2-4. Replaced head gasket with Felpro. Replaced water pump impeller and thermostat (Motorad) and cleaned EGR cooler and manifold. Also new spark plugs and coils.

Early Apr 2026 – No cooling issues or lost coolant in town, no stumbling. Attempted interstate road trip, but found coolant on and around the reservoir at highway speeds. Replaced reservoir and cap (Dorman) out of desperation to try to continue the trip, but still spitting coolant at highway speeds.

Recent – Test drives with diagnostic tool with real-time data at 80 mph on I-10. Temperature spikes to low 220+F and spits small amounts of coolant from reservoir at 80 mph and hills when cruise control accelerates, water pump rpms high 4k to low 5k with actual slightly higher than commanded, fan to high speed at 205F. Replaced water pump (Autozone Duralast) with no change in behavior at speed. Research showed possible thermostatic control valve problem on the exhaust heat exchanger. Bypassed exhaust heat exchanger down near the exhaust. Still no change in behavior at speed at 75F ambient, with the exception that the high temperature recovers to below 200F when slowing down much, much quicker than with the exhaust heat exchanger connected.

Never had coolant system sealant in it and the antifreeze is fresh and proper mix. System purged of air per manual after refills.

To recap, new radiator, radiator hoses, thermostat, head gasket, water pump, reservoir and cap, bypassed exhaust heat exchanger, still have temperature spikes to low 220+F and spitting coolant from the reservoir at 80 mph and hills. Also had a hybrid system fault and a weak 12VDC battery, and the battery was replaced between the water pump replacement and the exhaust heat exchanger bypass.

I never hooked up the diagnostic tool and ran at interstate speeds prior to this month to check real time temperatures when initially looking for disappearing coolant, and from what I recall, I wasn’t spitting coolant. I am perplexed. I feel like I’m looking at either having messed up something during the head gasket repair (but what??) or I have a defective radiator or thermostat. If anyone has ideas, I am more than game to listen.