I was recently driving down the highway and saw the engine overtemp light come on on my dash. I pulled over and looked under the hood and the coolant was boiling in the reservoir. Bad sign. Coolant was slightly below the low line from evaporating and boiling coolant coming out of the reservoir cap. I have a 2011 Toyota Prius Level 5 with 77,000 miles with no modifications of any kind. PROBLEM: Engine coolant overheats up to 235F while driving at high speeds (65mph) under heavy load (going slightly uphill on highway at high RPMs). At low speeds under 60mph or when the car is idle, no overheating occurs. Sketch-1550725530879 by Sergey K posted Apr 1, 2019 at 3:59 PM REPAIRS TRIED: 1. Replaced coolant reservoir cap: I started with the most inexpensive part to replaced: the reservoir cap. I got a replacement from Toyota dealer. If the cap is not holding the correct pressure in the reservoir, the coolant can boil faster than necessary. I replaced the cap and filled up the reservoir to the max line. Replacing this made no effect, I had the same overheating problem. 2. Replaced thermostat: Got a new thermostat from Toyota and replaced it. When draining the coolant, it all came out clean, debris free. Checked the old thermostat in boiling water and old one still worked as well. Installed the new thermostat regardless. Replacing this made no effect, I had the same overheating problem. 3. "Bled" the cooling system: Spent about a couple hours running the car up to temp and squeezing all of the coolant hoses. Had the heater on high with lowest fan speed. Doing this made no effect, I had the same overheating problem. 4. Checked the coolant temp sensor: Removed the coolant temp sensor and tested with an ohm meter according to the Prius service manual. Sensor is working as normal. No issues found. Screenshot_20190319-134352 by Sergey K posted Apr 1, 2019 at 3:59 PM 5. Replaced the radiator: Some friends kept telling me the radiator is probably partially clogged and is likely the issue. Ordered a new radiator and installed it. Again, old coolant came out clean, debris free. Filled new coolant into new radiator and bled the system again. Replacing this made no effect, I had the same overheating problem. Screenshot_20190321-204436 by Sergey K posted Apr 1, 2019 at 3:59 PM 6. Replaced the water pump: I left the costliest part for last. But my Prius started showing me 1 engine code: P148F: Engine coolant pump over revolution. I'm not sure if this code was here the whole time or not and I'm not sure what it really means. I bought a brand new water pump and installed it, filled system with new coolant, and bled the system. The engine code went away and I'm not seeing it at this time. Replacing the water pump made no effect, I still have the same overheating problem under heavy load at high speeds. Screenshot_20190322-205152 by Sergey K posted Apr 1, 2019 at 3:59 PM I checked the service manual and the 3rd gen Prius normal operating temp is 176 to 212F. Mine spikes up to 235F (and would prob go higher if I didn't pull over right away). Sketch-1553314740704 by Sergey K posted Apr 1, 2019 at 3:59 PM Some more notes: 1. I dont think it's a blown head gasket because the coolant does not leave the system except when it boiled over. It stays put at the max line now that I pull over before it boils and the oil is clean and not getting mixed with coolant. 2. Both radiator cooling fans work and blow the correct way. 3. The cabin heater works fine and blows very hot air within several minutes. 4. I'm also using the Toyota brand pre-diluted 50/50 pink coolant. Never used anything besides Toyota coolant. 5. No other engine codes or issues come up. 6. When the car is heated up (over 165 or more), the coolant is going around as the left thick coolant hose stays cool and the right thick coolant hose gets fairly warm. HELP! I've spent so much money and time and literally replaced and/or tested everything I can!!! I need this car for work. I don't know what else to test. It only overheats under load at freeway speeds. If I drive 60mph or less on flat roads, I see no overheating issues and the coolant always stays under 200F. Am I missing something? Could I still have an air bubble somewhere in my hoses? What can I do???