Hello! I bought a 2010 mk7 fiesta earlier this year. It has 115k kilometers on it (72k mi), 1.2 82hp petrol engine. First thing I did was timing belt + water pump + thermostatic valve + radiator replacement + new coolant since the timing belt was 14 yrs old and the radiator was pretty dirt and corroded.

Radiator fan is coming on correctly, when I turn on AC the fan instantly kicks in and turns off within a couple of minutes when AC is off. Similarly the fan turns on when the water temp hits 105C (I don't have temperature gauge so I read this data from the test menu). If I drive around the city, also in slow traffic, also in extreme hot temps (average of 35 degrees slow driving for 40 minutes) nothing happens, AC works correctly and everything is fine.

However, last time I drove it on the highway (100-140kmh or 80-90mph) after less than 30 minutes the water temp reaches almost 120C, and I think that the AC goes in protection mode (past 112C the compressor turns off and the fan pushes outside air in the vehicle). Warning temp light did not show up because I went easy on the gas and I used the AC fan set on heat to redirect engine heat in the cockpit to get to my destination.

I really dont know why the car is overheating only on the highway, where the airflow is much stronger and the radiator should cooldown easily. Any ideas?

It's like if the engine's cooling system can handle the slow-paced-in-hot-weather city drive but can't handle high speed/rpm highway driving.

Thanks in advance

Remove the thermostat and see if the car now runs cold. If so then at least you've elimated a faulty stat