I own a 2000 Ford Focus sedan 2.0 litre, 5 speed manual transmission, 58000 miles on the odometer. While driving I noticed that at highway speeds the car would overheat to just under the red line. At normal surface street speeds the temperature is normal.
I went my car to the mechanic, and he said the cause was the thermostat being stuck closed. After changing the thermostat and driving again on highway about 20 minutes later the temperature would spike to ¾ mark and then after a few seconds recover to below ½ mark. This spiking behavior would continue happening at highway speeds, especially during hard acceleration or hill climbing. When stuck in traffic or stopping I could also hear the radiator fan blowing.
Returning to the mechanic again he said that the cause was the electrical interruption in the plug in one of the two radiators fans. Repairing this and again the same happened. Returning to the mechanic, he changed the coolant hose which is connected with the thermostat housing, the thermostat housing, the coolant and he checked the whole electrical cooling system (two radiator fan relays and resistor pack of the cooler) and I also washed the outside part of the cooler but nothing happened.
I returned my car to the mechanic, and he said he could not find anything wrong. I was told to return the car if it happens again. Well, it still happens.
I would appreciate your opinions on what the cause may be.