What are the effects of a faulty thermostat on a Toyota Vios?

[McPiper]

Toyota Vios engine will malfunction if the thermostat is faulty. Below are the details about the car thermostat: 1. Coolant temperature exceeds 110 degrees: Park the car, turn off the engine, and open the engine hood. Touch the upper hose of the coolant radiator—it should be very hot. Then touch the lower hose of the radiator; it should also be very hot. If there is a significant temperature difference between the two hoses, it confirms a thermostat failure. 2. If the engine takes a long time to reach normal operating temperature: Park the car and let the engine temperature drop to match the ambient temperature. Restart the engine and drive until the dashboard temperature reaches around 70 degrees. Then, park the car, turn off the engine, and open the engine hood. Touch the upper and lower radiator hoses. If there is no temperature difference, it confirms a thermostat failure. 3. Using an infrared thermometer to inspect the thermostat: Aim the infrared thermometer at the thermostat housing. When the engine starts, the inlet temperature will rise, and the thermostat should close. Once the coolant temperature reaches 70 degrees, test the outlet temperature—it should suddenly increase. If there is no temperature change, it indicates poor thermostat performance, and replacement is needed.

[LeJoy]

I remember when I was driving a Toyota Vios, I encountered a faulty thermostat that almost wrecked my car. The engine started overheating frequently, and during traffic jams, the temperature gauge would spike into the danger zone, forcing me to pull over multiple times to cool it down. It turned out the coolant wasn’t circulating properly, which caused wear on the cylinder components. The mechanic later told me that this part in Toyotas tends to degrade over time—if it gets stuck, it can either cause overheating, leading to engine deformation or stalling, or remain open, resulting in poor cold starts, increased fuel consumption, and accelerated oil contamination. Now, I make sure to regularly inspect the cooling system and replace the thermostat before summer as a precaution. After all, an old car can’t handle much abuse, and driving around in constant worry just isn’t worth it.