The thermostat, which is usually located in a housing where the upper radiator hose connects to the engine, controls the operating temperature of the engine. It does this by blocking the flow of coolant from the engine to the radiator until the engine reaches a certain temperature (usually 190 to 195 degrees Fahrenheit). When this temperature is reached, the thermostat opens and allows coolant to circulate from the engine to the radiator. If the thermostat fails to open, which can happen due to mechanical failure or if a steam pocket forms under the thermostat due to incomplete filling of the cooling system or coolant loss, no coolant will circulate between the engine and radiator, and the engine will quickly overheat.
On most front-wheel drive cars, the fan that cools the radiator is driven by an electric motor. A temperature switch or coolant sensor on the engine cycles the fan on and off as additional cooling is needed. If the temperature switch or coolant sensor (or the relay that routes power to the fan motor is bad), the fan will not come on when it is needed and the engine will overheat. Likewise, if the fan motor itself is bad, the fan will not work. The system needs to be diagnosed to determine where the problem is, so the correct component can be replaced.
Leaks in radiator or heater hoses, the water pump, radiator, heater core or engine freeze plugs can allow coolant to escape. No engine can tolerate the loss of coolant for very long, so it usually overheats as soon as a leak develops. A visual inspection of the cooling system and engine will usually reveal where the coolant is going. If no leaks are apparent, the radiator cap should be pressure tested to make sure it is holding the specified pressure.
If the spring inside the cap is weak (or the cap is the wrong one for the application), the engine will lose coolant through the overflow tube every time it gets hot. If there are no visible coolant leaks, but the engine is using coolant, there may be a crack in the cylinder head or block, or a leaky head gasket that is allowing coolant to escape into the combustion chamber or crankcase.
When you are driving your car in summer, or even if you are lounging in the backseat, enjoying the flow of cool air around you, keep an eye on the temperature gauge. Generally, in all modern engines, the temperature stays at around half. This is normal. In most Japanese and Korean cars, like the Hondas, Mitsubishis, Hyundais, etc., the needle goes to the half mark within a few minutes of moving off. In certain European cars like the Mercedes, the initial temperature reading might be below half, which can then gradually creep up over the half mark. A slight upward indication is not a problem, unless and until the needle strays into the red zone. If this happens, the first thing to do is to switch off the air-conditioner. Then slowly try to move to the side and stop the car. Switch off the engine immediately, but whatever you do, DO NOT try to open the radiator cap as you could get scalded by the hot steam. Just try and get professional help.
Another problem with certain cars, especially older Mercedes and BMW cars, is that the engine temperature starts to go up when you are idling at a red light especially with the air-conditioner on. What you need to do is to tropicalise the car by fitting a larger radiator and another electric cooling fan.
Get this done in any competent garage and you will notice an immediate difference. Uprating your water pump is also a good idea, but at times, these parts are not available and it can create quite a problem. If you plan to continue using your existing radiator, at least get the radiator flushed at a good place, as this will usually remove the scales that form as a result of corrosion over time and prevent the efficient circulation of water or coolant.
You might have noticed that throughout the article, I have sometimes used the term water and sometimes coolant. Contrary to what anyone might tell you, a coolant does not necessarily keep an engine cool. You can use plain, tap water to keep your engine coolas long as you have sufficient quantities in your radiator and expansion bottle. A coolant merely raises the boiling temperature of water to some extent. For the past 10 years, I have run cars like Mercedes Benzes, BMWs and Audis on plain water and have never ever had an overheating problem even in Delhis harsh summers.