They say that where's there's smoke there's fire, well if its white smoke from under your bonnet on a summers day chances are that the engine has reached boiling point especially in heavy traffic and stop start motoring where very little air flow can get under the bonnet.
To keep the engine at its optimum operating temperature it is cooled by coolant (apart from air cooled cars like the VW Beetle) circulating round the engine through the thermostat and then through the radiator. The radiator is where the heat exchange takes place and the coolant is cooled down, without the radiator the coolant would get hotter and hotter. Under normal driving conditions air is forced through the radiator cooling it as it goes. When the car is stationary then the cars fan should either push or pull air through the radiator to keep things cool. The original way as to have a fan that was driven directly from the engine (usually with a belt) but in more recent times electric fans have been fitted. Electric fans have the advantage that they are not running all the time so don't over cool the engine and waste engine power running the fan thus increasing MPG as well.
1. If you have an electric fan run the car at half throttle until the temperature gauge has reached about 3/4 of the temperature gauge. Around this area the fan should switch on and you should see it rotate and here the hum. If it does not switch on then drive the car to stop it from getting too hot. Alternatively you can find the connections for the switch that turn on the fan (check your manual) and short out the connections, this should switch on the fan. If not then the fan is defective and should be replaced.
2.If the fan is belt driven check that the drive belt is correctly tightened and is in good condition. This should be changed as per the service interval for your car.
3.If the car has been restored or you know the fan has been removed check to make sure it has been fitted the correct way round as if it has been fitted wrong it will not cool correctly.
During the winter months make sure you have the correct level of antifreeze in the system otherwise the coolant will freeze and you could crack the engine block or split some hoses. Most antifreeze now also works well as a summer coolant and corrosion inhibitor for the system. Certain cars especially with aluminum heads etc need a 50% concentration.
Top Tips for coolant
1. Never unscrew the radiator or expansion tank cap on a hot engine, this can result in a lethal fountain of scalding hot water, let it cool down for at least fifteen minutes then remove the cap with a cloth covering it very slowly.
2. If you can easily pinch the cooling water hoses then the pressure in the system is low and the cap can unscrewed.
3. Regularly check the coolant level (do it the same time you check oil and tyre pressures) and if the level is low then there is a reason such as a leak which must be investigated.
4. Check the concentartion of the antifreeze/summer coolant regulally(a tool can be bought for a few pounds to do this.
5. If the engine temperature gets too high as an emergency measure you can set the interior heater to full and put on the blower, this will get rid of some heat through the interior of the car bt it might get a bit warm for the ocupants.
6. If the heating system does not get warm then this could be a sign that there isn't enough coolant in the cooling system.
7.Coolant should be replaced every 2 or 3 years and the system flushed through, this prevents the build up of rust in the system and reduce the chances of blockages in the system.
8. The air needs to be bled out of the system, some cars have a bleed valve while others you have to keep filling and undo the highest hose on the system to let the air out.