Overheating cars used to be a familiar sight, stopped at the road side, bonnet up, steam pouring from the engine. Thankfully it’s not so common now as cars in general have become more robust.
But overheating – when the engine’s water literally boils – does occur. Here we look at why, what you can do if your car does overheat, and offer some tips on how to prevent it.
Why do cars need water?
Just as staying hydrated is vital for humans, so fluid is vital for cars, if not in quite the same way. Most cars now are water cooled. This means they have channels built into their engine block that water circulates through. This is designed to stop too much heat building up from all the engine’s moving parts. The water then passes through the radiator where it’s cooled by the air forced through it as the vehicle moves forwards. Whether or not this cooling water is pumped through the engine is controlled by a thermostat.
Why do cars overheat?
There are numerous reasons why engines get too hot and the liquid boils over. It could be that there isn’t sufficient coolant to take the heat away from the engine properly. If the engine overheats at low speeds, it might be that there’s something wrong with the fan. This draws air through the radiator when the car isn’t moving fast enough for the air to cool the radiator. It might be the fan belt if the car has one. Or the switches that turn the fan on and off.
If the engine overheats at high speeds, it could be that the thermostat telling the water pump to send water around the cooling system has packed up. Or the water pump itself might be on the blink. Airflow alone won’t be sufficient to cool an engine that’s working hard.
It could be that the radiator has a hole in it and is leaking water. Or there might be a leak elsewhere in the system through a hose or faulty connection. The radiator might even be blocked. I’ve seen a car’s engine temperature rise alarmingly fast because an old plastic bag had got stuck to the radiator grille.
What happens if your car is overheating?
First, make sure the steam you can see is coming from your car. If you’re in a traffic jam, it might be from a car in front. Then look for somewhere safe to pull over. An engine that’s running too hot could cause itself serious damage. Assuming pulling over is possible, there probably isn’t much you can do for around 30 minutes.
Pop the bonnet from inside the car, then make sure you and any other occupants are standing a safe distance from any moving traffic. You then need to let things cool down while you wait for your breakdown operator. There’s every chance the steam from the engine’s boiling water will have made the bonnet catch super hot. But it should have cooled by the time a breakdown professional arrives. Sadly, some of the causes of overheating mentioned above are impossible to fix roadside, even by one of our skilled technicians.
And if you can’t stop safely
The first thing to do is turn off the air-conditioning. This is another motor working away beneath the bonnet, taking heat away from the cabin and thereby making things under the bonnet even hotter. Then turn the heating up to full. It might make things inside the car pretty sweaty, but it’ll draw some heat away from the engine. Even so, you should stop as soon as you safely can, particularly if there are any red lights illuminated on the dashboard.
What precautions prevent overheating?
The most preventable cause of overheating is a lack of coolant. To ensure you’ve got enough, simply open the bonnet and look for a bottle with ‘min’ and ‘max’ markings (above). If you’re in any doubt about where it is, look at your car’s owner’s manual. Coolant is a mixture of water and anti-freeze and the level should be between the markers. If it’s below the ‘min’ line, you can top it up with water. But make sure there’s a proper concentration of anti-freeze before winter. You could also have your car serviced before you go on holiday. The diagnostics should show if there are any problems with the thermostat or the fan not kicking in when it’s supposed to. It really is a case of prevention being better than cure
Nick Reid is head of automotive technology at Direct Line Group and a fellow of the Institute of the Automotive Industry
There is a lot you can do if your car’s engine overheats. First pull in and stop. Next find a large cloth or similar. Pull the bonnet release. Touch the bonnet area to see if it is hot. If it is hot, wrap the cloth around the hand and prepare to open the bonnet. A bit of hot metal is hardly the most dangerous thing in the world. Stand back as you open the bonnet. The steam will blow away.
Then look to see where the steam is coming from. Once the steam has stopped escaping, place the cloth over the coolant reservoir cap and very slowly release the cap a fraction. Any pressure in the system will be released, and you are left with only the hot coolant left in the system.
Hose fittings may need retightening. You can then refill the system, if it is intact. But do so very slowly, a few drops at a time, to prevent the cool water from boiling and bubbling back out. If you do not have plain water with you, or you cannot get plain water, and depending on the situation then perhaps cool drinks might be used as a temporary solution.
If you have had a fan belt failure, an old trick was using a ladies’ stocking in its place. A hole in the radiator can be filled with Radweld – always carry a can.
Regrettably few cars these days have temperature gauges, so the driver cannot tell when the engine is starting to get too hot. But driving gently to when the system can be refilled completely with plain water is one option. Once home, drain and flush the system and refill it with proper coolant to prevent future freezing.
And always carry a five litre can of water in the boot in case of overheating.
In the past few decades, nearly every problem I have had with my (older) cars repeatedly overheating, despite all the normal checks ( hoses, fluid, radiator, thermostat, water pumps, fans etc), the underlying cause has been the engine coming to the end of its life due to wear. Big ends going, blown head gaskets, cylinder wear all seem to have been diagnosed by garages. The honest ones tell you the car needs scrapping and replacing. The less honest ones try to fix the faults at great cost. Know when to stop nursing that beloved but repeated overheating old banger- send it to that great scrapheap in the sky! The alternative is a sudden catastrophic and unfixable engine failure,often on a motorway or in the middle of nowhere.
If your car is prone to over heating on steep hills, a tip is to open the windows and turn on the heating to divert coolant via the heater matrix which contains on average 2 to 3 letters plus what in the pipe system.Note put the heating on slowly so a to avoid the sudden extra presure serge on the system.