If the coolant reservoir is low or empty, its time to call roadside assistance.

With most of the country experiencing unusually high temperatures this summer, there's a real risk that more drivers than usual may wind up stick at the side of the road with an overheated car.

According to les McMaster, chairman of the Motor Industry Workshop Association, while it's rare for modern cars to overheat just because it's hot out there, high ambient temperatures increase the risk of overheating if the car's coolant level is low, whether due to neglect or leaking.

"When last did you check your car's coolant level?" he asked.

To prevent overheating, it's advisable to check regularly that your car has fresh coolant in its radiator - not just water - and also have the radiator checked for corrosion. However Mc Master cautioned that even a well-tuned car may start to sizzle in hot conditions, particularly in stop-go traffic or when climbing a steep hill.

The first sign that your car is overheating will be the needle of your car's temperature gauge starting to climb, or a warning icon (it usually looks like a thermometer!) in the instrument cluster lighting up.

"The first thing you need to do is turn off the air conditioner and open the windows," McMaster said. "That will reduce the load on the engine and help it cool off.

"Although it sounds odd, it will also help to turn on the heater>”

“That draws heat from the engine to heat the inside of the car. You might cook but it will drop the engine's coolant temperature a few degrees.

“If you're stuck in heavy traffic, shift into Neutral or Park and gently rev the engine just a little to speed up the water pump and fan; increasing liquid and air circulation through the radiator.”

Mc Master said brake drag also increased the load on the engine, causing yet more heat.

"Rather crawl along slowly just above idle, than move up and brake repeatedly," he advised. "Even better, leave the car in neutral until the gap is too large and only then move up.

The most serious indication that your car has already overheated, is when the needle hits the red zone or you see steam coming from the engine. At that point, find a safe place to stop immediately op, pull over and turn off the engine.

"Just pulling over but still idling the engine is not enough. Engines have to work harder to keep cool at idle than at cruising speed, so turn it off as soon as you can," he said.

Ideally you should then open the bonnet and wait at least half an hour for the engine to cool off.

"Only then is it safe to check the coolant level."

Look for the coolant reservoir (usually a plastic bottle) near the radiator; your car owner’s manual will show you exactly where it is.

If the level of coolant is normal - ie between the 'MIN' and 'MAX' marks on the side of the bottle - you might just have a malfunctioning temperature gauge, but if it's low or empty, the system is probably leaking somewhere, usually underneath where you can't see it.

"That means it's time to call roadside assistance."

"But if you have to keep going,” he said, “make sure the engine is cool, wrap your hand in a rag or a glove and twist off the radiator cap. Refill both the radiator and the reservoir, using coolant or, if necessary, water.

"Replace the cap and gently start the engine; as soon as the fresh coolant begins to circulate the temperature should come down - but don't relax, as you probably have a serious leak somewhere.

"If the temperature starts rising again, pull over and repeat the process.

"But this is by no means a long-term solution," he warned. "Get to an accredited workshop as soon as possible for the professional help your engine needs."