When should I replace my car's engine coolant?

Today's technologically advanced combustion engine demands a technologically advanced coolant. Water just doesn't cut it anymore – and particularly not in an age when virtually every car on the market has an alloy head.

Engine coolant now comes out of an off-the-shelf plastic container – not a tap – and is mixed with propylene glycol, which lowers the solution's freezing point to as low as -37 degrees and raises the boiling point as high as 129 degrees when used in a pressurised cooling jacket.

Manufacturers will stipulate in the owner's manual for each vehicle how often the engine coolant should be checked, topped up and/or flushed. If unsure whether your car is due for its engine coolant to be checked – the service techs at the nearest dealership will look after this for the vehicle's next service. But it is important that the vehicle be serviced and any consumable items, including the coolant, be replaced or topped up at the time.

In years past many cars fell victim to corrosion in the cylinder head after prolonged exposure to tap water in the engine's (cast-iron) cooling jacket. This process – corrosion escalating when two differing metals are in contact with each other – is known as galvanic corrosion. The two different metals in this case are the aluminium alloy of the engine's cylinder head and the cast-iron cylinder block. Even separated by a gasket the two different metals can corrode in such close proximity.

Over time, the coolant can pick up and carry metal particles from the engine around the cooling jacket. The anti-freeze/anti-boil properties are diminished with this extra 'plaque' carried along, making it important for the engine's long-term durability that the coolant is flushed and replaced in accordance with the manufacturer's recommendations.

The build-up of corrosion within the cooling jacket can restrict the flow of coolant around the engine. This can increase pressure within the jacket, placing stress on radiator hoses, and reducing the ability of the engine's cooling system to bleed off heat from the engine.

As a consequence, the engine runs consistently hotter, which is not necessarily good for thermal management and efficient combustion. And if a radiator hose bursts under pressure, the entire contents of the cooling system may be discharged and the engine may overheat catastrophically before the driver even becomes aware there's a problem.

Overheating will warp the very expensive alloy head, it will lift away from the engine block and the cylinders will lose all compression immediately. Apart from the expense of removing, machining and refitting the cylinder head – with a new gasket too – there's also the possibility of valves and pistons being damaged.

That's costly, but there's also the inconvenience factor to consider.

Not only is the car off the road for days, undergoing repairs, it's just as likely to have broken down in the middle of nowhere – possibly late at night as well – in an area without adequate mobile phone reception.

So we cannot stress enough how important it is to ensure the engine cooling is checked regularly, in accordance with the manufacturer's maintenance schedule – and if need be, topped up or flushed and replenished.

It's easy to tell when engine coolant is fresh and new – it's usually fluorescent green in colour, containing silicates (combining silicon and oxygen) or phosphates based on phosphoric acid, or organic acid technology compounds.