How your radiator cap helps your engine beat the heat

auto radiator cap

auto radiator cap

How your radiator cap helps your engine beat the heat

Most people have no trouble locating and identifying the radiator cap in their vehicle’s engine. But do you know how it works?

While the radiator cap, also known as a pressure cap, is just a small component of your vehicle’s cooling system it plays a vital part in helping the engine keep its cool and providing you with trouble-free motoring.

Unfortunately, most people never spare a second thought for their radiator cap, sometimes even for the entire life of their vehicle. However, this is not something that we at ADRAD National Radiators recommend you do.

If you notice an unexplained water loss from your vehicle’s cooling system or you find your vehicle’s engine is running a little hotter than usual, there could be a problem with the radiator cap. A radiator pressure cap is a relatively cheap automotive item, making the cost a very small price to pay for insurance against the more dire consequences of overheating.

That’s why, as members of a specialist radiator repair company, our nationwide network of radiator agents advise that you replace your vehicle’s radiator cap every two years, when you get your radiator service done, and drain and flush the radiator.

Understanding the basics of your vehicle’s cooling system

When in operation, a vehicle’s engine generates a lot of heat. That’s OK to a point, because engines are designed to operate efficiently within a certain temperature range – usually between 85° C and 96° C. It’s when things heat up beyond that range that the engine gets into trouble – big trouble that can end up costing you a bundle in repair costs.

That’s why engines are also designed with a pressurised cooling system to keep them operating within their optimum temperature range. The pressurised cooling system is designed to do 3 main tasks, namely, to:

  • Enable the engine to quickly warm up to its ideal operating temperature.
  • Circulate coolant through the engine to maintain that ideal operating temperature.
  • Maintain the right pressure and level of circulating coolant by directing excess coolant to and from the overflow tank as the coolant heats up and cools down.

Why is the right mixture of cooling liquid important?

The liquid in an engine’s cooling system should be around a 50/50 mix of water and coolant or antifreeze.

The job of the water is to take the heat out of the engine. However, water’s boiling point is too low and too close to the optimum operating temperature of the engine for it to be able to cool the engine effectively on its own. Water’s freezing point is also too high, and water also allows corrosion to happen within the system.

So, we have to add coolant or antifreeze to the mix. The coolant enables a higher boiling temperature and a lower freezing point, and it also contains inhibitors that prevent corrosion.

With the right water-to-coolant ratio in the system, a vehicle’s cooling system can manage temperatures as low as -37° C and as high as 106° C.

But wait, the story doesn’t end there. In fact, this is where it starts to get really interesting, and leads eventually to the little old radiator cap suddenly taking on a starring role.

Why does the cooling system need its radiator pressure cap?

Remember how we said that engines are designed to operate efficiently within an 85° C to 96° C temperature range? And how the right water/coolant mix will increase this to about 106° C?

OK, so that gives the engine a little bit of wiggle room, but not very much or nearly enough, especially if it has to work hard or in the warmer months.

Now this is where the beauty of the pressurised cooling system takes centre stage, because when you put a liquid under pressure the point at which it will boil goes up.

As we said, the boiling point of the right water/coolant mix was around 106° C, but that is when the liquid is only under 1 psi (or 1 pound of force per square inch) or 6.9 kPa (or kilopascal – 6900 newtons per square metre) of pressure. However, most automotive cooling systems are designed to operate at around 15 to 18 psi or 103 to 124 kPa.

As the engine gets going and its coolant heats up, the coolant also begins to expand and create additional pressure. By allowing that pressure to increase, the coolant’s boiling point also increases to around 125° C at 15 psi / 103 kPa, which means the fluid in the cooling system can absorb even more heat from the engine without boiling.

At 125° C the cooling system and the engine now have a lot more wriggle room than they would have at 106° C for staying safe and being able to operate at maximum performance. However, heat and its corresponding pressure know no bounds and they’ll party on together until disaster strikes, unless something can control them.

The radiator cap – a pressure-releasing gizmo

So what happens when the contents of the cooling system continue to heat up, building up even more pressure to the point where it exceeds the cooling system’s psi/kPa rating? If there’s no way to regulate the increase of pressure the end result would be an explosion.

Cue the radiator cap.

When the heat causes the pressure in the vehicle’s cooling system to increase above its psi/kPa rating, the radiator cap, by means of a spring-loaded valve, releases some of the coolant from the system into the overflow tank. As coolant exits the system, the pressure drops. Once enough coolant has left the system, the valve in the radiator cap closes. This is how the vehicle’s cooling system is able to maintain its optimal pressure rating while the engine is doing its job.

Later, when the engine is turned off and starts to cool, the pressure in the cooling system drops, thereby creating a vacuum in the system, which is not something you want. At this point another valve in the radiator cap opens to allow the coolant that was pushed by the pressure into the overflow tank to return to the cooling system.

Simple but very, very effective.

Have you checked you radiator cap?

Now that’s a lot of work – and work under pressure – for your vehicle’s little radiator cap to have to do day in day out and right in the heart of all that heat too. This hard work can easily make it tired and worn out, and, over time, cause those pressure-releasing valves to malfunction. When that happens, your vehicle’s cooling system will no longer be able to do its job properly, and the engine could be facing a potentially life-threatening situation.

That’s why at the start of this article we said that as radiator specialists we recommend that you replace your vehicle’s radiator cap every two years, at the same time you drain and flush the radiator.

Simply pop into one of our nationwide network of radiator repair specialists and we’ll give your vehicle’s cooling system a thorough check-up, including the radiator pressure cap. Call us on 0800 723 428 and we’ll get it sorted for you.