How does a car radiator cap work?
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Straightforward answers and explanations for DIY mechanics
We believe that every car owner should know a few basics about car maintenance. Being an amateur mechanic can get you out of a jam and save you quite a bit of money if you apply your knowledge correctly. Understanding your radiator and the importance of the radiator cap is a good place to start.
In this article, we’re going to answer some frequently asked questions about how car radiator caps work and how to work safely with them.
With workshops dotted around Australia, you’re probably never too far from one of our Natrad radiator specialists. If you’re having issues with your radiator cap or your cooling system, feel free to come in for a fix — or just call us for advice.
What do radiator caps do?
A radiator cap isn’t just a lid — it’s a complex pressure valve that maintains safe pressure levels within your vehicle’s cooling system.
As the coolant draws heat out and away from the running engine, it expands and pressurises the entire cooling system. The cap’s internal pressure spring helps seal the cooling system until it reaches a certain pressure level. This allows over-pressure to be relieved as the expanding coolant is allowed back into the overflow tank (also called the expansion tank).
When the engine cools down again (bringing the coolant temperature down with it), the cap’s vacuum valve opens to allow the coolant fluid from the overflow tank to return to the radiator system without letting in any air.
Why do radiators need to maintain pressure?
The pressure increases the coolant’s boiling point, making it able to provide cooling for the engine at higher temperatures. A top-notch radiator and cooling system will allow your car’s engine to maximise its full potential.
Can a bad radiator cap make your car overheat?
Yes. A faulty radiator cap can fail to hold pressure. If this happens, your engine will overheat if its temperature exceeds around 135°C as the coolant will boil. Bubbles and air pockets diminish the effectiveness of the cooling system and can deprive some components of cooling altogether.
This excess heat building up in your engine block or in certain components can cause irreparable, expensive damage.
A faulty radiator cap can even contribute to your radiator exploding.
Can you start a car with the radiator cap off?
Yes, you can start a car with the radiator cap off. In fact, turning the engine on with the cap off can help remove air bubbles which is part of the process when topping up your radiator coolant level.
However, we strongly recommend only removing your radiator cap while the engine is cool since hot, pressurised coolant can sputter and burn you.
Can I open a radiator cap when a car is running?
It is possible to open a radiator cap while the car’s engine is running, but it can be dangerous. Do not open the radiator cap if the engine has been running for a while or if the radiator feels hot. The hot, pressurised fluid could erupt and scald you.
You may have the cap open while the engine is running if you’re bleeding the radiator after filling it.
Why is my car overheating, but my radiator cap is cold?
Your radiator cap remaining cold after your engine has been running for some time is a symptom of a significant problem. A serious cooling issue should be inspected by a radiator specialist or mechanic.
We can say, however, that the reason your radiator cap isn’t heating up might be because of one of two reasons: there isn’t enough coolant in your cooling system, or your coolant isn’t circulating properly.
We recommend you bring your car into your local Natrad store for our radiator specialists to review. Take extra care while driving to avoid overheating. We can help you learn more about how long you can drive your car without a working radiator.
How much is a radiator cap for a car?
Car radiator cap replacements typically cost $20-30. Specialist radiator caps or those with a high PSI rating can cost more.
Should I just buy the highest PSI-rated radiator cap available?
Radiator caps for passenger cars are typically rated at 15 PSI or thereabouts. Ratings vary by engine, radiator, and vehicle make and model.
We’ve explained how increasing the pressure within the cooling system raises the boiling point of the coolant inside and how this makes the engine cooling more effective. But there’s an upper limit to how much pressure your system can bear.
While a high PSI-rated radiator cap may be able to withstand high pressures, other components in your car may not. If the pressure inside your cooling system exceeds the limit that fittings and connections are rated for, it can cause a significant breakdown.
Buying the highest PSI-rated radiator cap will not necessarily improve your cooling system. In fact, it could do just the opposite. The safest thing to do when buying a replacement radiator cap is to purchase one that matches your system. Check your car’s manual to find the right radiator cap for your vehicle.
How do I know what PSI rating my radiator cap needs to be?
Not all radiator caps are the same. If you need to buy a replacement radiator cap, you should check your car’s manual to see what type you need to buy. Our technicians can also help you figure it out if you bring your car in.
Got more questions about radiator caps? Let’s talk!
Natrad has been a leading cooling system specialist for 40 years. We know radiators and cooling better than most! Whether you need advice or assistance with your DIY autocare project, don’t hesitate to give us a call.
