Radiator Cap Functions and Malfunction Symptoms

Although the name suggests that the radiator cap is just a normal cap holding the coolant inside the radiator, that’s only half of its function. The radiator cap is actually a two-way valve, holding the coolant inside, like we said, but also maintaining system pressure.

How does a radiator cap work? 
When you heat a liquid to a certain point, it starts to boil and eventually vaporize, changing its properties by going from a liquid to gas. In normal conditions, water boils at 212 degrees Fahrenheit, but by pressurizing it we can raise its boiling point substantially. Cooks do this all the time. They use a pressure cooker to cook food faster thanks to the hotter water inside. This is the same concept with the radiator cap and the radiator.

The pressure inside the radiator is kept at 16 pounds per square inch (in most cases), and it’s all regulated by the radiator cap. It’s a very delicate system. If the pressure isn’t maintained, the coolant can boil and evaporate, putting the engine at risk of overheating. On the other hand, if the pressure builds up too much you better hope that a hose lets go before the radiator ruptures. If that happens, you’ll be looking at a brand new radiator, and those aren’t cheap.

Radiator cap malfunction symptoms
As such, detecting a bad radiator cap early can be extremely good on your wallet and watch. A skilled mechanic can check whether your radiator cap is fit for the job in less than five minutes, but if you want to do it yourself or simply want to learn about inspections and the symptoms of radiator cap, continue reading this article.

The first symptom is coolant leak, whether it be from the radiator or the cap itself. A punctured radiator can be the result of a bad cap. The cap doesn’t just let go suddenly. In most cases, there are some tell-tale signs, the first one being coolant escaping from your cap.

Another obvious symptom is engine begin to overheating. If the coolant has escaped or isn’t allowed to move freely, you might notice your engine temps rapidly rising. If that’s the case, you want to immediately stop or you risk overheating the engine, which is never a good thing.
Read also: Symptoms of a bad idle control valve

Radiator cap inspections
Check the leaking in cap area

If you notice liquid around the edges of the cap, investigate further. With a bad cap, you can even hear antifreeze bubbling away sometimes. But be careful. Do not open the cap until the car and the coolant have a chance to cool off. Opening the cap when the car is hot can seriously scar you if the hot coolant sprays all over you (and it usually erupts like a geyser).

Check the plunger condition

The spring-loaded plunger in the cap can get jammed, trapping the coolant in a certain place, restricting access from the cooling system to the overflow reservoir. A quick visual inspection of the overflow reservoir will show if that’s the case

Test the radiator cap using pressure gauge

The cap can also be tested with a pressure gauge. If the pressure drops when testing it, try to clean the cap and test it again. If it gives the same results again, it needs to be replaced. Replacement caps are easy to come by. They’re very cheap and all you really have to do is buy one with the exact same pressure rating. Again, make sure that the car has cooled down before replacing the radiator cap if you want to avoid burns.