Radiator and Transmission Maintenance

Why and How to Replace a Radiator Cap


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While a simple component of your cooling system, your radiator cap plays a crucial role in maintaining your vehicle’s cooling efficiency. Radiator caps are cheap, easy to find and take only a few seconds to replace, but ignoring a faulty one can cause overheating problems and potential engine damage, especially during the hot summer months.

Pressure is Crucial

A radiator cap generally contains a spring loaded plunger, and this plunger serves two purposes in your cooling system. First, should internal pressures exceed the spring pressure of the plunger, coolant will be diverted to an overflow reservoir. This is important because if this simple feature did not exist, pressure would simply continue to build, causing sudden leaks or even explosions as the building pressure desperately tries to find a way to relieve itself. Second, this spring pressure helps to maintain a specific pressure inside your cooling system. As atmospheric pressure rises, the boiling point of a fluid also rises. Boiling fluid isn’t very good at transferring heat, so by increasing the pressure inside the system, you increase the amount of heat the cooling system can remove before it begins to boil.

Diagnosing a Faulty Cap

A faulty radiator cap will show a few common symptoms that are usually easy to see if you are looking for them. Coolant should move back and forth between the overflow reservoir and cooling system as pressure dictates. If you are overheating and coolant is not moving effectively between these two areas, the spring loaded plunger may be stuck or jammed in place. A cap that is unable to maintain pressure will often show coolant boil at normal operating temperatures. This boiling can be heard quite easily, and witnessed by looking closely at the overflow reservoir. Leaking from around the radiator cap is also a common and easy to notice symptom, but be careful not to confuse this with freshly filled coolant, as slight spillage is common during refills.

Replacement Tips

Replacing the cap on your radiator is an easy thing to do and may only require a few seconds of your time. It is extremely important to always make sure to allow the engine to cool down before removing the radiator cap, as removing the cap from a hot system can cause sudden coolant spray and severe burns. Pay close attention to the pressure rating on your cap, and select one with the same pressure rating. Once you have a replacement and your engine is cool, simply unscrew the old cap, and install the new by screwing it on.

A faulty radiator cap can have a huge impact on the cooling performance of your vehicle. If your coolant is boiling at even normal temperatures, or you seem to be having problems with your coolant moving back and forth between the cooling system and your overflow tank, consider taking a look at your radiator cap. Replacements are cheap and easy to obtain and can sometimes be the simplest solution to radiator overheating problems. While a cap may be easy to replace however, it is not a substitute for proper radiator maintenance. Proper coolant changes and cleaning are key components to a healthy cooling system.

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