Radiator Cap

Radiator Cap

A radiator cap is a safety device that allows for protecting the engine cooling system from damage, while keeping it properly pressurized for higher efficiency. A failing radiator cap may cause a variety of engine problems and expensive repairs if not fixed in time. Here are the most common of them:

Coolant leaks

The job of a radiator cap is to prevent excessive pressure buildup in the cooling system. If your radiator cap gets clogged, the valve inside the cap will be blocked and not open when necessary.  As a result, the excessive pressure will find its way through the cap itself or a cooling hose, or another cooling system part.

Engine overheating

Your radiator cap is designed to withstand high pressure needed to increase the boiling point of the coolant inside the system. If the cap fails to maintain the pressure, the coolant will boil at lower temperatures, losing its ability to absorb heat from the engine, so it will quickly overheat. On the other hand, a poor cap seal may allow air to enter the cooling system, which will prevent proper coolant circulation inside the system and cause overheating.

Whichever the root of the problem, you should shut down the engine once you notice it is getting overheated. Keep in mind that the engine and other parts under the hood can be extremely hot, so you should not open the hood and remove the radiator cap until the vehicle cools down in order to avoid personal injury.  

Your radiator hose has collapsed

If your radiator cap doesn’t allow coolant from the expansion tank to be drawn back into the cooling system, the negative pressure inside the system will cause a collapsed hose. 

Steam from under the hood

As it was described above, a malfunctioning radiator cap can reduce the boiling point of the coolant. When coolant boils, it transforms into a lot of steam that is released through the overflow tank and the radiator cap. If you see steam coming from under the hood, don’t try to open it. Shut down the engine and wait until it cools down. Moreover, you shouldn't start the engine if the coolant level is low. Top it up and call your mechanic for instructions.

How a radiator cap works

Placed on the top of a radiator, a typical radiator cap features a spring-loaded plunger which serves as a one-way release valve. When the engine is cold, the cap can be removed to add coolant or to check its level.

Under normal conditions, a radiator cap allows for keeping the cooling system pressurized, which increases the boiling point of the coolant and, therefore, its physical properties. This helps to protect the engine from overheating at higher operating temperatures, which improves its performance and fuel economy. When the pressure exceeds a set value, the cap opens to release excessive pressure by redirecting it into the expansion tank.

How to fix a bad radiator cap

A bad radiator cap is not repairable. However, it is not expensive as well as easy to take off and replace. When purchasing a replacement, make sure it has the same pressure rating as your original cap.

A radiator cap can be visually inspected for cracks and other kind of damage. If you notice any signs of damage, replace it with a new cap. Besides, even if the cap has no signs of damage, but its plunger provides abnormal resistance, it also must be replaced. The resistance of the cap can be checked by means of a pressure tester.  

Any signs of rust or corrosion on your cap should also lead to its replacement. Fortunately, it is an inexpensive part that is easy to replace.

Oil, foam or incorrect color coolant found on the cap can alert you of more serious engine problems, like mixing oil with coolant. If so, call your mechanic for advice and don’t try to start the engine. 

Radiator cap maintenance 

While a radiator cap is an inexpensive part, the damage it can cause may cost you a pretty penny. That’s why if you notice any signs of a bad radiator cap, replace your cap as soon as possible to avoid damage to more expensive components, such as your radiator, expansion tank, and engine.

When replacing a bad radiator cap, make sure it has been the only source of the problem. It’s always a good idea to check the entire cooling system for leaks and possible damage. Another great idea is to read the manufacturer’s recommendations before working on the cooling system. Some engines require purging all the air from the cooling system before filling it.