What Is a Radiator Cap? Functions, Symptoms, Replacement, and More

A car’s cooling system keeps the engine running at a safe temperature to prevent overheating. One component that aids in maintaining proper temperature is the radiator, which disperses a mix of antifreeze and water to release the engine’s heat.

The radiator has another component attached to it called the radiator cap. Despite its small size, the cap plays a big part in keeping the engine safe.

What Is a Radiator Cap?

The radiator is fitted with a filler neck that goes on top of the radiator, which is called the radiator cap. The cap has a spring-loaded valve that closes the cooling system vent, causing pressure to build up to the pressure setting of the cap.

Once it hits a certain threshold, the valve releases the excess pressure to prevent damage. Essentially, the cooling systems are pressurized to raise the boiling temperature of the coolant.

• The boiling temperature increases by 3°F for each pound of increase in pressure.
• At sea level, water boils at 212°F. With a 15 PSI (pounds per square inch) pressure cap, water boils at 257°F, the maximum operating temperature of engines.

This process serves two functions.

First, it allows the engine to run at an efficient temperature, negating the potential dangers of boiling the coolant.

Second, the higher the coolant temperature, the more heat the coolant system transfers. The transferred heat is proportional to the temperature difference between the coolant and outside air.

What Happens If Radiator Cap Pressure Is Too High?

Excess pressure in the radiator cap can damage the radiator because it puts strain on the components, which could lead to cracks or leaks.

The increased pressure could also cause the engine to overheat. This occurs because, as mentioned, the cap regulates the pressure inside the system. Once it goes above the intended threshold, the engine might not cool down properly.

How to Tell If the Radiator Cap Is Bad

Some symptoms of a bad or faulty radiator cap include an overheating engine, coolant leaks, air pockets in the cooling system, a collapsed radiator hose, and steam coming from under the hood.

Overheating

As mentioned, a defective radiator cap can cause overheating.

You might notice it once you hear an audible ticking or knocking sound coming from the engine, signifying a loss of lubrication. This is often due to the excess heat causing the oil to get thinner, negating its lubricating properties.

In the end, overheating can result in wear and damage due to the increased friction between parts.

Coolant Leak

A faulty rad cap can cause excess pressure buildup in the cooling system, forcing the coolant to find an escape route, either through the radiator hoses or the water pump seal.

As a result, coolant leaks, which leads to low coolant levels and eventually overheating. Once the coolant dries and burns off, it typically leaves white stains or streaks on the radiator.

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Air Pockets in the Cooling System

Air may enter the cooling system through the damaged radiator cap, creating air pockets within the cooling system. The pockets could reduce the coolant’s effectiveness, create bubbling sounds from the dashboard, or fluctuate the temperature on the gauge.

There are a handful of solutions for air pockets in the cooling system, but the most effective one is bleeding it to remove the trapped air. This involves running the engine and topping up the coolant as air escapes.

Collapsed Radiator Hose

The pressure in the cooling system drops when the engine is turned off. This creates a suction effect that could result in the radiator hose collapsing if the rad cap is defective. It’s mainly due to the pressure inside the system being lower than the atmospheric pressure.

The worst-case scenario is a burst radiator hose due to the excessive pressure in the cooling system. While it’s typically uncommon, it is still possible, especially with a bad cap. A leaky coolant below the car is one of the probable symptoms.

Steam From Under the Hood

Steam from under the hood could indicate different issues, one of them being a faulty radiator cap. This occurs when a radiator hose leak drips coolant onto the hot engine, resulting in vapors or steam.

Similarly, a faulty rad cap may not maintain the necessary pressure and cause the liquid coolant to boil, resulting in steam coming through the coolant system. It also decreases the coolant level over time, which could result in overheating.

How to Remove and Replace a Radiator Cap

A bad radiator cap is repairable, but it might be better to replace it, as some solutions to physical damage are only temporary. To replace a faulty rad cap, follow the steps below:

• Step 1: Let the car sit for around 30 minutes to cool the engine down and prevent potential accidents or injuries caused by the hot engine. Afterward, look for the radiator, which is usually at the front of the engine compartment behind the vehicle’s grille.
• Step 2: Remove the cap by either pushing down and turning the cap clockwise or twisting it counterclockwise. The proper way of removing the cap depends on its type.
• Step 3: Inspect the cap for damage by checking the rubber disc inside to look for cracks. If there are, replace the rad cap immediately. Look for a replacement with the same PSI rating as the original for better compatibility.
• Step 4: Put the replacement cap back onto the radiator by pushing and twisting it in or by twisting it back clockwise, depending on how you removed the cap in the first place.

If you can’t replace the radiator cap on your own, you can always visit an auto shop. This way, you’re guaranteed to get a proper replacement, as the mechanics will have more knowledge and experience working with rad caps.

The price of replacing the cap differs depending on the car model, but expect it to cost around $10 to $50. New rad caps cost $5 to $25, while labor fees are usually free of charge due to how quick it is to replace them.

How Often Should You Replace the Radiator Cap?

Replace the radiator cap every four or five years. Rad caps naturally wear out over time and won’t be as efficient even if they haven’t failed yet. This could affect your cooling system and result in damaged components.