Bad Radiator Cap Symptoms, Causes, and How to Replace It

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The coolant or antifreeze cap, commonly known as the radiator pressure cap, is one of the most important parts of your vehicle. It holds significant importance within your vehicle’s cooling system. Its primary function is to maintain pressure and effectively contain the coolant within the radiator. A leaked or bad radiator cap produces different signs. This article explains the bad radiator cap symptoms, causes, and how to replace it.

What is the Function of a Radiator Cap?

The radiator cap plays a vital role in your car’s cooling system. The primary function of the radiator cap is to securely seal the top of the radiator filler neck, preventing any loss of coolant, pressurizing the cooling system, and effectively increasing the boiling point of the coolant.

Depending on the specifications of the vehicle manufacturer, radiator caps can be made of plastic or metal.

The radiator itself acts as a reservoir for carrying hot coolant, allowing it to cool down before recirculating through the engine for further cooling.

The caps on the tanks or reservoirs, including the radiator cap, serve as protective covers that not only keep the liquid contained but also serve a crucial purpose in managing temperature.

When the coolant temperature rises, it eventually reaches a boiling point, causing it to convert into vapor. By maintaining the system pressure at around 16 PSI, the radiator pressure cap prevents the coolant from boiling off, even when subjected to high temperatures.

Furthermore, this cap allows for the expansion of the coolant as it heats up. As the liquid expands with increased temperature, the cap enables the excess coolant to enter a reservoir tank. This ensures that the radiator of your vehicle maintains the proper amount of coolant during expansion.

Another important function of the radiator pressure cap is preventing the buildup of air within the cooling system. The presence of air can lead to inadequate cooling performance and corrosion to various parts of your cooling system.

Overall, the radiator cap of your vehicle assists as a vital part in maintaining the efficiency of the cooling system, managing sufficient pressure, preventing coolant loss, and protecting against possible cooling system problems.

Symptoms of a Bad Radiator Cap

When the radiator cap goes bad, it produces one or more of the below-given symptoms:

Leaking Coolant
Radiator Hose Collapses
White Streaks on Radiator
Radiator Hose Bursts
Low Coolant Level
Air Inside the Cooling System
Overheated Engine

1) Leaking Coolant

The presence of coolant pooling beneath your car is one of the clear symptoms of a bad radiator cap. When the radiator cap of your car becomes stuck, it stops the release of coolant, leading to a buildup of pressure within the radiator. Consequently, this increased pressure can cause hoses to leak or even rupture.

If there is coolant near the radiator or the cap, it indicates a coolant leak. It is important to inspect the radiator for any visible holes or signs of wear and damage on the cap. If such issues are detected, it is advisable to replace the cap.

Read More: Causes of Coolant Loss

2) Radiator Hose Collapses

The hoses connected to the expansion tank and radiator play a crucial role in circulating coolant and assisting the engine in maintaining a normal operating temperature. But if the radiator cap is damaged, it can lead to different problems, such as hose collapse.

When a radiator hose collapses, it resembles a flat tire and occurs due to unnecessary internal suction. If you open the hood and observe a bent radiator hose leading to the expansion tank, it is likely that the cap is preventing the coolant from returning to the cooling system from the expansion tank.

3) Overflowing Reservoir

An overflowing reservoir is also one of the clear signs of a bad radiator cap.

Under normal circumstances, when the coolant reaches a certain temperature, it produces sufficient pressure to open the pressure valve on the radiator cap. This allows the coolant to flow from the engine to the reservoir.

However, if your cap is malfunctioning, it may cause an excessive amount of fluid to be redirected to the reservoir, resulting in coolant being expelled through the overflow hose.

4) White Streaks on Radiator

If coolant escapes from the radiator filler neck and evaporates, it commonly leaves behind visible white streaks. Even if you don’t directly observe coolant leaking beneath the radiator cap, it’s important to be vigilant for these white streaks.

These white streaks can serve as an indication that your vehicle’s cap may be experiencing leaks under pressure or irregularly.

5) Radiator Hose Bursts

If the pressure within the cooling system of your vehicle becomes too high, there are high chances of witnessing one or more hoses spraying coolant in the engine bay.

However, it is uncommon for the pressure to be high enough to cause the hose to tear apart. Instead, you may notice a small pinpoint leakage that only leaks coolant once the vehicle engine heats up.

A hose with a small hole might initially seal effectively when the engine is cold. But when the vehicle run and the pressure in the cooling system builds up, the pressure ultimately becomes sufficient to force coolant to leak through the small hole, resulting in a slow loss of coolant over time.

In such a condition, you may observe a coolant leak in the engine bay or on the floor as you complete your journey, but not immediately upon starting to drive.

Pay attention to whether your overflow reservoir is gradually emptying after driving a few miles, as this can provide a clue to identify this issue.

Read More: Bad Radiator Symptoms and Causes

6) Low Coolant Level

In a properly functioning cooling system, the coolant remains sealed, meaning that if you had enough coolant one day, you should still have enough the next day.

If you find yourself consistently adding coolant, it indicates that your engine is either burning or leaking coolant.

The coolant is not disappearing without a cause—it is escaping from somewhere. If you regularly experience low coolant levels, it is worth inspecting the radiator cap to determine if it is the source of the coolant loss.

Read More: Low Coolant Symptoms and Causes

7) Air Inside the Cooling System

One of the common symptoms of a failing radiator cap is the introduction of air into the cooling system. However, detecting air in the system is challenging until the hoses and tubes begin to develop cracks.

When air spaces are present in the radiator system, especially during the vehicle’s shutdown and cooling process, the low pressure and vacuum conditions can lead to cracking of the hoses and tubes, resulting in coolant leakage.

8) Overheated Engine

A coolant leak or the presence of air in the cooling system may result in engine overheating. If you observe steam emanating from the engine due to excessive heat, it is important to avoid looking under the hood until you have completely shut off the engine. Allow the engine to cool down for a sufficient period before opening the hood.

By allowing the engine to cool, you ensure that it remains at a safe temperature while you inspect it. If you notice coolant near the radiator cap, it is possible that there is damage to the pressure cap. Take a closer look and change your cap if necessary.

Causes of a Faulty Radiator Cap

The radiator cap of your vehicle becomes damaged due to one or more of the following causes:

Age and wear
Damaged or stuck pressure-relief valve
Cracked or damaged cap seal
Corrosion
Contaminated coolant
Wrong cap type

Radiator Cap Location

Although the radiator cap is often overlooked, it is generally straightforward to locate. To begin, you should find the radiator itself, which is typically positioned at the front of the engine bay.

The radiator cap is located on the top of the radiator, commonly found on either the left or right side, although occasionally, it can be located in the middle of the radiator.

Radiator Cap Replacement Cost

The replacement cost of the radiator cap depends on your vehicle model and labor cost. The average replacement cost of the radiator cap is from $10 to $50. The radiator cap costs from $5 to $30, while labor only takes 10 to 20 seconds and is often free.

How do you Check a Radiator Cap?

Follow the following steps to check a radiator cap:

Start your vehicle and allow it to run for approximately 8 minutes before opening the hood. During this time, observe the cap and hoses for any signs of coolant leaks or the presence of bubbles.
If there are no visible indications of leaks, take precautions by wearing gloves or using a thick cloth. Slowly loosen the radiator cap, but avoid removing it completely to prevent the risk of burns.
As the coolant radiator cap loosens, it should release small amounts of steam and bubbles, indicating that the radiator coolant is operating within the recommended pressure range. Failure to release steam suggests a faulty radiator cap with a pressure leak.

How to replace a Radiator Cap?

Follow the below-given steps to replace your radiator pressure cap:

Ensure that your car is turned off and allow the engine to cool down completely. Don’t open the hood and touch engine parts while the engine is hot because they may cause injuries.
Find the radiator cap and carefully loosen it by slowly turning it counterclockwise. As you unscrew the cap, expect a slight release of pressurized steam from the radiator. Always use a thick cloth or safety gloves to handle the cap and prevent burns. After unscrewing the cap, gently remove it.
Take out the mounting shield bolts to reach the entire radiator.
After accessing the radiator, loosen the drain plug to initiate the drainage of the radiator coolant fluid. Use sand or baking soda to absorb any spills and dispose of the antifreeze properly. Clean up any remaining mess as thoroughly as possible, using towels or suitable absorbent materials.
After draining all the coolant, detach the mounting bolts from the reservoir and detach the coolant overflow tube from the radiator neck. It is essential to remove all coolant fluid and inspect the radiator components to make sure that no further damage has occurred.
Verify that the replacement radiator cap matches the specifications provided by the manufacturer.
Refill the radiator with fresh coolant and install the new radiator cap in place of the old one.
Tighten the mounting bolts securely, and start your car to test the repaired cooling system. Keep an eye out for any leaks or bubbles around the newly installed cap.

Frequently Asked Questions

How to temporarily fix a leaky radiator cap?

If you discover that your radiator cap is leaking due to improper sealing, using a temporary fix, like adding a tablespoon or two of black pepper, can help stop the coolant from leaking. The pepper acts as a temporary sealant, allowing the coolant to flow without leakage.

What happens when a Radiator Cap goes bad?

When the radiator cap fails to function properly, your car is at risk of overheating, which can lead to more severe issues in the future. The radiator cap’s primary role is to maintain the internal pressure of the cooling system. If the pressure becomes too high, it can cause components of the cooling system to break. Conversely, if the pressure is too low, the coolant may boil off, leaving the system without sufficient cooling capacity. Therefore, addressing a faulty radiator cap is crucial to prevent potential damage and maintain proper cooling performance for your vehicle.

Can a bad radiator cap cause bubbling?

Yes, a bad radiator cap can cause bubbling in the cooling system. The radiator cap is designed to maintain the proper pressure within the cooling system. If the cap is faulty or not functioning correctly, it can lead to a loss of pressure, resulting in coolant boiling and creating bubbles. This bubbling is a sign of excessive heat and pressure, indicating a potential issue with the radiator cap that should be addressed promptly.

Can a bad radiator cap cause overheating?

Yes, a faulty radiator cap may lead to engine overheating. The radiator cap assists in maintaining the ideal pressure in the cooling system, which is important to prevent coolant from boiling at lower temperatures. If the cap is faulty, it may not hold the required pressure, causing the coolant to boil at a lower temperature and leading to overheating.

How do I know if there is air in my radiator?

Use the radiator key to turn the valve located at the top of the radiator. Attach the key to the square groove in the center of the valve, as illustrated in the diagram, and slowly turn it in an anticlockwise direction. During this process, you may hear a hissing sound, which indicates the release of trapped air.

How long do my radiator caps last?

Naturally, radiator caps do not need frequent replacement. However, it is recommended to replace them every five years to avoid potential problems.

What is the pressure due to the radiator cap?

Mostly, the radiator caps are designed to maintain the system pressure at 16 PSI, allowing the coolant to reach higher temperatures without the risk of boiling off. Insufficient pressure in the cooling system can lead to coolant boiling.

What is an airlock in a radiator?

Airlock may occur when vapors are trapped in your vehicle’s heating system, obstructing the flow of water into the radiator and resulting in insufficient heat distribution. The heating system circulates hot water throughout the radiator to warm the interior of your vehicle. But when the water heats up, it may produce additional water vapor at times.

Written By

Engineer Rahela

Rahela is the owner and main author of Mechanicalspecial. She is an automotive expert writer. She have over four years of experience in the automotive writing niche and have completed over 700 pieces of content from 40 projects. She have good knowledge and skill in vehicle repairing, all mechanical work, car upgrades and maintenance.