What Is A Radiator Pressure Cap? Its Function And Diagram

A radiator pressure cap seals the top of the radiator filler neck to prevent coolant loss, pressurize the cooling system, and raise the coolant’s boiling point. Depending on the vehicle manufacturer’s specifications, they are manufactured with a metal or plastic design.

In this reading, we will explore what a radiator pressure cap is, its function, its diagram, how it works, and its symptoms.

Let’s get started!

What Is A Radiator Pressure Cap?

A radiator pressure cap is a small mechanical device that helps to control the pressure of the hot coolant in the radiator. The radiator is a reservoir that carries the hot coolant to cool it before returning to the engine for cooling. The caps on containers serve as cover that keeps the liquid in, but not just that.

As the temperature of a coolant increases, it will begin to boil eventually and then turn into vapor. The boiling point of the coolant is about 212 degrees F, which will rise when it’s under pressure. The radiator cap increases the boiling point of the coolant by about 45 F.

Functions of Radiator Pressure Cap

Sealing and protecting the coolant from the air outside and maintaining coolant pressure inside the radiator are the two major functions of the pressure cap. It features a two-way spring-loaded disc valve that opens and closes in response to the radiator’s coolant pressure.

The coolant pressure causes the valve to open at the maximum pressure point, which is typically between 15 and 16 psi, allowing extra coolant to flow into the overflow tank. By releasing the extra liquid, the pressure decreases and the cap valve returns to its closed position.

As a result, when the engine is running, the pressure cap helps to maintain the coolant’s ideal pressure. The coolant temperature will be at its typical level when the engine is shut down. There is less pressure in the system because the hot liquid is not moving through the radiator.

A vacuum is created within the system as a result of this pressure drop, which pushes coolant in the reverse direction. This means that the secondary radiator cap valve is responsible for returning the overflow tank’s surplus coolant to the water pump.

Diagram Of A Radiator Pressure Cap

A radiator pressure cap typically consists of several key components, including a cap body, rubber seal, spring, pressure relief valve, vacuum relief valve, and check valve.

Radiator Pressure Cap

Related: What Is A Cooling System? Its Diagram and How It Works

How A Radiator Pressure Cap Works

The workings of a radiator cap are considerably easier to understand as they increase the boiling point of the coolant. This is done in the same way a pressure cooker increases the boiling temperature of the water. The device is a pressure release valve that is set to 15 PSI on a car.

The boiling point of water increases when the water is under pressure. The fluid in the cooling system expands when it heats up, causing the pressure to build up. In the whole coolant system, the cap is the only place where this pressure can escape.

The spring on the cap determines the maximum pressure in the cooling system. The pressure pushes the valve open when the pressure reaches 15 PSI. This allows the hot coolant to escape to an overflow tank through a thin hose.

Air cannot be present in this coolant-bleeding arrangement. As soon as the radiator cools, a vacuum is created in the system that pulls another spring-loaded valve, sucking water back through the bottom of the overflow tank to replace the water that was earlier saved.

Symptoms Of Bad Radiator Cap

A bad radiator cap can lead to several symptoms that indicate it’s time for a replacement. Here are some common signs:

Leaking Coolant

When the radiator cap gets stuck, pressure will build inside the radiator. This will lead to a leak or burst of the cooling system components. This issue can be obvious; a DIYer would be able to replace the pressure cap and even some leaky parts when such occurs. It’s important to check the cap regularly to prevent any potential issues.

White Streaks On The Radiator

Coolant leakage from the radiator filler neck often leaves a white streak when it dries. This is another way to know the radiator pressure cap is faulty; just check for these white streaks. The problem is known to be the cap leaking under pressure or intermittently.

Overflowing Reservoir

As the pressure increases, the coolant expands, and the pressure cap allows some of it to flow into an external reservoir tank. A bad cap will delay or quickly release the coolant, which will cause overflow to happen. Always ensure the overflow tank is also in good working condition.

Radiator Hose Collapses

The radiator cap may become faulty if the radiator hose collapses. This is because the cap won’t properly release the vacuum, which could also lead to the radiator hose breaking down during the cooling process.

How To Test a Radiator Cap

Make sure the rubber or fiber sealing ring is in good condition, and test the large pressure relief valve by pressing it in against its spring with some effort. Also, ensure that the small valve in the center moves freely.

Warm up the engine to perform a rough check on the working efficiency of the cap. Wear a stout glove and wrap the cap in a thick cloth. Now, loosen it to its first stop without removing the cap. There should be a hissing sound of air or steam.

But if there’s none, the cap isn’t maintaining the proper pressure in the system. It needs to be replaced. Get a radiator pressure cap that has the correct pressure rating. When the boiling point of water in a pressurized system rises, a weak valve will not be able to maintain the pressure.

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