fairly used radiator cap

Understanding Radiator Pressure Cap

Radiator cap despite how small, its design is so functional and adds good benefit to the working of an engine cooling system. You, reading this article you’re expected to have knowledge of how automotive cooling system works. If you don’t, we’ve got you covered in a fresh post that you should check out.

Most drivers overlooked the function of a radiator pressure cap in their car but a mechanic can tell the benefits. Well, apart from being a cover that prevents the radiator from pouring out, what else could be its function, you’ll get to know?

fairly used radiator cap
Used radiator cap. The picture is taken on white background.

Today we’ll be looking at the definition, functions, components, diagram, working principle, bad symptoms as well as advantages and disadvantages of a radiator pressure cap.


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 in order to be cool 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 actually increases the boiling point of the coolant by about 45 F.

Functions of Radiator Pressure Caps

Below are the functions of radiator caps in automotive engines:

  • A radiator pressure cap keeps the system pressure at 16 PSI, which prevents the coolant from boiling off even when it gets hotter considerably.
  • Because liquid expands as it gets hotter, the pressure cap allows coolant to escape to a reservoir tank when it expands. This is to keep a proper amount of coolant in the radiator when it expands.
  • The radiator pressure cap prevents the trapping of air in the cooling system. The air in the system will cause poor cooling and cause corrosion to the cooling system components.
  • The coolant in the external reservoir will be pulled back, leaving no room for air.
  • The pressure cap is an important component of the proper working of an engine. it should be periodically inspected and considered a replacement if it fails. Ensure 5o/5o water and anti-freeze agent are used.

The components of a radiator pressure cap consist of a shaped tin metal sheet at the top. Its functional parts include a spring, valve, thin rod, and flat rubbers. All these will be explained in the working principle.

The complete diagram of a pressure cap:

radiator cap diagram

Working Principle

The working of radiator pressure caps is considerably easy 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 actually a pressure release valve that is set to 15 PSI on a car. The boiling point of water increases when the water experiences 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.

In this coolant bleeding, air is not allowed in the 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.

The reason why this article is published is for users and professionals to see the essence of this device. They tend to overlook the radiator cap when troubleshooting vehicle cooling systems for overheating.

Watch the video to learn more about how radiator pressure cap works:

How to Test a Radiator Cap

Testing a radiator cap can be done in just two easy steps which include:

Removing the pressure cap when the system is cool and when the engine is not running. The seals should be inspected for damage as the heat cycling could cause the seal to harden the rubber. Damage seals are the common source of leaks which is why they should be replaced. A broken recovery valve is another common damage that must be examined when testing a radiator cap and a rusted spring should be replaced.

In the second step, the device should be installed into a radiator cap adapter supplied with the tester set. The adapter is like a radiator filler neck on both ends. The cap should be placed on one end, and the other end of the adapter should be attached to the pressure tester. Since radiators are available in different sizes depending on the engine so as the caps. The proper size and shape of the adapter as the radiator filler neck should be employed.

The pressure tester should then be pumped to the pressure stamped on the radiator cap. The cap is faulty if you notice the pressure releases the pressure before reaching the correct. This means the cap does not hold pressure. The process should be trying to confirm the cap is faulty.

Symptoms of Bad or Failing Radiator Cap

Below are the common signs or symptoms of a failing radiator pressure cap:

Leaking Coolant:

When the radiator cap gets stuck, the pressure will be built 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 leak parts when such occurs. The cap should be checked on a regular basis to avoid such problems.

White Streaks on 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 for the cap leaking under pressure or intermittently.

Overflowing Reservoir:

As the pressure occurs the coolant expands and the pressure cap allows some portion 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 get faulty if the radiator hose collapses. this is because the vacuum won’t be properly released by the cap and can also cause the radiator hose to break down during the cooling down period.

Related Article:

In conclusion, the radiator pressure valve is one of the vital components in the engine cooling system. We’ve seen its function which one it is to pressure the coolant. We saw how it works and how it can be tested in two steps. Finally, we also examined some symptoms of a bad radiator cap. I hope you enjoyed the reading, if so, kindly comment, share, and recommend this site to other technical students. Thanks!


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