How Does a Radiator Cap Work?

These days cars and motorbikes have radiators to help cool down the engine. Cars usually have high-power engines, which require an effective cooldown to prevent any damage to the engine or its performance. The fluid inside the radiator allows air to pass through the small gaps and then flowing air cools down the water and rest of the engine.

Among all those, there is the radiator cap, which holds a significant role and most people doesn’t pay attention to the same. If you have ever come to the discussion of radiator cap working and want to know more about the same, then have a look at below given four points –

radiator cap

1. Helps in Increasing the Boiling Point

In the closed cooling system of a radiator, the cap helps in building the pressure by not allowing air to pass through. As the pressure rises, the boiling point of fluid present in the radiator also increases which further helps in effective cooling down.

The boiling point increase by three degrees. It may not seem that high, but in practical life, there is a big difference in performance. When 16 pounds of pressure is increased in the system, then the cap will prevent again boil offs and the boiling point of fluid rise to almost 260 degrees.

2. Allow Fluid to Flow

As the coolant heats and expands, the volume starts increasing which leads to building more pressure in the system. Radiator caps are rated up to a degree of pressure mark and when the threshold reaches, the cap allows fluid to flow. Thus, fluid can flow into the reservoir tank to cool down.

In simple words, the radiator cap is helping in many ways. It is keeping the pressure in control and when the temperature rises, it further helps in allowing the fluid to flow for the pressure to remain constant.

3. Vacuum Valve

To flow back fluid from the reservoir tank to the radiator, there is a need for any motor pump or vacuum for the same. Thus, the radiator caps have a simple mechanism of the vacuum valve. This vacuum valve plays a crucial role in flowing back fluid into the radiator. As the fluid cools down, the pressure starts going low and it starts contracting.

The level in the radiator start going up and the radiator will function properly. The entire system works on maintaining the level of fluid and offering the perfect balance. The radiator cap helps in maintaining the level of fluid, pressure and cooling down the fluid effectively for proper functioning.


  1. What’s the cost of a radiator cap?

A radiator cap usually cost between $7 to $15 based on the car model and manufacturer.

  1. What’s the Pressure maintained by the radiator cap?

A radiator cap is rated for 15 PSI (Pound-force per square inch) and it can handle more, but the cap allows pressure to stay under 15 to 16 psi by allowing fluids to pass through.

  1. Do radiator caps have a constant size?

No, pressure caps are of different sizes based on the car model, size, and manufacturer.

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