Cooling System Pressure Caps Explained

Pressure caps are among the smallest components in a cooling system, yet they have a direct influence on temperature stability, coolant behaviour, and long-term reliability.

They’re often misunderstood, incorrectly uprated, or blamed for problems they didn’t cause.

Although commonly referred to as radiator caps or header tank caps, both perform the same function. The only difference is where the cap is located within the cooling system.

What a Cooling System Pressure Cap Actually Does

A pressure cap is not just a lid. It is a calibrated valve designed to control internal system pressure.

Its function is to:

Maintain a specified operating pressure

Relieve excess pressure safely

Allow coolant to return to the system as it cools

By controlling pressure, the cap raises the boiling point of the coolant, allowing the engine to operate at stable temperatures without vapour formation.

Why Pressure Matters

Coolant boiling point increases as pressure rises.

At atmospheric pressure, water boils at around 100 °C. In a pressurised system, the boiling point increases significantly, allowing:

More consistent heat transfer

Reduced risk of vapour pockets

Stable operating temperatures under load

Without sufficient pressure, coolant can locally boil even when the temperature gauge appears normal.

How a Pressure Cap Works

A typical pressure cap contains two valves:

Pressure relief valve

This spring-loaded valve opens when system pressure exceeds the cap’s rated value, allowing coolant to flow into an overflow or expansion circuit.

Vacuum valve

As the system cools and pressure drops, this valve opens to allow coolant to be drawn back into the system instead of collapsing hoses or pulling in air.

Both functions are critical. A failed vacuum valve often causes just as many problems as a failed pressure valve.

Common Pressure Ratings and What They Mean

Pressure caps are usually rated in bar or psi.

Typical values include:

0.9 bar — lower pressure or older systems

1.1 bar — common OEM specification

1.3–1.5 bar — performance or motorsport applications

Higher pressure raises boiling point, but it also increases stress on hoses, radiators, tanks, and seals throughout the system.

The Uprated Cap Myth

Fitting a higher-pressure cap is often seen as a simple “upgrade”. In most cases, it isn’t.

An uprated pressure cap:

Does not improve cooling efficiency

Does not remove air from the system

Does not fix bleeding issues

Does not compensate for weak components

Increasing system pressure on ageing or plastic components can accelerate failures, often at:

Plastic header tanks

Radiator end tanks

Hose joints and clamps

If coolant is being pushed out, the underlying cause is usually:

Trapped air

Poor bleeding

Component fatigue

Localised boiling

Not the cap itself.

Choosing the Correct Pressure Cap

The correct pressure rating depends on:

OEM system design

Material of the header tank or radiator

Hose condition and age

Intended use (road vs track)

As a general rule:

Match the OEM pressure unless there is a clear engineering reason not to

Only increase pressure when all components are known to tolerate it

Reliability and consistency matter more than maximum pressure

With aluminium header tanks, maintaining the correct pressure protects the rest of the system rather than pushing it harder.

Header Tank Caps vs Radiator Caps

The function of the cap is identical — location is the difference.

Header tank–mounted caps

Used when the header tank is the highest point in the cooling system. The cap becomes:

The pressure control point

The fill point

The main bleeding point

Radiator-mounted caps

Used when the radiator remains the highest point, with the header or expansion tank handling only volume changes.

What matters most is:

The cap is at the highest practical point

Air cannot collect below it

Incorrect cap placement can trap air regardless of pressure rating.

In Summary

A cooling system pressure cap is a control device, not a performance upgrade.

Correct pressure:

Stabilises coolant temperatures

Prevents vapour formation

Protects system components

Incorrect pressure masks problems — or creates new ones.

Like most reliable cooling systems, longevity comes from getting the fundamentals right, not chasing higher numbers.