What pressure number is a radiator cap?

In most passenger vehicles, the radiator cap carries a pressure rating measured in psi (pounds per square inch) or bar. The typical range is about 13 to 16 psi (roughly 0.9 to 1.1 bar), with the exact rating stamped on the cap and specified in the vehicle’s manual. The cap’s pressure rating is designed to control the cooling system’s pressure and raise the boiling point of the coolant.

How radiator caps work

The radiator cap acts as a pressure-relief valve as the engine heats coolant and it expands. By maintaining a specific pressure, the system prevents coolant from boiling at normal operating temperatures and directs excess coolant to the expansion reservoir. Choosing the correct cap helps maintain proper cooling system pressure and performance.

Common pressure ratings you may encounter

Caps come in several ratings, and the exact number depends on the vehicle’s cooling system design. The following are typical values you might see on caps in common vehicles.

• 7 psi (about 0.5 bar) — common on older, low-pressure systems


• 8–10 psi (about 0.5–0.7 bar) — vintage or some early-design cooling systems


• 13 psi (about 0.9 bar) — very common in many modern cars


• 14–15 psi (about 1.0 bar) — used on a number of current vehicles


• 16 psi (about 1.1 bar) — found on some newer or performance-oriented designs


• 20 psi (about 1.4 bar) — used on certain high-temperature or heavy-duty systems

In practice, the exact rating for your vehicle should be checked on the cap itself or in the owner’s manual or parts catalog. Using a cap with the wrong pressure rating can lead to overheating, coolant loss, or reduced cooling efficiency.

Find the right cap for your vehicle

To ensure you install the correct cap, follow these steps. The following guidance helps you identify the exact pressure rating required by your cooling system.

• Look at the current cap for a stamped pressure rating (e.g., 13 psi, 15 psi) and note the number.


• Consult the owner’s manual or the manufacturer’s parts catalog for the exact recommended rating for your engine and model year.


• If replacing after a repair, choose an OEM (original equipment) cap or an aftermarket cap with the same pressure rating as specified for your vehicle.

Using the correct cap protects the cooling system and helps maintain optimal engine temperature. If in doubt, verify with a dealer or a trusted mechanic.

Testing and safety considerations

Testing a radiator cap’s integrity and its pressure-relief function should be done carefully, especially since the system is under pressure when hot. Use proper equipment and follow safety precautions to avoid burns or coolant spray.

1. Ensure the engine is completely cool before removing or testing the cap.


2. Attach a radiator cap tester or a pressure tester to the filler neck or expansion tank cap.


3. Increase pressure gradually to the cap’s rated value and observe whether the cap holds and releases at the correct pressure.


4. Check for leaks around the cap seal and the neck; replace if the cap is damaged or fails to hold pressure.

If the system repeatedly overheats or you notice coolant loss without visible leaks, the cap may be faulty or the cooling system may have another underlying problem. Have a professional inspection if problems persist.

Summary

radiator caps are rated to a specific pressure, typically between 13 and 16 psi for many modern cars, though ratings can range from about 7 psi to 20 psi depending on the design. Always verify the exact rating from the cap itself or the vehicle documentation, replace with the correct cap, and test the cap if you suspect a leak or improper pressure. Correct cap pressure helps maintain coolant boiling point, prevents overflow, and supports reliable engine cooling.

What is the number on the radiator cap?

The numbers on an automotive radiator cap indicate its pressure rating in pounds per square inch (PSI) or kilopascals (kPa), which is the maximum pressure the cap will hold before releasing excess pressure. This rating is crucial because a higher pressure cap raises the boiling point of the coolant, allowing the engine to run at a higher temperature without the coolant turning to steam. When choosing a new cap, always select the correct pressure for your vehicle, as a cap that is too high can damage the cooling system.
 
You can watch this video to learn what radiator cap numbers mean in a car: 58sPeter Finn the Car DoctorYouTube · Jun 18, 2016

• Pressure Rating: The numbers on the cap, such as "151515" or "161616", show the pressure rating in PSI (or kPa). For example, a "161616" PSI cap will maintain 161616 pounds of pressure per square inch in the cooling system. 
• Boiling Point: Increasing the pressure in the cooling system raises the boiling point of the coolant. For every 111 PSI of pressure, the boiling point is raised by approximately $3$°F. A higher pressure cap can prevent the coolant from boiling and turning to steam. 
• Pressure Release: If the pressure in the system exceeds the cap's rating, the cap's internal valve will open to vent the excess pressure. This is a safety feature, but it's important to have the correct cap for your vehicle's system. 

This video explains the role of radiator caps in the cooling system: 58sMotoRadYouTube · Oct 24, 2018

What does 0.9 on a radiator cap mean?

A "0.9" on a radiator cap refers to the cap's pressure rating of 0.9 bar (or approximately 13 psi), which indicates the maximum pressure the cooling system will hold before releasing excess coolant. This pressure rating is crucial for raising the boiling point of the coolant and preventing the engine from overheating. It is often used in older vehicles, motorcycles, or smaller engines where a lower pressure cap is recommended to prevent damage to the cooling system components.
 
What the pressure rating means

• Function: The radiator cap's main job is to maintain pressure in the cooling system. 
• Pressure release: The 0.9 bar rating means the cap will open to release pressure once the system reaches 0.9 bar to prevent over-pressurization. 
• Higher boiling point: By holding pressure, the cap raises the boiling point of the coolant. A 0.9 bar cap allows coolant to boil at around 118∘C118 raised to the composed with power cap C118∘𝐶 (244∘F244 raised to the composed with power cap F244∘𝐹) with a 50/5050 / 5050/50 antifreeze mix, instead of its normal boiling point of 100∘C100 raised to the composed with power cap C100∘𝐶 (212∘F212 raised to the composed with power cap F212∘𝐹). 

Why you should use the correct rating

• For older vehicles: A 0.9 bar cap is often suitable for older vehicles, as their radiators and hoses are not designed to handle the higher pressures found in modern systems. 
• To prevent damage: Using a 0.9 bar cap on a car that requires a higher pressure cap (like 1.1 bar) can cause the coolant to boil at a lower temperature, which can lead to overheating and system damage. 
• To prevent leaks: Conversely, using a cap with a higher pressure rating on an older system can cause hoses or the radiator to leak or fail due to the excessive pressure. 

What psi should a radiator cap be?

You need a radiator cap with the pressure rating recommended by your vehicle's manufacturer, which can often be found on the original cap or in the owner's manual. If you have an aftermarket radiator, check the radiator itself for a pressure rating. For most stock vehicles, this is between 121212 and 161616 PSI, but some performance and modern vehicles may require a higher rating. 
How to find the correct PSI

• Check your owner's manual: This is the most reliable source for the factory-recommended pressure. 
• Look at the old cap: If the original cap is still on your vehicle, its PSI rating should be stamped on it. 
• Inspect the aftermarket radiator: Some performance radiators will have the maximum pressure rating stamped directly on them. 
• Consult the manufacturer: If you can't find the information on the cap or in the manual, contact your vehicle's manufacturer.

Why the PSI matters

• Raises the boiling point: A higher PSI cap increases the pressure in the cooling system, which raises the coolant's boiling point. This allows the engine to run hotter without the coolant boiling over. 
• Avoids damage: Using a cap with a PSI rating that is too high for your system can put excessive strain on hoses, seals, and the radiator itself, potentially leading to leaks or failure. 
• Don't guess: It is crucial to use the correct pressure for your specific cooling system to ensure it functions properly and efficiently without causing damage. 

How many psi is a 1.1 radiator cap?

A 1.1 bar (bar) radiator cap is equivalent to approximately 15-16 psi and raises the boiling point of the coolant, which can help prevent overheating, especially in high-performance applications or hot climates. This pressure rating is higher than many factory caps (which might be 0.9 bar or 13 psi) and can improve cooling efficiency, but it should only be used if your cooling system is designed to handle the increased pressure to avoid causing leaks in old hoses or seals.
 
Function and benefits

• Increases boiling point: The primary function is to increase the boiling point of the coolant. A 1.1 bar cap raises the boiling point to about 122°C (252°F).
• Improves efficiency: It can improve cooling efficiency, especially in high-stress situations like track driving or in hot weather.
• Reduces coolant loss: The higher pressure reduces the chance of coolant boiling over and escaping into the overflow tank. 

Potential drawbacks

• System compatibility: It's crucial that the cooling system components (radiator, hoses, and seals) can handle the extra pressure. 
• Risk of leaks: If your system is old or has weak components, a higher-pressure cap can cause leaks. 
• Not a substitute for a healthy cooling system: A higher-pressure cap won't prevent general overheating if there's another problem, like a faulty water pump or a clogged radiator. 

Before installation

• Check your vehicle's manual: Always confirm the correct pressure specification for your vehicle in the owner's manual to ensure you buy the right cap. 
• Inspect system components: Make sure all parts of your cooling system are in good condition. If components are old or weak, it is safer to use the original pressure cap.