Today, you will learn what a radiator cap is and its main functions. We also discuss its working and how it controls the cooling system and engine temperature of your vehicle.
Furthermore, you will see radiator cap pressure range, available sizes, common problems and signs of a bad cap. After reading this guide, you will easily identify the right radiator cap and make a better choice if you want to buy or replace this cap.
What is a Radiator Cap?
A radiator cap is an important component of a vehicle cooling system used to control coolant pressure and regulate its flow. You can use it to seal the radiator and prevent the coolant from boiling too early. Moreover, this pressure valve prevents coolant from escaping and engine overheating.
In most vehicles, the radiator cap can handle pressure level around 13 to 16 psi. Suppose the pressure exceeds this limit, then this cap will send extra coolant to the overflow or expansion tank. This way, it keeps radiator and radiator hose safe from damage.
What does a radiator cap look like? You will find this metal/plastic cap small and round with a flat top. On its sides, you see two metal tabs. You can hold them to twist the cap on and off. Most caps mostly show pressure ratings such as 13 psi, 16 psi or 20 psi on the top. If you look at the underside of a radiator cap, you will find several important parts which are listed below.
| Radiator Cap Parts | Role |
| Pressure spring | Controls coolant pressure |
| Pressure valve | Releases extra pressure |
| Lower seal | Seals radiator neck |
| Upper seal | Prevent leakage |
| Main rubber seal | Keep overall system sealed |
| Vacuum valve | coolant return to the radiator |
| Locking part | Holds the cap in place |
| Cap body | Holds all parts together |
Where is Radiator Cap Located?
You will generally find the radiator cap on top of the radiator (near the front of the engine). A hose connects the radiator to the expansion or overflow tank. Some cars do not have a cap directly on the radiator. Instead of the radiator, this cap will be present on a coolant reservoir or expansion tank.
Radiator Cap Types Explained
There are different types of radiator caps in market and not all work in same way. Your car needs a cap that matches its cooling system perfectly. This table will show you radiator cap types based on its function and design.
| Types of Radiator Cap | Detail | Valve Design |
| Standard radiator cap (non vented) | Best to control normal cooling system pressure | Two way |
| Vented radiator cap | Releases excess pressure from cooling system | One way |
| High pressure radiator cap | Used to hold higher cooling system pressure | Two way |
Radiator Cap Function
What is the purpose of a radiator cap? To keep a vehicle engine cool and running smoothly, radiator cap perform these important roles.
- Prevent vacuum & return coolant: The vacuum valve allows coolant to flow back into the radiator, which prevents air pockets and damage to hose.
- Seal system: This component tightly seals engine cooling circuit. This way, you can prevent coolant loss and keep dust, dirt plus other contaminants out.
- Pressure regulation: The primary job of this cap is to control the pressure inside the cooling system and keep it at the right level. This prevents leakage and improves performance.
- Raise coolant’s boiling point: Spring of radiator cap acts like a pressure controlled door. Because high pressure can make the coolant boil at a higher temperature. Radiator cap will add extra pressure to car cooling system. This will give the engine more room to get hot without the coolant boiling.
How Does a Radiator Cap Work?
A radiator cap is a spring-loaded valve that mainly controls pressure in the cooling system of a vehicle. It works in two stages which we discuss below.
1. Pressure Cycle
Suppose you start your car, then the engine will run and produce heat. The coolant will absorb this heat as it moves through engine. This liquid will also expand and increase pressure inside sealed cooling system. Well, the radiator cap mostly has a pressure rating between 13 psi to 16 psi. The calibrated spring of the cap will hold the valve closed until the pressure reaches 16 psi.
In case pressure goes above this range, the spring will compress and open the valve. As a result, extra hot coolant will flow through a radiator hose into the overflow or expansion tank. This will reduce excess pressure and protect the radiator & hoses.
2. Vacuum Cycle
After you turn off the vehicle engine, it will cool down again and cooling fluid will now no longer absorb heat. It will start to shrink and create a vacuum inside the cooling system.
If the vacuum becomes too strong, it can collapse the radiator hose. At this point, the vacuum valve in the radiator cap will open. As a result, coolant from the overflow tank will flow back into radiator. This will keep the radiator full of cooling liquid as well as prevent air from entering car cooling system.
List of Radiator Cap Sizes
This radiator cap size chart will show you common specifications of this component, so you can choose the right radiator cap for your vehicle.
| Type | Filler Neck Outer Diameter | Filler Neck Inner Diameter | Depth / Height | Overall Cap Diameter |
| Small radiator cap | 41 mm (1.6 inch) | 32 to 38 mm | Approx. 16 to 19 mm | 45 – 59 mm |
| Large radiator cap | 68 mm (2.69 inch) | Around 54 mm (2.13 in) | 19mm, 21 mm, 27 mm, 29 mm | 70 mm to 71 mm |
Our recommendation: You should not buy a radiator cap only because it fits the opening. Instead, a safe approach is to measure the cap you already have. You can check its diameter, depth plus PSI rating. Then you can buy a replacement that exactly matches those measurements.
Radiator Cap Pressure & PSI Chart
If we talk about radiator cap pressure, it is the actual amount of pressure that a radiator cap allows the cooling setup to hold before it releases extra pressure. You can measure this pressure level in PSI (pounds per square inch).
| Radiator Cap Pressure | Suitable Application |
| 4 to 7 psi | Vintage & small engines |
| 13 to 16 psi | Older passenger cars |
| 16 to 20+ psi | Modern or standard passenger vehicles |
| 22 to 32 psi | Performance & racing motorcars |
What Happens If Pressure of Radiator Cap is Very Low?
If the pressure of the radiator cap in your car is too low, the coolant can boil at a lower temperature. This situation can convert coolant into steam. As a result, it will lead to engine overheating, coolant loss or air pockets in your vehicle’s cooling circuit.
What Happens If Radiator Cap Pressure is Too High?
A cap with a high pressure rating can put extra stress on the engine cooling system and cause a more serious failure. In fact, some drivers who increased cap pressure in their cars reported problems with hoses, radiator end tanks & expansion tanks. Based on expert instructions, you must always use the radiator cap pressure specified for your vehicle.
Fact: The pressure rating of the radiator is not only a number on the cap. A quality cap with a unique PSI range guarantees you that it will hold that specified pressure. It releases coolant only if the system exceeds the limit of pressure. But, a faulty cap or damaged filler neck can cause premature pressure loss and cooling problems.
Can You Drive Without a Radiator Cap?
We will never recommend you to drive without a radiator cap. As we mentioned above, the cap seals the radiator and controls pressure inside the engine cooling system. So driving without a radiator cap is an extremely dangerous driving practice and you should avoid it at all costs.
What Happens if You Drive With Radiator Cap off?
Without it, coolant can leak or boil more easily and overheat your car engine. In case you drive your car with the radiator cap off, it can cause serious problems. For example: the coolant can spill out and boil faster. Because the system cannot build pressure. This can make the engine overheat quickly and can harm it too. Furthermore, it can allow dirt and air to enter the cooling system.
Helpful reading: how long can you drive without a radiator cap?
Radiator Cap Hose
A radiator cap hose or pipe is a small tube that creates a link between radiator filler neck and coolant overflow tank/expansion tank. You can use it to transfer extra cooling fluid to the tank in case the radiator gets hot. It will also return coolant to radiator if the engine cools. Radiator cap pipe is also known as overflow hose, coolant recovery hose or reservoir hose.
Note: This hose is different from the main radiator hoses such as upper and lower radiator hose. Basically, these large hoses carry coolant between the engine and radiator rather than excess coolant to the expansion tank.
Difference between Good vs Bad Radiator Cap
Are you a car owner or auto parts buyer and have a plan to purchase radiator caps in future? Then you must be aware of symptoms of a well engineered and faulty radiator cap.
| Feature | Good Radiator Cap | Bad Radiator Cap |
| Vacuum valve seal | Soft and in good condition | Deformed, sticky / stuck |
| Metal parts | Clean with less or no buildup | Rusty, corroded and covered with heavy buildup |
| Rubber seal | Soft, flexible and undamaged | Cracked, hard, brittle, can be damaged |
| Engine temperature | Prevent overheating | Can contribute to overheating |
| Pressure spring | Firm and springs back quickly | Weak, soft or stuck |
To Summarize
So, a radiator cap is used to seal the radiator, control pressure, manage coolant flow and support proper engine cooling. Our advice is to always match the cap with your vehicle’s required pressure and size.
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Frequently Asked Questions
Can a bad radiator cap make your car overheat?
Absolutely, yes. A bad radiator cap can fail to hold the correct pressure or allow coolant to escape. This can reduce the ability of the system to keep the engine cool and lead to overheating.
Will thermostat open with radiator cap off?
Yes, the thermostat can still open. Actually, it responds to the coolant temperature but not the pressure in the cooling system. So if coolant reaches the thermostat’s set temperature, then the thermostat will open and allow coolant to flow.
How To Open Radiator Cap?
First of all, you should let the engine cool completely. Then, you need to cover the cap with a thick cloth, press it down and turn it slowly. You can press down again and turn the cap to remove it. Watch this video for a practical guide on how you can safely open radiator cap.
When I open my radiator cap, should I see fluid?
Suppose the engine is completely cold; only in this case, you will see coolant near the top of the radiator and below filler neck. If you see very little or no coolant, it means your vehicle has lost coolant or has a leak.
What Happens if a Radiator Cap is Bad?
A faulty radiator cap can not keep the right pressure or seal the radiator properly. This can cause the coolant to boil sooner and the engine to overheat. It can also lead to coolant leakage, overflow tank and collapsed radiator hoses.
