Do you notice a sudden jump of temperature gauge into the red zone? You think the reason can be a bad radiator cap. Yes, it is possible because a faulty radiator cap can overheat the engine of your car.
In this scenario, you need to do radiator cap replacement which is not a complex task. You can do it easily after reading this helpful article. First of all, you will learn radiator cap testing & then six step process to replace it.
First You Should Do Radiator Cap Testing Before Replacement
How to test a radiator cap at home? These are some basic tests you can perform to check the real problem.
Visual Inspection: You can do a visual inspection at home. This chart will help to inspect the radiator cap in an accurate manner.
| Condition of the Cap | Its Meaning |
| Cracked rubber seal | Cap does not hold the pressure & coolant leaks out |
| Rust or corrosion | Seal is not in condition to hold the cap tightly |
| White residue | The cap was not fixed on the radiator and leak coolant |
| Flat and hard seal | It is worn out and not suitable to hold pressure |
| Bent edges of cap | Cap is not fitted on the neck in correct way |
Radiator Cap Pressure Test: For this test you need to buy a snap-on pressure tester pump. You fit the pressure gauge on the radiator cap to seal it. Then pump the pressure into the gauge & test it. You notice what kind of pressure it creates. You have to check whether it holds the pressure that is advertised on the cap or not.
| Test Result | What It Means |
| Holds the pressure | The cap is perfect and holding pressure completely |
| Pressure drops slowly | There can be a small leak; change the cap |
| Needle drop fastly | Cap is not suitable to use; must replace it |
Squeeze Hose Test Method: This is a very easy test to check the current condition of the radiator cap. First, let the engine warm up a little, then turn it off. After that, gently squeeze the upper radiator hose with your hand.
| Result of the Test | Explanation |
| Firm hose and slight resistance | Cap is holding pressure in the right way |
| Soft hose, squish easily | The vacuum valve of the cap is out of order |
| Too hard hose | It shows the pressure valve is stuck |
Expert Advice: If the result of the test shows the radiator cap is faulty, you must replace it because it is not very expensive. But if the result indicates a good radiator cap, the problem can be present in other parts of the cooling system.
How to Replace Car Radiator Cap in 6 Easy Steps?
1. Let the Engine Cool Completely
The very first thing you have to do is to turn off the car engine. You should wait for at least 30 minutes to cool down the engine. As you know, when you drive the vehicle engine becomes very hot.
This time duration allows the heat to escape into the atmosphere. After thirty minutes, check out the temperature by putting your hand slightly on any part of the engine.
2. Locate Radiator Cap
You will look at the top of radiator. The cap is located near the front of the engine bay. You just have to search for a round metal or plastic cap. You often see a red and white warning sign on this cap. For example, you notice a written warning about hot & pressurized liquid.
Some modern vehicles have the radiator cap under a plastic engine cover. You can also find it in a separate coolant expansion tank not on the radiator. Still, if you did not find the cap read the manual guide of the car or read discussion of the drivers regarding this.
3. Release Pressure Safely & Remove Old Cap
If the cap is identified, do not open it suddenly. The hot pressurized water in it can burn skin or damage your hands/other parts of the body. You should cover the cap with a thick towel or rag. In this way, your skin gets protected from hot splashes.
Push down the radiator cap (in some models) and turn the cap counterclockwise. But do not open it fully just hold at first stop. Allow the pressure or trapped steam to vent out from the overflow tube. At this time, keep your hands away from the cap because hot steam can harm skin. When the pressure is gone, turn off the cap and lift it upward.
4. Select Right Radiator Cap
After removing the radiator cap, you must check for the radiator cap size and its pressure rating. If it is not mentioned, read this table. These two things are very important if you want to install a suitable radiator cap for your car.
Radiator Cap Size
You must select the right size of radiator cap for the radiator. Because when the cap does not fit tightly on the radiator, it is not able to build pressure inside. Therefore, the coolant will leak & fail to protect the engine and other cooling components.
| Radiator Type | Approx. Size | Best Use Case |
| SAE “A” | 32 mm ID, 54 mm OD | Passenger car, older vehicles, aftermarket radiators |
| Small Diameter | 32 mm ID, 45 mm OD | Compact vehicle, mini radiators, Japanese performance engines |
| Metric Standard | 40 mm ID, 50 mm OD | Commonly used in some models of Honda, Toyota, Mazda, Nissan and Subaru |
Radiator Cap Pressure Rating
Pressure rating of the radiator cap is very important to understand. The reason is its control on the boiling point of coolant of your vehicle. Furthermore, it protects the components of the cooling system from damage.
| Rating | Specification | Common Usage |
| 13 PSI | Opens at lower pressure, reduces stress on aged hoses | Older vehicles, smaller cars |
| 16 PSI | Open at higher pressure, best for hot weather | Modern passenger cars |
| 18 PSI | Keep the engine cool in case of heavy load and hot weather; protect hoses | Modern vehicles, heavy-duty vehicles, Jeep |
| 20 PSI | Work with a closed system; prevent hoses from heat | High-performance cooling systems (BMW, Mercedes-Benz, Audi) |
| 24 PSI | Built to handle higher temperatures; control semi-custom performance setup | Efficient cooling system, modified engines, racing vehicles |
| 29+ PSI | Not suitable for older vehicles and standard systems | Racing car, heavy-duty engine, customized track car |
5. Install New Cap
So, you have chosen the required radiator cap. You should place it on the radiator neck and push it down. After that turn it clockwise until it locks in place. You will feel or hear a small click. This click sound means the cap is fully secured. You should not force it to be fixed. If it does not turn in the right way, check the alignment again.
6. Check Coolant Level and Test Drive
You must check coolant level before you drive the car. For this purpose, open the reservoir tank and look at the level line. If you notice a low coolant level then add more in it. After it, turn on the engine and let it run for a few minutes in a parked position. At the moment, go on a short drive and be attentive. You must check these signs to make sure the cap is replaced in the right way.
Signs of Successful Radiator Cap Replacement
- The temperature of the car is normal.
- There is no steam or coolant smell.
- Do not notice any coolant leak.
- The coolant level is stable
Symptoms You Need to Check the Cap Again
- The engine gets hot during this short test drive.
- You hear a hissing sound.
- Coolant level drops below the level line.
- You feel the smell coolant from the engine bay.
- The heater blows cooled air.
Radiator Cap Replacement Cost
Well, the replacement cost of the radiator cap depends on the model of your car and type of radiator cap. On average, $10 to $50 is needed if you want to change radiator cap.
Its labor cost is very low because an expert mechanic can fix it in only a few minutes. Actually, the radiator cap is among the least expensive repairs of your vehicle. I suggest doing it yourself because it is an easy task that can be done by a driver.
People Also Ask
What Happens if Radiator Cap Pressure is Too High?
Too high radiator pressure is very dangerous, it will build excessive pressure on the system. This pressure can burst or swell the radiator hoses. Rubber hoses can weaken, modern plastic tank splits, head gasket can blow and you see external leaks.
What Happens if Radiator Cap Pressure is Too Low?
If pressure of the radiator cap is very low, the boiling point of engine drops. The engine gets overheated and car struggles under heavy load and hot weather situations.
How Can I Test a Radiator Cap in Boiling Water?
This process is used under the visual inspection test. Many workers keep the radiator cap in hot water to remove old, crusty coolant deposits or debris. As a result, the inspection becomes easier and rubber seals get softened, you can see visible cracks and damage. But you can not check the pressure of the cap with this method.
To Summarize
I hope now you are able to replace the radiator cap of your car. But if you feel any confusion or do not want to take any risk, take your car to a mechanic. In case the radiator cap is broken and you can’t go to repair shop, use this radiator cap temporary replacement technique.
This technique uses aluminum foil, a plastic bag, a rag, or plastic wrap. Simply, you cover the opening area with foil and place a rubber band on it. Now, slowly take your car to the repair shop.
