How to Pressure Test a Radiator Cap

As the coolant in your cooling system increases in temperature, the pressure in the system increases as well. Normal operating temperature of a cooling system is around 220 degrees Fahrenheit and the boiling point of water is 212 degrees Fahrenheit.

By increasing the pressure in the cooling system, the boiling point of the coolant is increased to 245 degrees Fahrenheit at 8 PSI. Cooling system pressures are controlled by the radiator cap. Radiator caps hold pressures between 6-16 PSI for most automotive systems.

Most cooling system pressure test kits come complete with everything you will need to pressure test most vehicles. This includes testing radiator caps as well. In order to pressure test the cooling systems of multiple makes and models of cars, adapters are required for each manufacturer.

Part 1 of 1: Pressure testing the radiator cap

Material Needed

radiator cap and neck with cap being removed

Step 1: Make sure the cooling system is not hot. Carefully touch a radiator hose to see if it is hot.

  • Warning: Extreme pressure and heat are at play. Do not attempt to remove the radiator cap while the motor is hot.

Step 2: Remove the radiator cap. Once the motor has cooled down enough to touch a radiator hose without it burning you, you can now remove the radiator cap.

  • Warning: There could still be hot pressurized coolant in the system so be sure to pay attention and be careful.

  • Tip: Place a catch pan under the radiator to catch any coolant that may come out when you remove the radiator cap.

cooling system pressure tester with radiator cap adapter

Step 3: Attach the radiator cap to the pressure tester adapter. The cap will fit onto the pressure tester adapter in the same way it twists onto the radiator neck.

Step 4: Attach the adapter with the installed cap onto the pressure tester.

Step 5: Pump the pressure tester handle until the pressure builds to the specification written on the radiator cap. It should not rapidly lose pressure, but it is normal to lose a little.

  • Tip: The radiator cap should be able to hold most of the max pressure for up to five minutes. You don’t need to wait five minutes, though. A slow loss is normal but a rapid loss is a problem. This takes a little bit of judgment on your part.

installing the radiator cap

Step 6: Install the old cap. Do this if it is still good.

Step 7: Buy a new radiator cap from the auto parts store. Make sure you know the year, make, model, and engine size before you head to the parts store.

It is often helpful to bring the old radiator cap with you.

  • Tip: It is a good practice to bring the old parts with you to buy the new parts. By bringing the old parts, you can be sure you are leaving with the correct parts. Many parts also require a core or an extra charge will be added to the price of the part.

Radiator caps are an essential part of the cooling system that many discount, keeping the cooling system in balance. If you would like one of YourMechanic’s professional technicians to pressure test your radiator cap for you, book an appointment today and one of our mobile mechanics will test it for you at your home or office.


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Recent Pressure Test Radiator Cap reviews

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Robert

12 years of experience
76 reviews
Robert
12 years of experience
Dodge Ram 1500 Van V8-5.9L - Pressure Test Radiator Cap - San Jose, California
From the beginning, I knew just from looking at Robert, that he would exceed expectations and I was not wrong! My suspension diagnostic revealed stripped wheel barrings. It was the source of my driving problems including knocking noises. Thank you Robert for your thorough and patient diagnostic and recommendations to the problems. I recommend knowledgeable, professional Robert for your vehicle needs and will be doing by my word also.
Nissan Altima - Pressure Test Radiator Cap - Alviso, California
Nice, knowledge. Very convenient.

Jose

12 years of experience
49 reviews
Jose
12 years of experience
Chrysler Pacifica V6-3.8L - Pressure Test Radiator Cap - Redlands, California
Very friendly and punctual. Knows what he is talking about. Appreciate Jose working on my car. I would recommend him to customers.

Hugh

11 years of experience
106 reviews
Hugh
11 years of experience
Subaru Outback H4-2.5L - Pressure Test Radiator Cap - Littleton, Colorado
Hugh was super honest and competent. I scheduled the service based on a repair shop and a dealer's recommendation. But Hugh came, did a quick inspection and determined a simple cap replacement should fix the overheating issue and it did. Obviously that saved me a lot of money and made feel a lot better about my car. Hugh was also very friendly, easy-going. I would recommend him in a heartbeat.

Trevor

22 years of experience
185 reviews
Trevor
22 years of experience
Honda Civic L4-1.7L - Pressure Test Radiator Cap - Oceanside, California
Trevor in Oceanside, CA is excellent. Correctly diagnosed my car’s problems, provided me with a fair quote for only what was needed (no scamming/upselling/etc), then came back a couple days later for a follow up to complete the repairs. Arrived on time, quickly fixed what was wrong, and my car is running perfectly after paying for repairs that cost only a fraction of what I was expecting. Would definitely recommend.

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