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When to Replace a Radiator Cap

Dan Ferrell writes about DIY car maintenance and repair. He has certifications in Automation and Control Technology and Technical Writing.

When to Replace a Radiator Cap

  • The radiator cap seals are damaged.
  • The radiator cap valve is stuck.
  • The radiator cap doesn't hold pressure.

Common Bad Radiator Cap Symptoms

  • Engine overheating
  • Coolant leaking
  • Temperature gauge reading high

Damaged seals may sometimes be an obvious visual clue that you have to replace the radiator cap. But sometimes, this is not too obvious.

Sometimes, though, it is hard to know if your cap has actually failed, or it's the culprit behind cooling system problems.

The following sections will help you decide whether you need to replace the cap.

It's a good idea to have on hand the repair manual for your particular vehicle make and model.

You may need to do more than just replace a bad radiator cap, if the cooling system has been affected. Probably, you'll need to replace the system's coolant, or top off the system with coolant, and purge it.

If you don't have your repair manual yet, you can buy a relatively inexpensive copy through Amazon.

Haynes manuals include:

  • Photographs
  • Systems descriptions
  • Troubleshooting
  • Parts location and replacement
  • Parts specifications
  • Electrical diagrams
  • Maintenance schedule

Additionally, with your manual on hand, you can keep up with your vehicle maintenance to avoid unnecessary breakdowns and save a lot of money in repairs.

The following sections will help you decide whether you need to replace the radiator cap in your vehicle.

If you need more help diagnosing your radiator, problems with your temperature gauge, or an overheating condition, check the Resources section at the bottom of this post for more help.

In This Article

1. How a Radiator Cap Works
2. Where Is the Radiator Cap
3. Removing a Radiator Cap Safely
4. Inspecting the Radiator Cap
5. How to Test a Radiator Cap
6. VIDEO: Testing the Radiator Cap
7. Radiator Cap Pressure Conversion Chart
8. Resources

Radiator cap vent valve.

Radiator cap vent valve.

1. How a Radiator Cap Works

Radiator caps are used on sealed, recovery type cooling systems or open, (older engine models) non-recovery cooling systems.

The cap seals the system to prevent coolant from splashing out of the radiator.

Also, the cap is designed to keep the cooling system within a certain temperature range, depending on the particular engine model. The cap accomplishes this by pressurizing coolant to a specific level, usually marked on the cap itself.

Most automotive radiator caps are pressure rated between 13 and 18 psi (pounds per square inch), or 90 and 124 kPa (kilo-pascals). This allows coolant to reach a higher-than-normal temperature without boiling. In turn, coolant absorbs more heat from the engine and releases it through the radiator.

The radiator cap accomplishes this through the vent valve located under the cap. The cap practically seals the upper and lower sealing surfaces of the radiator neck, pushing the relief valve against the lower seal.

When you start the engine, coolant pressure begins to build up as temperature rises.

  1. When the cooling system reaches the pressure marked on the cap, the pressure overcomes the cap's valve spring force, opening the cap's relief valve.
  2. At this point, excess pressure exits the system through a hole in the radiator neck located between the cap's upper and lower seals. Coolant flows into the expansion tank.
  3. Once pressure has been released, the relief valve begins to close and coolant temperature drops.
  4. This lower pressure causes the cap's vacuum relief valve to open, drawing coolant from the expansion tank to refill the radiator.
  5. And the cycle starts over.
Look for the cap along the top of the radiator.

Look for the cap along the top of the radiator.

2. Where is the Radiator Cap

Usually, you'll find the cap somewhere on top of the radiator, screwed onto the radiator neck.

On many late-model engines, though, you'll find the radiator cap fastened to the coolant expansion tank.

If necessary, consult your vehicle repair manual to locate the radiator cap in your model.

3. Removing a Radiator Cap Safely

Removing a radiator cap should be done safely. Never remove a radiator cap on a hot engine.

If you do, the suddenly released cooling system pressure will cause coolant to boil immediately and squirt out of the radiator.

In turn, this will cause thick coolant to stick to your skin and severely burn you.

  1. First, verify the radiator is not hot to the touch. If it is hot, wait a few minutes.
  2. Using a piece of cloth, turn the radiator cap counterclockwise a quarter of a turn to its safety stop.
  3. Allow the cap to release system pressure.
  4. Once pressure is released, press the cap down and turn it counterclockwise to remove it.
Check cap seals and radiator sealing surfaces.

Check cap seals and radiator sealing surfaces.

4. Inspecting the Radiator Cap

Sometimes, a careful visual inspection of the radiator cap is all you need to know there is a problem with it.

A radiator cap needs good seals to do its job. Refer to the picture above when inspecting your cap.

Clean all rubber seals and sealing surfaces using a soft brush, if necessary, on both the radiator cap and radiator neck.

Examine the Radiator Cap

  • Check gaskets and seals for cracking, deterioration, and brittleness.
  • Closely inspect the under vacuum valve.
  • Inspect the sealing surfaces around the radiator neck.

If seals or sealing surfaces show any signs of damage, replace the radiator cap.

A damaged radiator neck may be repaired in a shop, but sometimes it's just better to replace the radiator.

Use a pressure tester to test your radiator cap.

Use a pressure tester to test your radiator cap.

5. How to Test a Radiator Cap

After years of service, the spring in the radiator cap weakens. This causes the coolant's boiling point to drop. One of the symptoms is the loss of coolant through the overflow hose every time the coolant gets hot.

However, you cannot detect a weak cap spring just by looking at it. However, you can easily test the radiator cap at home using a radiator pressure tester. Most car owners don't have this tool at home, though. Visit your local auto parts store and get one through their loan service.

Test the Radiator Cap

  1. Fit the head of the radiator pressure tester with the correct adapter for your radiator cap.
  2. Wet the cap seals with coolant.
  3. Mount the radiator cap to the tester head.
  4. Check the cap's pressure rating. Usually, you'll find this number on the cap itself. Also, you can find the rating in your vehicle repair manual.
  5. Pump the tester until it reaches the cap's rating pressure, as indicated on the tools dial.
  6. Check that the cap holds the pressure for at least one minute; otherwise, replace it.
  7. Then, pump the tester some more to exceed the cap's pressure rating by 30 percent. For example, if your cap is rated at 14 psi, apply 18 psi of pressure.
    • The cap should vent the excess pressure applied and return to the rated pressure, often plus or minus 1 or 2 psi.

Replace your radiator cap if:

  • It doesn't hold pressure.
  • It doesn't fall back to cap rated pressure when pressure exceeded.
  • It holds pressure outside the rated pressure, plus-minus 1-2 psi.

If you need to replace your radiator cap, make sure to buy one of the same pressure range.

6. VIDEO: Testing the Radiator Cap

7. Radiator Cap Pressure Conversion Chart

If you need to replace the radiator cap in your vehicle, install one with the rating recommended by your car manufacturer. You can find this number marked on your radiator cap or your vehicle repair manual.

Each pound of pressure above the rating specified for your application will increase the coolant's boiling point by 1.4C or 2.5F.

Most vehicle cooling systems use a pressure cap rated between 13 and 18 psi. If your country uses the metric system, you can use the conversion table below to find the corresponding rating for your radiator pressure cap.














8. Resources

This article is accurate and true to the best of the author’s knowledge. Content is for informational or entertainment purposes only and does not substitute for personal counsel or professional advice in business, financial, legal, or technical matters.

© 2022 Dan Ferrell