How to Tell If a Radiator Cap is Bad and Replace It

By | May 11, 2023

How to Tell If a Radiator Cap is Bad and Replace It
A radiator cap is a necessary part of any cooling system to not only keep fluid in but let coolant out if it becomes too hot.

If a radiator cap goes bad, it can be replaced by most DIY type individuals.

Remember to never to remove a radiator cap while the engine is hot as the cap is under pressure, and boiling water can spray out.

Only remove the cap when the engine is cold so the pressure has dissipated.

What is the Function of a Radiator Cap?

What is the Function of a Radiator Cap
The basic purpose of a radiator cap is to give a pressure outlet when an engine becomes hot and overheats.

It does this with a spring-loaded plunger or spring that has a set pressure point to give way.

When the pressure is released, water will spill out into the reserve or from the cap itself.

A radiator cap also is an access point to pour fluid into the cooling system.

How to Tell If a Radiator Cap is Bad?

How to Tell If a Radiator Cap is Bad
When a radiator cap goes bad, it will usually leak and have discoloration from the fluid, such as rust.

Sometimes the cap will only leak when the engine is hot, which indicates the pressure spring has failed.

The spring has a set pressure to release fluid; when a vehicle becomes too hot, if the spring fails, it may leak fluid.

Symptoms of a Bad Radiator Cap

As a cap ages, the spring that controls the pressure can release early, letting fluid spill out.

This can leave a vehicle low on fluid and lead to overheating, which in turn makes the cap leak out more.

Air also is let into the system, which passes air bubbles into the cooling system. Since air is not good at transferring heat quickly, a motor can overheat more easily when air bubbles are in the system.

Symptoms can include overheating, burnt stains around the cap, and loss of fluid.

Here white spots around the cap show where fluid has been leaking out.
Radiator Cap is Bad

This radiator cap is rusted and needs replacing.
Radiator Cap is Bad rusted

How To Replace a Bad Radiator Cap

  1. The first step is to buy a new radiator cap from an auto part store or online.
  2. Be sure the engine has cooled down, and the cap is no longer under pressure.
  3. Remove the cap by pressing it down and turning it counter-clockwise.
  4. Replacing a Bad Radiator Cap Turn to Left

  5. Match the old cap to the new cap making sure they are the same.
  6. Replacing a Bad Radiator Cap

  7. Put the new cap on by matching the notches, pressing down, and turning it clockwise.
  8. Replacing a Bad Radiator Cap Turn to right

A bad radiator cap will not seal properly and can cause a leak.

When a cap goes bad, it usually will have rust or other discoloration.

Fluid also usually will leak out from around the cap, although in some cases, it could be the radiator.

If you think you have a bad radiator cap, it can be replaced by a DIY person.

Of course, if in doubt, be sure to take your vehicle to a professional.

Have you had issues with a bad radiator cap? Let us know your thought below.

3 thoughts on “How to Tell If a Radiator Cap is Bad and Replace It

  1. Merril kirchner

    2012 Dodge Avenger drove 45 minutes, very little heat. And when I take radiator cap off there is no pressure and I can stick my finger in the fluid. Luke warm only. Is it a radiator cap?

  2. Tom

    Was losing radiator fluid but no fluid was on my driveway. I changed the radiator cap after watching your video and the problem was solved. I did find the white residue you were talking about near the radiator cap.

  3. doug

    Awesome job maybe mention rubber deteriorates so quickly now days causing problems


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