Defective Radiator Caps

Defective Radiator Caps

A bad radiator cap can cause a lot of cooling system problems. You may think, the only purpose of your truck radiator cap is for adding coolant to the radiator, but it is not. Your radiator cap is much more important than most people understand and while adding coolant to prevent overheating is important, it is not a true picture of all your radiator cap does for you.

Your radiator cap is key to the proper functioning of your cooling system. It is important for every driver to know how a radiator cap works.


  • Allows you to add fluid
  • Seals your cooling system
  • Allows pressure to build up to a specific point in your cooling system
  • Raises your cooling system boiling point to prevent overheating
  • Vents excess pressure to protect cooling system components from damage
  • Transfers overheated coolant into an overflow bottle to prevent fluid loss and air pockets
  • Creates a vacuum to recapture vented coolant as your system cools

Radiator Cap Seals

Radiator caps have a couple of seals that are necessary to control your coolant. The main seal is on the inside top of the cap and works with a spring-loaded plunger. As your vehicle heats up, your coolant will expand, and pressure will increase. If the pressure keeps building up and surpasses the limit held by the radiator cap seal the internal spring and plunger will start to move. The seal then allows the heated coolant to pass into the overflow reservoir tank. When the pressure is sufficiently relieved, the radiator cap spring will allow the plunger to close stopping the venting. The second seal is used to create a vacuum. As your truck cools down this seal will allow vented coolant to draw back from the overflow reservoir into the radiator, helping to maintain the proper coolant level.

Radiator Cap Pressure

Your radiator cap must hold and release pressure for it to work properly. It is important for you to have the recommended pressure cap for your cooling system, not too much pressure and not too little. You should also know how to pressure test a radiator cap to make sure it is holding and releasing pressure properly. A defective radiator cap can cause a lot of cooling system problems, including bursting hoses.

Radiator Cap Vacuum

If you look at the bottom center of the radiator cap, you will find a brass or stainless steel plate. This little plate should always be hanging down and seem loose. This circular plate acts as a check valve that operates in only one direction. As the temperature in your engine starts to cool down, the coolant shrinks in volume. As the volume of the coolant contracts, the pressure in the radiator creates a vacuum that forces the circular plate to be pulled down and opens the check valve.

Once the check valve on the radiator cap opens, the coolant is sucked through the tube that leads to the radiator reservoir tank and refills the car’s radiator. This is a very effective system in that it pushes and pulls the fluid at each temperature cycle. It pushes fluid into the reservoir tank when the radiator is too hot and pulls fluid from the radiator reservoir tank back into the radiator as the engine cools. It is important that the vacuum is working as you need to bring all vented coolant back into the radiator to prevent air pockets or air bubbles.

Diagnosing a Cooling System Problem

  1. A radiator cap that is stuck closed and not releasing fluid or pressure may allow pressure to build up to unsustainable levels. This high pressure can cause a radiator leak or a hose to burst.
  2. If the radiator cap is stuck open, it will not allow your cooling system to build up to the proper pressure. Your boiling point will be lower, and your cooling system will not be properly pressurized.
  3. If you have a collapsed radiator hose, it could be because of your radiator cap. A defective radiator cap that does not release the vacuum properly and draw the coolant from the overflow bottle can cause radiator hoses to collapse as your system cools.
  4. A stock radiator cap normally holds between 14 to 16 PSI. The extra pressure created by custom or higher performance caps is not normally needed, and extra pressure created by these types of caps can cause radiator failure or other cooling system failures. If your cooling system is weak, and you have small seep leaks within your radiator, head gasket or hoses, you may wish to lower the amount of pressure your cap will hold. Maybe a 7-pound cap would be better but only until you can get your system fixed. For safety reasons, we only recommend using the factory recommended parts and settings on your vehicle.

During any Truck cooling system inspection, it will be wise to check your radiator cap. Look at the rubber seals and the spring and make sure everything looks good. It is also a good idea to do a pressure test on the cap to make sure it holds and then releases pressure at the proper settings for your vehicle.

If you have a radiator cap leak or any radiator cap problem just change the cap. Most radiator cap replacement prices are below $10, and it is not worth the risk to not change the cap. If you’re not sure if your radiator cap is bad, replace it anyway. At $10 there is absolutely no reason 2015-03-26 13.56.17a radiator cap replacement will ever be a bad idea.

Also, during your vehicle cooling system inspection make sure your overflow bottle has fluid in it. A cracked overflow bottle will remove your safety net and may allow you to overheat. There is a cool fill line. Make sure there is coolant at this line when the vehicle is cool.



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