Bad Radiator Cap – A Cheap Fix For A Potential Disaster

When was the last time you checked your radiator cap ?

The radiator cap is a very important part of the engine’s cooling system, yet it is often overlooked when troubleshooting the system.

Radiator caps are designed to ensure that the prescribed pressure on the cooling system is maintained at all times.

Some of the issues that accompany a bad radiator cap can cause damage to other parts of the cooling system as well.

Cooling systems are under pressure to increase the boiling point of the coolant.

This allows the system to operate efficiently without boiling off the coolant and overheating the engine.

A loose or bad radiator cap will cause the system not to pressurize, resulting in overheating.

In this case, the coolant is more likely to be lost to boiling off than by simple evaporation.

A bad radiator cap cannot effectively seal off the system nor keep it properly pressurized which will compromise the proper functioning of the cooling system.

So, when performing routine cooling system maintenance or repairs, make sure to inspect the caps and replace any that are defective.

There are a few signs that will let you know if you have a bad radiator cap.

  • Leaking Coolant
  • Overflowing Reservoir
  • Radiator Hose Collapses
  • Air Inside the Cooling System
  • Overheated Engine

So, The radiator cap does several things.

  • Seals the system against the outside world (main seal function)
  • It will keep the system pressurized when needed, also raising the boiling point of the coolant
  • Allows excess pressure and coolant expansion to vent to the expansion reservoir (pressure seal function)
  • Allows coolant to return to the radiator when the engine cools down (return seal function)
Bad Radiator Cap
Bad Radiator Cap

As you can see from the above section, the radiator cap has three seals, any of which may fail independently of the others:

  • The main seal is the one that seals the cap against the top of the filler neck. Just a rubber gasket that operates just like one on the lid of a pickle jar. Simple and reliable.
  • A failed pressure seal will allow the coolant to boil at a lower temperature.  This will allow coolant will be able to travel freely and foamy to the expansion reservoir. It will cause localized hot-spots inside the engine. This will lead to premature head warping, and may hasten head gasket failure. It will cause the rad coolant level to be low, just like a failed head gasket.
  • A failed return seal may even prevent the coolant from returning to the radiator as the rad cools off. It could cause a vacuum that can collapse the radiator’s hoses. This will prevent the coolant from circulating if the hoses don’t re-expand as the engine warms up.

A bad rad cap may even cause similar symptoms of a failed head gasket, so it’s a cheap first step to try before bringing it in. If you replace the rad cap and you still have bubbles in the coolant (or foam in the reservoir), then suspect the head gasket. If the engine starts to overheat at idle, or in heavy traffic, and the gauge goes down when you rev it, the coolant is probably low.

Moreover, a neglected cooling system can load up the cap with crud and corrosion, preventing proper coolant flow in and out through it. Peel the seals back with your fingernail to check for goop. If you find any, a blast with a garden hose and probing with a toothpick should clear most of it out.


A new radiator cap is usually less than $20. You should change it every 5 years, just in case.


About Danny Bender 175 Articles
I have been into cars for as long as I can remember but the engine is what interested me the most. Now with over 40 years in the engine rebuilding and machine shop industry I have retired. I will be spending my time sharing what I have learned while keeping up with the latest Technology - Helping Solve Engine Problems !


  1. I have to say you are quite professional. What I need to do now is to know as much automotive diagnosis knowledge as possible because of my job.I think your posts are very useful for me. Thanks a lot!

  2. I am glad to know that it is important that you let the radiator cool down, so that the coolant can return to the radiator properly. That is interesting that a radiator cap actually has three different seals. I think I’ll research more about how to care and maintain my car radiator. Thank you for all your help!

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