Bad Radiator Cap – A Cheap Fix For A Potential Disaster

Do you have a Bad radiator cap ?

So, The radiator cap does several things. It

  • seals the system against the outside world (main seal function)
  • It will keep the system pressurized when needed, also rasing 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 foamily to the expansion reservoir. It will cause localized hot-spots inside the engine. This will lead to premature head warpage, 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 146 Articles Is The Premier Automotive Engine Troubleshooting Resource Site. Shares Information On Engine Troubleshooting, Engine Rebuilding, Engine Repair Tips, Tech Info, Basic Machining, Automotive Testing, In Addition To Possible Solutions.


  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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