Bad Radiator Cap – A Cheap Fix, For A Potential, Disaster

Bad Radiator Cap - A Cheap Fix, For A Potential, Engine Disaster
Bad Radiator Cap - A Cheap Fix, For A Potential, Engine Disaster

When was the last time you checked to see, if you had a bad radiator cap ?

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

So, 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.

So, cooling systems are under pressure, to increase the boiling point of the coolant. Therefore, this allows the system to operate efficiently, without boiling off the coolant and overheating the engine. However, a loose or bad radiator cap, will cause the system not to pressurize, resulting in overheating.

Radiator Cap Illustration
Radiator Cap Illustration

In this case, the coolant is more likely to be, lost to boiling off, than by simple evaporation. Also, a bad radiator cap, cannot effectively seal off the system, nor keep it properly pressurized. And, will compromise the proper functioning of the cooling system.

So, when performing routine cooling system maintenance or repairs, always inspect the radiator cap and replace it if necessary.

Common Failure Signs Include:

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

So, The Radiator Cap, Actually 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 coolant recovery tank (pressure seal function).
  • Allows coolant to return to the radiator, when the engine cools down (return seal function).
The Radiator Cap Has Three Seals
The Radiator Cap Has Three Seals

The Radiator Cap Has Three Seals, Any Of Which May Fail, Independently Of The Others:

  1. 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.
  2. A failed pressure seal, will allow the coolant to boil, at a lower temperature. This will allow coolant to be able to, travel freely and foamy, to the coolant recovery tank. It will cause, localized hot-spots inside the engine. This will lead to early, head warping and may hasten, head gasket failure. Finally, it will cause the rad coolant level, to be low, just like a failed head gasket.
  3. A failed return seal, may prevent the coolant from, returning to the radiator, as the rad cools off. It could cause a vacuum, that can collapse the radiator hoses. This will prevent the coolant from circulating, if the hoses don’t expand, as the engine warms up.
Leaking Radiator Cap Seal
Leaking Radiator Cap Seal

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.

But, if you replace the rad cap and you still have bubbles in the coolant (or foam in the reservoir), then suspect the, head gasket.

So, if the engine starts to overheat at idle, and the gauge, goes down when you rev it. Then, the coolant is probably low. Moreover, a neglected cooling system, can load up the cap, with crud and corrosion. As a result, stopping proper coolant flow in and out of it.

Peel the seals back, to check for goop. So, if you find any, a blast with a garden hose and brush, should clear most of it out.


So, a new radiator cap, is usually less than $20. In other words, just change it every 5 years, just in case.

Thank You !