Radiator Caps – Are Cheap, But More Important Than You Think

Radiator Caps - Are Cheap, But More Important Than You Think
Radiator Caps - Are Cheap, But More Important Than You Think

Radiator Caps, are designed to ensure that, the prescribed pressure in the cooling system, is maintained at all times.

And, this is done by allowing air to escape, when pressure exceeds a preset amount.

That’s why, radiator caps should be considered “the safety valve” of the cooling system.

Radiator caps do more than just one thing and therefore, can fail in a verity of different ways.

So, Radiator Caps Do Several Things:

  • Seals the system against, the outside world (main seal function)
  • Keeps the system pressurized when needed, so as to raise the boiling point of the coolant
  • Allows excess pressure and coolant to be able to vent, to the expansion reservoir (pressure seal function)
  • Allows coolant to return to the radiator, when the engine cools down (return seal function)

Radiator Caps Have Three Seals, Any Of Which May Fail Independently Of The Others:

Radiator Caps Illustration
Radiator Caps Illustration
  1. Sealing the cap against the top of the filler neck, is the job of the main seal. The main seal keeps the whole system sealed and is considered, the most important seal.
  2. Radiator caps with a leaking pressure seal, will allow the coolant to boil, at a lower temperature. And, coolant will be able to travel freely, to the expansion reservoir. This could result in, localized hot-spots inside the engine. This may lead to premature cylinder head warping, and may hasten head gasket failure. It will also cause, the engine coolant level to be low, just like a failed head gasket.
  3. A faulty return seal, may prevent coolant from, returning to the radiator. This may lead to, collapsed radiator hoses due to, the excessive vacuum. This will prevent the coolant from circulating, if the hoses don’t re-expand as the engine warms up.

Also, Bad Radiator Caps Can Cause Symptoms, Similar To A Failed Head Gasket

Damaged Leaking Radiator Caps
Damaged Leaking Radiator Caps

If you replace the radiator caps and you still have, bubbles in the coolant (or foam in the recovery tank). Then, suspect the head gasket. So, 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.

Neglected Cooling System
Neglected Cooling System

Moreover, a neglected cooling system, can load up radiator caps with crud and corrosion. As a result, preventing proper coolant flow in and out through it. Peel the seals back with your fingernail, to check for goop. Consequently, if you find any, just replace the cap.


So, if you suspect the cap is failing, give it a thorough visual inspection. Start by looking for any damaged, cracked, hardened or worn out seals. Finally, if the seals look bad, just replace the cap. Because, radiator caps are cheap!

Thank You !