Radiator Caps – This Fairly Cheap Part, Is More Important Than You Think

Radiator Caps - This Fairly Cheap Part, Is More Important Than You Think
Radiator Caps - This Fairly Cheap Part, Is More Important Than You Think

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

Radiator Caps do this, 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 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. 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.

A Bad Rad Cap Can Cause Symptoms, Similar To A Failed Head Gasket

Damaged Leaking Radiator Caps
Damaged Leaking Radiator Caps

If you replace the rad cap 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 the cap 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; a blast with a garden hose and probing with a toothpick, should clear most of it out.


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, replace the cap. They are cheap !

Thank You !