Common Symptoms of a Bad Radiator Cap

Many symptoms of a faulty radiator cap can also be signs of another problem, such as the much-dreaded head gasket failure.

If you suspect you have an issue with your head gasket(s), it’s not a bad idea to replace the radiator cap first since it’s so quick, cheap, and easy, before throwing more time and money at a potentially expensive problem.  

•  Coolant Leak

If you attempt to use a radiator cap that holds more pressure than the system was designed for, you may introduce a leak, especially if it causes an old coolant hose to fail. This could also happen if the radiator cap fails to allow excess pressure to bleed into the coolant reservoir when the system reaches the target pressure.

Since substances naturally take the path of least resistance, the weakest point will leak coolant if the system is overpressurized. The leak may be at the cap itself, gaskets, hoses, water pump, or the actual radiator.

You can determine where the leak is by warming up the vehicle to pressurize the system and then carefully looking for leaks under the hood. Since different vehicles use different types of coolant, it can be either bright green, orange, red, yellow, or blue.

Sometimes these leaks will only manifest at operating temperature or after driving for a while. Since the system is under pressure, you may notice a leak spraying from a coolant hose into the engine bay, or even straight up into the air! 

To locate the source of a leak, wait until the engine has cooled then pinch hoses in the vicinity of the suspected leak. If you notice coolant bleed out when you pinch a hose, that’s the one you need to replace.

Coolant hoses are cheap and easy to replace. Just be sure to bleed or burp the cooling system when you’re finished; air in the system could cause your vehicle to overheat.

• Overheating Engine

If you’re wondering whether a bad radiator cap can cause overheating, the answer is a definite yes.

Air pockets in the cooling system from an ineffective seal (such as one in a bad radiator cap) or a lack of sufficient pressure can cause the engine to overheat. The latter lowers the boiling point of coolant which makes it unable to absorb enough heat from the engine.

If you see the temperature gauge on the dashboard rise, it’s wise to stop driving as soon as it’s safe to do so and wait for the engine to cool before continuing (or call a tow truck).

Allowing the engine to overheat will eventually cause engine failure as the parts to begin to take damage or warp due to excess heat.

• Steam from Engine

As coolant boils, it evaporates, turning into a gas that looks like steam. If your engine begins billowing steam, the coolant is boiling and escaping through a bad seal which may be in your radiator cap.

The engine will also eventually overheat so don’t wait to get your vehicle taken care of.

• Collapsed or Torn Radiator Hose

Internal pressure fluctuations can cause warping and/or tearing of the hoses. When the pressure gets too low, it causes a vacuum effect which collapses one or more hoses. Overpressurization can also cause cracks in the line. 

When inspecting the cooling system, check that no hoses are hard or feel spongy. Spongy hoses are weak and may collapse, while hard hoses are brittle and can’t take much more abuse before they crack under pressure. 

• Low Coolant

If the coolant overheats and evaporates as visible steam, it’s leaving the system and won’t come back. Leaking works the same way, of course. If you see that your coolant level is dropping, you know you may have a problem. 

Make sure never to let your coolant run below the recommended level. This is usually indicated by a low mark on the coolant reservoir. In a pinch, you can use distilled water to top off the coolant level until you are able to refill it with actual coolant. Note: using water that contains minerals may corrode the cooling system.

• Overflowing Reservoir

When there is a problem with the radiator cap, the coolant may go to the reservoir without the normal trigger of excess pressure. This can cause the coolant overflow tank to, well, overflow or release prematurely.