Last Updated on February 19, 2020
Combustion engines generate a lot of heat just from normal operation. Without removing heat from the engine, the vehicle will encounter a slew of problems that eventually lead to engine failure. A failure of the cooling system could leave you stranded, and may even cause total engine failure in extreme cases.
Fortunately, a common and easy to fix cause of cooling system issues is a bad radiator cap. Knowing when to replace a radiator cap is pretty simply when you know the signs.
How Does a Radiator Cap Work?
The radiator cap is the gatekeeper of the cooling system. The cooling system uses pressurized coolant (also known as antifreeze) to take accumulated heat away from the engine. Having a functional radiator cap is critical to this process, as it maintains the proper amount of pressure.
Each radiator cap is rated for a certain amount of pressure it can hold (e.g. 0.9 bar, 13 PSI, etc). Different cooling systems call for different pressures. It is important to buy a radiator cap that matches the factory specification.
Pressurizing the cooling system is necessary to increase the boiling point of the coolant. This enables the liquid to hold more heat before evaporating.
Most engines have a normal operating temperature right around the boiling point of water (212° F). If the coolant in the engine were to boil, it would not effectively cool the engine since a gas will not remove heat from the system nearly as well as a liquid will.
A cooling system at optimal pressure enables the engine to run more efficiently and protects it from damage.
Coolant is usually made up of water and antifreeze in a one-to-one ratio. The water absorbs the heat from the engine and the antifreeze further raises the boiling point of the coolant.
Evaporation of coolant is undesirable because gaseous coolant will either leave the system entirely through a leak or will take up more space than it should which causes overpressurization.
To avoid overpressurizing, which can damage components of the system, a spring-loaded valve on the radiator cap vents excess coolant into the expansion tank (also known as reservoir). Under lower pressure, the cap draws the coolant back into the cooling system.
Please note: since the cooling system is hot and under pressure when the vehicle has been running, it is very important that you never open the radiator cap until the vehicle has cooled completely.
If you attempt to remove the radiator cap while the engine is warm, the coolant (likely as hot as boiling water) would shoot out and could seriously injure you; third degree burns are a very real possibility.
If you’re unsure, carefully feel the hoses. If they are hot, wait longer (or overnight to be safe) before opening to avoid a scalding geyser of coolant in the face.
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.
#1 – 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 yourself. Just be sure to bleed or burp the cooling system when you’re finished; air in the system could cause your vehicle to overheat.
#2 – 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 begin to take damage or warp due to excess heat.
#3 – Steam from Engine
As coolant boils, it evaporates, turning into a gas which 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.
#4 – 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.
#5 – 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.
#6 – 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.
How to Tell if Your Radiator Cap is Faulty
If you suspect your radiator cap is leaking, you can first remove it and inspect it. Of course, never do this when the vehicle is still hot to avoid getting a bad burn! In the same way, avoid opening the hood of the engine if steam is coming out as that can also cause a scalding injury.
The radiator cap may have build-up of debris causing the malfunction. You can try cleaning it with water and a toothpick, though it’s usually better to just replace it since it’s relatively cheap and engine failure is an expensive potential alternative.
You can also get a radiator pressure tester to check for pressure leaks in the system. Always replace the radiator cap with one of the same pressure rating.
If you replace the cap with one that takes a higher pressure before the valve opens, the vehicle can over-pressurize which causes other issues such as overheating or hose rupture.
It’s also a good idea to check the other parts such as the reservoir and hoses for cracks and other issues while you’re in there.
Since the symptoms of a bad radiator cap mirror those of other cooling system problems, there could possibly be two weak points at the same time. Then once you replace the radiator cap, the other weak point is likely to break.
See Also: Tips to Flush a Radiator (and Cost)
Radiator Cap Replacement Cost
Radiator caps are relatively inexpensive to replace. You can easily do it yourself so there are no labor costs. The caps themselves usually cost less than $25 at an auto parts store, though you can expect to pay more if you want a model with a built-in thermometer to indicate the coolant temperature.