Combustion engines create significant amounts of heat because of the normal operation of the vehicle. If the heat is not taken from the engine, the vehicle will have many issues, causing failure of the engine.
A failure of the cooling system could leave you stranded, and in extreme cases, it could cause total failure of the engine.
The good news is that one of the common and easy fixes for cooling system problems is a bad radiator cap. Knowing when to get a new radiator cap is relatively easy if you recognize the signs.
How Does a Radiator Cap Work?
The radiator cap is the gatekeeper of the cooling system. The cooling system uses pressurized coolant (antifreeze) to transfer excess heat away from the engine.
A functioning radiator cap is critical for this process because it provides the right amount of pressure.
Every radiator cap is rated for a specific amount of pressure it can withstand (0.9 bar, or 13 PSI, for example). Different cooling systems require different amounts of pressure, so it is important to buy a radiator cap that matches the factory specification.
Pressurizing the cooling system elevates the boiling point of the coolant. This allows the liquid to absorb more heat before evaporating away.
Most engines’ normal operational temperature is very near the boiling point of water (212°F). If the coolant in the engine were to begin boiling, it would not be able to remove heat from the engine properly. A gas does not remove heat from the system nearly as well as a liquid.
A cooling system at its proper pressure allows the engine to run more efficiently and protects the engine.
Coolant is generally water and antifreeze at a 1:1 ratio. The water absorbs heat from the engine, and the antifreeze raises the boiling point of the coolant.
It is not favorable for the coolant to evaporate out of the system, because coolant that is in gaseous form will either escape the system completely (leak) or it takes up too much space and causes overpressurization.
To prevent overpressurizing and damaging the various components of the cooling system. Finally, the radiator cap has a spring-loaded valve that is able to vent excessive coolant to the expansion tank (or reservoir). At lower pressure, the cap is able to pull coolant back into the cooling system.
Please note: It is very important that you never open the radiator cap until the vehicle has cooled down completely if the cooling system is hot.
If you are to attempt to remove the radiator cap when the engine is warm, the coolant (likely in thousands) would shoot out dangerously toward you, and you could be seriously injured and could obtain third-degree burns.
If you are not sure, feel the hoses. If hoses are hot, feel free to wait longer (if you need to, even overnight) to avoid the geyser of coolant spraying into your face.
Signs of a bad radiator cap
A lot of signs for a defective radiator cap can have other causes, such as the dreaded head gasket failure.
If you think you have a head gasket issue, I don’t think it can hurt to replace the radiator cap first. It’s quick, cheap, and easy to swap out a radiator cap, and it saves you time and money if you do it before you pour money and time into a serious issue.
#1. Coolant Leak.
If you attempt to run a radiator cap with more pressure than your system was designed for, you might create a coolant leak, especially if the pressure makes an old coolant hose fail.
If the radiator cap does not bleed off excess pressure into the engine’s coolant reservoir when the engine reaches the target pressure, you also just created a coolant leak.
Substances under pressure will always take the path of least resistance, and when over-pressurized, coolant will leak at the point of least strength. The leak could take place at the radiator cap, head gaskets, hoses, water pump, or the radiator.
You can determine where the leak is by warming up the vehicle to allow the system to pressurize, and then carefully looking under the hood for the leaks.
Coolant leaks can only be determined visually because each vehicle’s manufacturer uses its own branded coolant. Your vehicle’s coolant could be bright green, orange, red, yellow, or blue.
However, it depends as well, sometimes the coolant leaks only show up after your engine has reached operating temperature, or from have driving the vehicle for a short distance.
Because the cooling system is pressurized, you may observe a leak spraying from a hose into the engine bay or even straight into the air!
To find the source of a leak, allow the engine to cool down, then pinch off all the hoses around the area of suspected leak. If you see antifreeze bleed out when you pinch a hose, that is the hose that is leaking and needs to be replaced.
Coolant hoses are cheap and very easy to replace yourself. Just remember to bleed or burp the cooling system after you are finished; air in the cooling system could lead to the engine overheating.
#2 Overheating Engine.
If you are wondering whether a bad radiator cap can cause engine overheating, the answer is a resounding yes.
Air pockets in your cooling system, caused by either insufficient pressure, resulting from a bad radiator cap seal, or an ineffective seal (a bad radiator cap), can cause an engine to overheat.
Insufficient pressure lowers the boiling point of the antifreeze, which prevents it from absorbing enough heat from the engine.
If you see that the temperature gauge on your dashboard is climbing, it is best to pull over when it is safe to do so and wait for the engine to cool down before continuing (or call a tow truck).
Overheating your engine can cause it to fail ultimately as the internal parts begin to take damage or warp due to excess heat.
#3. Steam from the Engine.
When the coolant boils, it evaporates, which will result in gas that looks like steam. If your engine starts blowing steam like this, the coolant is boiling and escaping due to a faulty seal, most likely in the radiator cap.
The engine will also eventually overheat, so do not wait to have your vehicle dealt with.
#4. Crushed or Ripped Radiator Hose.
Pressure fluctuations can warp or rip the hoses. When the pressure decreases to the point it causes a vacuum effect, one or more hoses can get crushed, whereas overpressurization can cause tears or even cracks in the line.
When inspecting a cooling system, you ensure that none of your hoses feel hard or spongy. A spongy hose is weak due to, while a hard hose is brittle and capable of giving a little more before it will crack under pressure.
#5. Low Coolant.
If the coolant overheats and evaporates as visible steam, it is escaping the system and will not return any time soon. It would follow that leaking would act in the same way, of course. So, if you see that your coolant level is dropping at all, you can assume you may have a problem.
Always remember never to let your coolant run below the recommended level, usually indicated by the “low” mark on the coolant reservoir.
If you end up in a pinch, you can always use distilled water to add to the coolant level until you can actually refill it with coolant. Note: Using water that has minerals may corrode the cooling system.
#6. Overflowing Reservoir.
If there is a problem with the radiator cap, it could send coolant to the reservoir without the normal trigger of increased pressure, causing it to overflow or prematurely discharge.
How to Tell If Your Radiator Cap Needs Replacing
There are a number of ways to identify if the radiator cap is leaking. The quickest method of identifying this fault is to simply remove it and inspect it, but remember, don’t do this with a hot engine, as you may burn yourself!
If you’re inspecting the cap when there is steam coming from the engine, don’t open the hood, as any steam can cause a serious scald.
Inspect the cap and you may find a build-up (debris) which may be the cause of the fault. You could carefully clean it using water and a toothpick. But, it is usually more effective to just replace it because the cost is low, and engine failure could be quite costly.
You could also check for pressure leaks in the system using a radiator pressure tester. Always replace the radiator cap with a new one that has the same pressure rating.
There is a risk that if you replace the radiator cap with one that has a higher pressure before the valve opens, the vehicle will over-pressurize, which can lead to issues such as overheating or hose rupture.
While you’re checking the radiator cap and making the replacement, you can also check the other parts, such as the reservoir and hoses, and inspect them for cracks or other concerns.
A faulty radiator cap has symptoms similar to other cooling system problems, so it is possible that there could be two weak points at the same time. Therefore, if you replace the radiator cap, it’s likely that the other weak point will fail.
Cost of A New Radiator Cap
Radiator caps have a relatively low cost to replace. Since you can do it yourself, it also avoids the labor costs of a mechanic.
The caps themselves are usually less than $25 at the auto parts store, although you may expect to pay more; the additional cost would be for a radiator cap with the built-in thermometer that indicates the coolant temperature.