A radiator cap is the part of a vehicle responsible for containing engine coolant and ensuring that the cooling system remains under pressure. Ideally, car cooling systems should maintain a pressure of around 13 to 16 PSI.
The radiator cap ensures that this pressure remains constant. There are several symptoms of a bad radiator cap like leaking coolant, the appearance of white streaks, and hose bursts caused by coolant pressure fluctuations.
Excessive pressure causes cooling system component failure while less pressure could lead to the liquid coolant boiling off. A car is likely to overheat when it has a cooling system failure.
Vehicle combustion engines dissipate a lot of heat while running and radiator cap malfunctions cause numerous car problems that can eventually lead to complete engine failure.
Take a look at some of the common symptoms of a bad radiator cap or head gasket that we come across.
What is the radiator cap?
The radiator cap sits on top of the radiator and contains a spring-loaded plunger. When the car is cold, the cap can be removed to check the coolant level inside the radiator, or to add coolant.
When working properly, the cap provides a tight seal that helps keep your car’s cooling system pressurized.
A properly pressurized cooling system raises the boiling point of the coolant, which allows it to remove more heat and protect your engine at higher operating temperatures. This increases the engine’s efficiency and improves gas mileage.
Too much pressure can damage components of your cooling system and cause leaks. Before this can happen, your radiator cap releases excess pressure by venting coolant into the expansion tank.
Once your engine cools, the resulting drop in cooling system pressure draws the coolant back into the radiator.
How A Radiator Cap Works?
The radiator cap plays a vital role in your vehicle’s cooling system. Its functions extend beyond just capping the radiator; it ensures the entire system operates under optimal pressure conditions, contributing significantly to engine efficiency and safety.
Here’s a detailed breakdown of how a radiator cap works:
Regulates Pressure
The radiator cap maintains a specific pressure threshold, typically 12 to 16 psi, which varies by vehicle, crucial for preventing coolant boil-over.
This pressure is crucial because it raises the boiling point of the coolant, allowing it to absorb more heat from the engine without boiling.
The cap contains a spring-loaded valve that responds to the pressure of the coolant: as the coolant expands with heat, the pressure builds, and the spring compresses. This mechanism stabilizes pressure, ensuring efficient engine cooling and heat dissipation.
Seals the System
Beyond pressure regulation, the radiator cap ensures that the cooling system is hermetically sealed. This seal prevents the loss of coolant via evaporation and maintains the system’s integrity, allowing it to function at the designed pressure.
Any compromise in the seal can lead to decreased pressure, leading to overheating and reduced efficiency.
Relieves Pressure
To protect against excessive pressure build-up that could damage the engine and cooling system components (e.g., hoses, radiator), the radiator cap’s valve opens when the pressure exceeds its designed threshold.
This release valve prevents the system from reaching pressures that could cause operational failures or catastrophic damage by venting excess pressure into an overflow reservoir.
Coolant Recirculation
The radiator cap also facilitates the recirculation of coolant. As the engine cools, the pressure in the cooling system decreases, causing the spring in the cap to expand.
This expansion pulls coolant back from the overflow tank into the radiator through a vacuum valve, ensuring that the radiator remains full and eliminating air pockets that could cause hot spots and overheating.
Common Causes of a Leaking Radiator Cap
The best way to prevent radiator cap leaks is to stop or prevent the factors that cause them. Here are the common culprits:
#1. Lack of Maintenance.
Poor maintenance affects the radiator cap like other components of your vehicle. It could damage the cap or aggravate any issues the part might have.
Is a busy schedule or a tight budget preventing you from taking your ride to an auto repair shop for proper maintenance?
Why not do it yourself? Include a radiator cap inspection in your maintenance routine or have it replaced if you have doubts. Check for leaks or overflowing coolant as well.
However, it’s still better to let a licensed mechanic do the maintenance. Doing so prolongs the lifespan of your vehicle and could even save you money in the long run.
#2. Wear and Tear.
Wear and tear on the radiator cap is unavoidable as your ride racks up mileage. So just keep an eye out for the symptoms of an aged cap, like rust, a loose fit, and cracks.
As mentioned above, it’s best to check your vehicle at least once a year to prevent wear and tear. Generally, the radiator cap should last as long as the vehicle’s lifespan.
When you pull the radiator cap off, you should see a spring sticking down. On some vehicles, the cap fails and the spring falls out.
#3. Damaged Radiator Hose.
Internal pressure that warps or tears the radiator hoses result in a vacuum effect that leads to a damaged or leaking radiator cap.
Similarly, the situation also occurs vice versa. Meaning, an already damaged radiator hose tends to rupture the hoses.
Symptoms of a bad radiator cap
#1. Leaking Coolant.
Manufacturers create every car cooling system to contain a specific range of pressure. Using a radiator cap that holds more pressure than the manufacturers intended for the car may cause the coolant hose to fail, causing coolant leakage.
Leakages also happen when the radiator cap resists additional pressure injection to the coolant reservoir after the cooling system achieves optimum pressure.
The coolant will leak from the weak points of the system due to over-pressurization. The coolant can therefore leak from the hoses, actual radiator, gasket, or the water pump.
You can tell that you have leaking coolant from visible colored deposits when you open your car’s hood or liquid sprays from the coolant hose.
#2. Overheating Engine.
Failing radiator caps that cannot sustain sufficient pressure lower the cooling liquid’s boiling point causing inadequate heat absorption from the vehicle engine. An engine can also overheat because of air pockets in the coolant because the radiator cap is ineffective.
The first faulty expansion tank cap symptom is the significant rise in the temperature reading shown on the dashboard gauge.
Overheating engines damage different car parts that can cause excessive damage to the vehicle. It is crucial to stop your car as soon as you notice your engine overheating. Let your car cool before checking under your hood to avoid injury.
#3. Steaming Engine.
Another symptom of a bad radiator cap is billowing steam from your vehicle’s hood. Billowing steam indicates that the coolant is boiling and escaping in gaseous form through a bad seal or radiator cap. A steaming engine is a red alert for an overheating engine.
Escaping steam is usually very hot and, as a driver, you should take precautions not to pop the hood while it is still steaming. Turn off your car and allow it to cool before you open your hood to inspect your engine problems.
The normal coolant loss through evaporation leads to overheating because the coolant reservoir lacks an adequate amount of heat capacity to accommodate the heat generated by the engine.
#4. Bursting or Collapsing Radiator Hoses.
Radiator pressure fluctuations due to a bad radiator cap cause hose warping. Low pressure creates a vacuum in the hose that has a collapsing effect on them.
Meanwhile, higher pressures tear the hoses. Torn hoses spray coolant at the engine bay and cause a reduction of coolant which leads to vehicle overheating.
More often, higher pressures don’t completely tear the hose in half but create small holes that only open when you are driving the car.
These holes are not visible when the car is off and has cooled down because the hoses contract under lower temperatures.
However, you can notice the holes because of constant coolant loss and leaks on the ground when you park your vehicle at your destination.
We advise that you check your overflow reservoir after driving short distances to see if you have leaking hoses.
#5. Constantly Overflowing Reservoir.
Ideally, the engine releases coolant to the overflow reservoir to balance the pressure when it exceeds the recommended amount.
However, when you have a bad radiator cap, the car may release coolant to the reservoir without the excess pressure trigger.
This causes pressure loss to the radiator leaving it with little or no pressure at all. Therefore, low pressure is one of the signs of a bad coolant reservoir cap.
As mentioned earlier, low pressure causes the radiator coolant to boil off leading to overheating and engine failure.
Therefore, you need to replace the radiator cap as soon as you notice a bad radiator cap overflow symptom.
#6. Air Entering the Cooling System.
Another sign of a failing radiator cap is air entering the cooling system. Unfortunately, you are unlikely to notice air in the radiation system until the hoses and tubes start cracking.
Air spaces in the radiation system cause cracking because of low pressure and vacuum spaces when the vehicle shuts down and cools off. The low pressure causes the tubes to crack bringing about leakages.
#7. Low Coolant Levels.
All the above-named factors can in one way or another cause low coolant levels in the radiator. However, you notice that the factors revolve around a failing radiator cap.
As much as broken hoses are the ones that cause leaks, stuck radiator caps also cause pressure build-up and leaks that reduce the coolant levels significantly.
Radiator caps that don’t seal the right way also cause pressure fluctuations that may also lead to leaks and overflowing reservoirs.
Having low coolant levels or your car losing liquid coolant quicker than usual is one of the main bad radiator symptoms that you should look out for.
How do you test a radiator cap?
The best way to test a bad radiator cap is to use a radiator pressure tester. It can also be used to test any other components of the coolant system, plus it will also determine the leakage points in the coolant path if there are any. Let us take a detailed look at how to conduct a pressure test.
- The first step is to arrange a radiator pressure tester. You can either buy it or rent it for some time.
- If you have just used the vehicle, wait until the engine cools.
- Open the hood of your car.
- Once the engine has cooled down, now open the radiator cap very carefully by pushing it downwards and rotating it counter-clockwise.
- Then you need to find the right cap adapter according to the size of your radiator.
- Now place one end of the cap adapter on the pressure tester inlet. Screw it carefully and ensure that no fluid can pass through it.
- Place the other end of the adapter on the radiator neck.
- Screw the cap adapter until it has become tight. Give it a twist or two even after it has become tight to ensure that there is no way any pressure leaves the system.
- Now perfect vacuum should have been created in the system. And if this is not the case try to find some other adapter for your radiator.
- Turn on the pump electric supply and by using a knob or handle on the pump slowly build up the pressure.
- Stop increasing the pressure once it has reached the pressure reading marked on your radiator cap.
- Now wait for 15-20 mins and notice if the system holds pressure.
- If you notice a pressure drop then look for leakage.
- If there is no pressure drop and still you are observing other symptoms, then chances are that the radiator cap has gone bad. But it has not caused any leakage or bursts so far.
- You must change the radiator cap immediately in this situation.
What happens when you drive a car with a bad radiator cap?
Because the engine depends on the heat absorption capacity of the coolant in the radiator, when you drive a car with a faulty radiator cap the car will overheat.
The car overheats because the pressure I the radiator is too low for it to contain all the heat generated by the engine.
Overheating can cause the head gasket to get blown o worse problems for the engine parts.
A faulty radiator cap can also lead to air entry in the car cooling system which may cause bubbling in the expansion reservoir.
Can a bad radiator cap cause a bubble?
A radiator cap needs to stabilize the coolant in the radiator. Therefore, faulty radiators cannot maintain the smooth flow of the coolant through the pump, block, and radiator.
Bad radiators leak coolant fluid through the radiator neck in the form of steam bubbles and burps.
Coolant leaks also cause overheating and gasket failure. Car owners should monitor and fix coolant leaks as soon as possible to avoid further damaging their engines.
Safety tips for opening a radiator cap
Vehicles produce an immense amount of heat when they are running. This massive amount of heat is regulated by the radiator to avoid overheating and damage to the vehicle’s engine parts.
To avoid burns and injury when you want to inspect your cooling system and radiator cap, you need to follow the following safety tips.
- Turn off your car and allow it to cool before popping your hood open. Although the hood might feel slightly cool, the radiator coolant and the radiator will be very hot if you are from driving your car. Always give the car a few minutes for the radiator fluid to cool from its boiling temperature before touching the radiator cap.
- Use thermal-resistant gloves or a heavy rag to unscrew and open the radiator cap. As previously mentioned, the radiator and its cap may still be hot even when the hood seems to have cooled down. Using thermal-resistant gloves will protect your hands from burns when opening the radiator cap.
- Tilt the cap away from you when inspecting it. Tilting the cap away from your body and face when you open it prevents the pressurized fluid from spraying your face when you open the cap.
How to Fix a Radiator Cap
- Turn off your car and let the engine cool down.
- Locate the radiator cap and slowly unwind it to open. Remove the cap by lifting it after you have successfully unscrewed it.
- Remove the mounting shield bolts to access the whole radiator.
- Loosen the radiator drain plug and drain the radiator coolant fluid.
- Remove all the mounting bolts from the reservoir and the coolant overflow tube from the radiator neck.
- Check that your radiator cap replacement matches the manufacturer’s radiator cap.
- Refill the radiator with fresh coolant and replace the old radiator cap with the new one.
- Tighten the mounting bolts as tight as you can and turn on your car to test your repaired cooling system.
How often should I change a radiator cap?
A vehicle’s radiator cap can last for an unlimited duration. However, certain conditions can spoil a radiator cap prematurely. These conditions include;
Unfavorable Weather
Several climate conditions, like high humidity and salinity in coastal regions, make vehicle parts susceptible to rust. Cold regions and snow-prone areas also subject metal parts to rust, making them wear out faster.
Common inside the Hood Damages
Several damages are difficult for you to prevent. Such damages include oxidation from rainwater, abrasives, and salt that creep underneath the hood cover during precipitation or when you take your car to the car wash.
We recommend that you check your radiator and radiator after six to twelve months to take note of small leaks and fix them to prevent further damage.
FAQs.
Q: What Is a Radiator Cap and Why Is It Important?
A radiator cap is a small component in your vehicle’s cooling system that maintains pressure and seals the coolant from the outside atmosphere.
It is crucial as it maintains the boiling point of the coolant higher, preventing your engine from overheating and keeping the system at the proper pressure level.
Q: How Often Should a Radiator Cap Be Replaced?
While radiator caps don’t have a universal replacement time, it is usually recommended to inspect your radiator cap every time you get your vehicle serviced and replace it approximately every five years or as recommended by your vehicle’s manufacturer.
Q: Can I Drive My Car With A Bad Radiator Cap?
Driving with a faulty radiator cap is possible, but it is not advisable. A bad cap can lead to a loss of coolant, overheating, and increased engine wear over time. It’s better to replace a defective cap as soon as it’s identified.
Q: How Do I Choose the Right Radiator Cap for My Vehicle?
It is essential to use a radiator cap that matches your vehicle’s specifications. You can find the right specifications in your owner’s manual or by contacting the manufacturer. Make sure to check the pressure rating and the size to ensure proper fitment.
Final Words
Radiators and radiator caps play a vital role in the operations of a vehicle engine. The radiator cap ensures the car’s cooling system is under optimum pressure for it to adequately cool the engine and avoid overheating.
Failed radiator caps can cause damage to the vehicle by hose leaks, overheating, and even engine knocking.
You should watch out for leaking coolant, bursts, and damaged pipes, and overheating tendencies from your vehicle to know if you have a bad radiator cap problem.
You should consult a mechanic to check your vehicle as soon as you notice the listed symptoms of faulty radiators.