Symptoms of a Bad Radiator Cap (How to Test and Fixes)

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.

Symptoms of a bad radiator cap

bad radiator cap overflow

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.

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.

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.

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.

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.

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.

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?

Here are a few steps of testing a radiator cap to know if it’s time for a replacement.

  • Start your car and leave it on for about 10 minutes before you open the hood. Check for coolant leaks or bubbles from the cap and the hoses.
  • If there aren’t any visible signs of leaks, use gloves or a thick cloth to slowly unwind the radiator cap. Don’t remove the cap or you risk burning yourself.
  • The loose coolant radiator cap should release small amounts of steam and bubbles to indicate that the radiator coolant is under the recommended pressure. Failure to release steam means that the radiator cap is faulty and has a pressure leak.
  • Alternatively, you can use the OEMTOOLS 27065 Radiator and Cap Test Kit to test for pressure faults and leaks from your radiator cap.
  • To use this device, you need to turn off your car and wait for at least 30 minutes before you remove your radiator cap.
  • Connect the pressure gauge or tester to the cap adapter and screw it tight.
  • Fill the gauge with pressure by pumping the pump handle of the tester until it is filled.
  • Check the tester gauge reading against the recommended PSI readings on your car manual.
  • You will know when to replace a radiator cap depending on whether your gauge readings are within your car’s recommended pressure indications in your user manual.

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.

How to fix a radiator cap

In case you notice that your radiator cap is leaking because it cannot seal properly you can use a tablespoon or two of black pepper as a temporary radiator cap fix to make the coolant course enough not to leak. However, you will need to rush to a repair shop to replace the cap.

Here are the steps to replace a faulty radiator cap.

  • Turn your car off and let the engine cool down. Opening your hood and handling engine parts when hot is dangerous and risks bodily injury.
  • Locate the radiator cap and slowly unwind it to open. You should expect a slight release of pressurized steam from the radiator as you unwind the cap. Please make sure you handle the cap using protective gloves or a thick rag to avoid getting burned. Remove the cap by lifting it after you have successfully unscrewed it.
  • Remove the mounting shield bolts to access the whole radiator.
  • When the radiator is accessible, loosen the radiator drain plug. At this point, radiator coolant fluid will start draining. You can dispose of the antifreeze by using sand or baking soda to soaking up the spillovers. You can also use towels to efficiently clean the mess as much as possible.
  • After all the fluid has drained, remove all the mounting bolts from the reservoir followed by the coolant overflow tube from the radiator neck.
    Removing all fluid coolant and checking the radiator parts is necessary to ensure that there aren’t other resulting damages from the bad radiator.
  • Now 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. Watch out for any discharge or bubbling around the newly fixed cap.

Please note that you should avoid purchasing generic radiator caps because they may not have the right pressure controls required by your vehicle. Buying your specific vehicle’s cap reduces the probability of it failing soon.

Safety tips for opening a radiator cap

when to replace 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 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.

Bad Radiator Cap Symptoms & Signs YouTube

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.

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Osuagwu Solomon

Osuagwu Solomon is a certified mechanic with over a decade of experience in the mechanic garage, and he has over five years of experience in the writing industry. He started writing automotive articles to share his garage experience with car enthusiasts and armature mechanics. If he is not in the garage fixing challenging mechanical problems, he is writing automotive repair guides, buyer’s guides, and car and tools comparisons.

3 thoughts on “Symptoms of a Bad Radiator Cap (How to Test and Fixes)

  1. Hey Paul,

    Your 1997 Ford F-150 should be pretty old and its expected to notice one or two issues down the line. Several things, including clogged radiator cap, collapsed radiator hose, overflowing coolant reservoir tank, weak fan clutch, and coolant leak are the most common possible reason the engine blows hot while sitting but clears up when driving.

    I’d suggest you check and flush the radiator, check for collapsed water hose, and the fan clutch.

  2. My 1997 Ford F-150 blows nice hot air sitting still until I drive it. It gradually cools way down as I drive. Can it be a bad radiator cap? I have tried and changed everything else.

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