Troubleshoot Engine

Symptoms Of A Bad Radiator Cap (And How To Check It)

symptoms of bad radiator cap

The radiator pressure cap on your vehicle plays an important part in keeping your engine at the right temperature by managing coolant system pressure. If the radiator cap is not working properly, then you’re going to notice some common symptoms.

As a general rule, the symptoms of a bad radiator cap include an overheating engine, low coolant levels, burst radiator hoses, steam from the engine when driving and coolant leaks.

Many modern vehicles don’t have radiator caps, instead there are bleed valves fitted to the coolant system that can help to regulate the pressure in the system.

7 symptoms of a bad radiator cap

If your car is losing coolant or is regularly overheating, then it’s a good idea to start your investigation by checking the radiator cap, if fitted.

The symptoms of a bad radiator cap can also be the symptoms of other failed parts of the coolant system. It’s worth checking for a faulty radiator cap first as it’s the cheapest and easiest fix.

1. Loosing Coolant from the vehicle

The radiator cap serves two main purposes: it physically seals the coolant system, preventing coolant from escaping and it acts as a pressure valve if the pressure in the coolant system gets too high.

Symptoms Of A Bad Radiator Cap
A typical radiator cap

Over time, the rubber seals on the radiator cap will harden and loose their ability to create a proper seal. If the seal is gone, coolant is going to leak from the radiator, leading to coolant loss from the system.

The other way that a radiator cap can fail is through failure of the pressure spring. The pressure spring is designed to control the flow of coolant to the coolant reservoir when the engine is hot. Coolant expands as it heats, causing an increase in pressure. When the pressure reaches a certain point, the pressure spring in the radiator cap will retract, allowing some of the coolant to escape into the coolant overflow tank.

As the radiator cap wears, the pressure spring can weaken thus allowing coolant into the reservoir at a lower coolant pressure when the engine is cold. This leads to a reduction of coolant flowing around the engine and a less effective coolant system.

2. Coolant Leaks Around the Engine Bay

A faulty radiator cap will increase the pressure within the coolant system when the engine is hot. This can lead to coolant leaks around the engine if the engine continuously increases above its recommended operating pressure.

Common places for coolant to leak from include the radiator hoses (especially at the joints), from around the water pump if the gasket fails, from the the radiator cap itself and from the coolant overflow reservoir.

It’s easy to diagnose a coolant leak by carrying out a pressure test on the system. It can be difficult to find the cause of the leak so it’s always a good idea to replace the radiator cap after fixing a leak. It’s a cheap fix that may prevent more leaks in the future.

3. An overheating engine

A bad radiator cap can cause an engine to overheat. There are a few reasons for this. If the radiator cap seal has failed, there may be air getting into the system as coolant leaks out. This decreases the efficiency of the coolant and how well it regulates engine temperature.

car overheating

An overheated engine can be caused by a bad radiator cap

The radiator cap also regulates coolant pressure and this has a direct impact on engine temperature. The higher the coolant pressure, the more heat it can absorb from the engine.

If the radiator cap is allowing coolant to escape into the coolant overflow reservoir or as a leak from the radiator, then this is going to reduce the pressure in the system and reduce the heat absorption of the coolant.

4. Burst or collapsed radiator hose

Radiator hoses and other rubber hoses that are part of the cooling system are sensitive to high and low pressures in the system. Their appearance can be a good indicator of possible problems with the cooling system.

If the radiator cap is causing a regular increase in coolant pressure, this can lead to deformed or burst hoses. Hoses can form small leaks or can burst entirely, depending on the age of the hose and the severity of the pressure in the system.

If you can see bulges in one or more of the rubber coolant hoses around the engine, this is usually a sign of excessive pressure build up when the engine is hot.

Likewise, if there is a coolant leak, a drop in pressure can lead to collapsed hoses. This usually won’t damage the hose as much as an increase in pressure. A collapsed hose is normally visible when the engine cools down and the pressure has dropped.

It can be a sign of not enough coolant in the system. Even though this may not permanently damage the hose, it can cause the engine to over heat, which can lead to expensive permanent damage to the engine.

5. Dried, flaky Coolant around the radiator cap

A classic telltale sign of a coolant leak is dried coolant at various places around the engine bay.

An easy to spot symptom of a radiator cap that is leaking coolant fluid is dried fluid around the cap itself. You may also see dried white streaks of coolant down the side of the radiator, indicating coolant loss from the top of the radiator when the engine is running.

6. Excessive amount of Air in the coolant system

As mentioned earlier, a leak in the radiator pressure cap will allow air to be sucked into the coolant system as the temperature of the engine decreases when the engine is cooling.

This can lead to the engine over heating as the coolant in the system has been replaced by air from outside.

7. Low Coolant level in the overflow reservoir

The radiator cap controls the flow of coolant to and from the overflow reservoir. It is designed to release coolant from that is circulating in the engine to the overflow bottle when the coolant reaches a specific temperature and pressure.

coolant leak when car is not running

 If there is a problem with the radiator cap it may not release the coolant back into the overflow reservoir when necessary, and this not only leads to a low amount of coolant in the overflow bottle, but can also cause leaks and other problems in the rest of the coolant system.

Likewise, if the radiator cap releases the coolant into the reservoir too often or at lower pressures, this can lead to a build up of coolant in the overflow reservoir.

How to tell if the radiator cap is gone bad?

A bad radiator cap doesn’t always look like it has gone bad. Usually there will be one or more of the symptoms I’ve talked about above present, that will help with your investigation.

If the radiator cap has totally failed then you may see signs of dried coolant leakage around the cap and down the side of the radiator. Dried coolant has a white, flaky chalk like appearance.

A bad radiator cap may not tighten properly onto the radiator and this can lead to a sweet smell of coolant coming from the engine by when the engine is running. There may also be a small amount of steam coming from around the radiator cap when the engine gets hot.

Related Questions

1. How Often should you replace the radiator Cap on your car?

You should replace the radiator cap on your vehicle every 3 to 4 years or after 50,000 miles.

The reason for this is that the radiator cap on your car will wear out over time. Even if it hasn’t failed completely, it will not be working as efficiently as it should and this can affect the performance of the cooling system.

The radiator cap is cheap and easy to replace costing from $15 to $25, so it’s a good idea to check it regularly and replace it before it fails completely.

2. Can you drive with a bad radiator cap?

No, it is not advisable to drive your car if the radiator cap is not operating properly. Doing so could cause the engine to overheat, leaving you with expensive or unrepairable damage.

About the author

The Motor Guy

The Motor Guy is a passionate car enthusiast with a love for troubleshooting and diagnosing all sorts of vehicle problems.

With years of experience in OBD diagnostics, he has become an expert in identifying and solving complex automotive issues.

Through, he shares his knowledge and expertise with others, providing valuable insights and tips on how to keep your vehicle running smoothly.

- 12 years experience in the automotive industry
- ASE Master Automobile Technician
- A Series: Automobile and Light Truck Certification, A9 Light Vehicle Diesel Engine Certification
- Bachelor's Degree in Information Systems