Top 6 Signs of a Bad Radiator Cap (Replacement Cost)

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The engine in every car runs constantly for the car to keep moving – the consistent rotating of the engine components generates a lot of heat, and that calls for the need for a cooling system in every car.

There are about 5 major components that make up the cooling system in modern automobiles. They include the radiator, water pump, thermostat, freeze plugs, and cooling fans. Particularly, the radiator has a cap that keeps the liquid inside from pouring out while the car is in motion.

However, just as in other parts of a car, the radiator cap can get bad – damaged – due to several reasons. Some of the signs of a bad radiator cap include engine overheating, coolant leaks, White streaks on the radiator top, low coolants, torn radiator hoses, and hot steam.

This article explains the symptoms of a bad radiator cap in all vehicles. Yeah, regardless of the vehicle type, these signs explained in this article are what you’d most likely experience when your car’s radiator cap is faulty.

What are the Signs of a Bad Radiator Cap?

1. Coolant Leaks

Isn’t that obvious? Of course, it is. One of the major signs of a bad radiator cap is coolant leaks. The radiator contains coolant liquid, which it passes – circulates – throughout the engine to keep the engine cool and running at operational temperature.

The moment the radiator cap is broken, cracked, or too loose to cover the radiator, then the coolant liquid would pour out as the car moves, especially when the car runs through bumps, or when the car gallops, due to potholes, probably. You’d see the coolant leak out from under the engine bay.

When you see this happening, it is advisable to check the radiator cap, and also the radiator itself, for cracks and physical damage. Notwithstanding, other things cause coolant leaks too.

2. Engine Overheating

When the radiator cap is broken or bad, it causes coolant to leak out, or evaporate faster as the car drives; this will cause a decline in the delivery of coolant liquid to the engine – by the radiator – to keep it at operating temperature.

Now, when the engine is not cooled down, due to a lack of coolant in the radiator (caused by a broken radiator cap), then the engine would overheat, and this is one thing you must try to avoid.

An overheating engine can cause a wide range of problems that are expensive to repair, and in worst cases, overheating can fray an engine, which would warrant buying a new engine for the affected car.

Again, engine overheating can be caused by a plethora of causes, but a broken/damaged radiator head is one of the major causes. If you’re able to detect this on time and fix it, you’d save a reasonable sum, which would have gone into repairs and parts replacement if the issue (overheating) was allowed to linger.

3. Busted/Torn Radiator Hoses

You changed the radiator hose for your car, and the new one got damaged pretty much earlier than you expected? Chances are that you’ve got a bad radiator cap. When the radiator cap is bad, it makes the internal pressure of the radiator fluctuate.

Coolant/radiator hoses are not sturdy, when pressure is high on them, they keep expanding, and when the expansion limit is reached, they’d burst or get torn. If this is the case with your car’s coolant hoses (getting torn always), then you need to check the radiator cap.

4. White Streaks on the Radiator Top

When coolant pumps up and escapes from the radiator (due to a bad cap), some remnant coolant liquid would liter around the radiator, and when they dry up, they’d form white streaks. That said, if you discover that there are white streaks around the radiator cap region, it could be a sign of a bad cap.

Well, the white stripes may not appear only on the top (cap) region of the radiator, you could see them all over the radiator’s body.

These white streaks/stripes are the chemical ingredients used in formulating coolant; when these chemicals react with a “very hot” metallic surface, they dry up immediately, and typically appear whitish.

5. Low Coolant Always

If you find out that your car seems to exhaust coolant more than it used to, then it’s actually a sign to check the cooling system. Since a bad radiator cap would cause coolant to leak away, or evaporate as hot steam when heated up, seeing the coolant level low at all times is an indication of a bad radiator cap.

Of course, the coolant liquid is not expected to go down, below a specified mark/level as indicated on the reservoir’s body. If coolant goes below the indicated level, you’re risking overheating, which could cause costlier damages and repairs.

6. Hot Steam

A bad radiator cap would allow coolant to escape as hot steams. This happens when the coolant heats up to boiling temperature. Yes, your car can heat up very high, and once a component of the coolant system is bad, a lot of anomalies are likely to occur.

What is the Function of Radiator Cap?

The radiator cap plays a crucial role in keeping the engine cool and functional. Basically, it performs five functions, which are explained below.

  • It keeps the coolant inside the radiator from escaping – leaking out or evaporating as hot steams
  • Protects the coolant liquid from outside contaminants that may be harmful to the system, and the engine too
  • The cap helps to maintain the internal pressure required for the cooling system to function effectively
  • When pressure increases inside the radiator, the cap allows the coolant liquid to flow into an expansion recovery tank to prevent the coolant hoses from blowing out, as well as prevent other damages
  • Air is an enemy to cooling systems, the radiator cap helps to keep air away – preventing it from penetrating the coolant system

How To Check a Bad Radiator Cap

Caution! Never attempt to remove the radiator cap after you just stopped the car. For the fact that you have driven the car, the radiator cap would be very hot, and touching it with bare hands can cause serious burns on your palms.

Also, even if you manage to open the radiator cap while it’s still hot, the already heated coolant pressurized inside would erupt, and may likely touch you, which would also cause serious burns.

That said, you should allow the car to cool down before performing any check on the radiator cap, or the radiator itself. Alas, when the car is now cool, remove the cap and inspect it carefully for cracks, physical damage, or broken part.

You can also get a radiator pressure tester to test the pressure level of the cap. When the radiator cap is unable to maintain a certain level of pressure, it’d need to be replaced with a new one, so that the radiator can perform well as expected.

Cost of Replacing the Radiator Cap

A new radiator cap costs between $5 to $30, and labor costs can be anywhere around $20. Therefore, the average cost of replacing a faulty or damaged radiator cap is $50.

Note that the cost of a radiator cap is based on the car model and make. You can do the replacement yourself and save on labor costs.


In summary, explained in this article are the top 6 signs of a bad radiator cap. If you experience any of these signs, do well to have the radiator cap checked, and if it is the culprit, getting it changed as soon as possible would save your engine and prevent a lot of potential damages that could ensue due to the bad cap.

However, you should be careful never to attempt opening the radiator cap while the engine is still hot; doing such would leave you with serious burns. To replace a bad radiator cap would cost you an average of $20 if you’d do it yourself, and $50 if you’d have a mechanic or someone else do it for you.

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