Blog Car Care Advice 8 Bad Radiator Cap Symptoms You Shouldn’t Ignore (+ the Fix)
Car Care Advice

8 Bad Radiator Cap Symptoms You Shouldn’t Ignore (+ the Fix)

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You noticed a bright green, sweet-smelling fluid under your car and knew it was a coolant leak at hand.

Is it a bad radiator cap at play here?

Keep reading as we familiarize you with bad radiator cap symptoms and what to do if you experience them.

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Let’s go!

8 Common Bad Radiator Cap Symptoms

Here are eight tell-tale signs pointing to a failing radiator cap:

1. Coolant Leak

A faulty radiator cap can cause excess pressure buildup within the cooling system. This forces the coolant (radiator fluid) to leak through radiator hoses or the water pump seal. A worn radiator cap gasket can also cause leaky coolant. 

Important: A coolant leak will eventually result in a low coolant level, leading to engine overheating and triggering the coolant warning light. This can cause severe engine damage if left unaddressed.

2. White Streaks on the Radiator

When the coolant leak dries, it leaves behind white stains or streaks. 

So, if you can’t see the radiator coolant leak when the engine has cooled down, you should check for white streaks. It indicates a failing radiator cap that’s leaking under pressure.

3. Air Pockets in the Cooling System

A damaged or loose radiator cap can allow air to enter the cooling system. This can create air pockets in the cooling system, reducing the coolant’s effectiveness and causing engine overheating.

4. Overheating Engine

As mentioned above, a radiator coolant leak or air in the cooling system can cause engine overheating. So, if you see the temp gauge (temperature gauge) over the red or the engine temperature dashboard light comes on, you should check for a bad cap. 

5. Overflowing Coolant Reservoir

A stuck-open radiator cap will result in low pressure in the cooling system, lowering the coolant’s boiling point. As the engine temperature rises, the coolant can boil and will be pushed out of the radiator into the overflow tank. When the excess liquid coolant and steam continuously overflow into the reservoir tank, it can fill up beyond its normal level.

6. Collapsed Radiator Hose

When you turn off the engine, the pressure in the cooling system drops, creating a suction effect. A radiator hose can lose its shape if the pressure cap is defective and doesn’t open due to this suction force. This is because the pressure inside the cooling system would be lower than the atmospheric pressure, resulting in a collapsed hose.

Additionally, the radiator fluid won’t be able to flow back into the cooling system from the overflow reservoir (coolant tank).

7. Radiator Hose Bursts

If the pressure in the cooling system builds up excessively due to a faulty radiator cap, a radiator hose may burst and release coolant all over the engine bay.

Typically, the pressure isn’t high enough to cause a burst. But as you drive with a bad cap, the pressure may build up and exacerbate a small radiator hose leak. In this case, you’ll likely notice leaky coolant below your car.

8. Steam from Under the Hood

A faulty radiator cap can fail to maintain pressure and cause the liquid coolant to boil and escape as steam through the coolant system. This can lead to a low coolant level over time.

Additionally, a radiator hose leak can drip coolant on the hot engine exterior, resulting in vapors rising from the engine bay.

Unsure if you have a bad radiator cap?
Let’s see how you can confirm and fix the issue.

How to Check and Replace a Bad Radiator Cap

Caution: A radiator cap should only be replaced when the engine has cooled down. Opening the radiator cap while the engine’s running or just after it’s switched off can result in severe burn injuries from the hot coolant rushing out. 

Here’s how to check for a faulty radiator cap:

  1. Check if there’s a loose radiator cap and tighten it.
  2. Replace the cap if it can’t be adequately tightened or its metal body is damaged.
  3. Check the seals on the radiator cap and replace them if worn out.
  4. Check the spring on the radiator cap for rust. Replace the radiator cap if the spring is rusted or compresses with difficulty.
  5. Pressure test the radiator cap to see if it holds the pressure specified for your car’s coolant system. For the pressure test:
    • Attach the radiator cap to the cap adapter for the tester.
    • Connect the cap adapter to the pressure tester.
    • Pump the pressure tester till a pressure equal to the cap’s rating is achieved.
    • The radiator cap should hold this pressure for about five minutes and shouldn’t lose pressure rapidly.
    • Replace the cap if it fails the pressure test.
  6. Check if the radiator cap’s vacuum valve functions properly. 

Replacing a radiator cap is an easy exercise if you’re familiar with the process. You just need to take off the old cap and put a new cap in its place. However, always use a radiator cap with a pressure rating specified for your car’s cooling system.

Here’s why:

Not sure about handling this yourself? 
You could leave it to a professional mechanic, like those at AutoNation Mobile Service.
We can also inspect the cooling system for a damaged hose, a faulty thermostat, or radiator problems like a clogged radiator to ensure a hassle-free drive.

Have more questions about the radiator cap?
Let’s address them.

4 FAQs about the Radiator Cap

Here are answers to four common queries about the radiator cap:

1. What Does a Radiator Cap Do?

The radiator cap lets coolant do two things:

  1. Absorb engine heat and expand by keeping the system pressurized.
  2. Contract and travel back to the radiator when cooled.

Here’s how:

The cap keeps your car’s cooling system “closed”, preventing air and contaminants from entering it. It acts as a pressure valve, allowing the coolant’s pressure and boiling point to rise. This helps the coolant fluid effectively absorb engine heat.

If the pressure gets too high as the hot coolant expands, the radiator cap directs some coolant into the overflow reservoir, maintaining the required pressure. Conversely, the vacuum valve in this pressure cap also lets coolant flow from the reservoir tank back to the main cooling system as the engine temperature lowers.

2. Can You Drive with a Bad Radiator Cap?

You shouldn’t drive with a defective radiator cap as it can damage your car’s cooling system and overheat the engine. However, if you have no option but to drive, keep checking the coolant level frequently and head to a mechanic for inspection

And if the coolant warning light comes on, pull over safely to the roadside and get your car towed to a workshop to avoid engine damage.

3. How Much Does Radiator Cap Replacement Cost?

A radiator cap can cost between $5 to $30, depending on the make and model of your car.

4. Does Every Car Have a Radiator Cap?

No, cars with a pressurized coolant reservoir (expansion tank) don’t usually have a radiator cap.

In this case, the expansion tank in the engine bay has a pressure-rated cap, unlike the plastic cap on an overflow tank. The coolant fluid goes through the expansion tank for every cooling cycle, just as it goes through the radiator.

Resolve Radiator Issues with AutoNation Mobile Service 

The radiator cap is a key component of most cars’ cooling systems and can go bad with age and use. When it does, you may experience several signs, such as coolant leakage or a collapsed radiator hose.

Is your car showing symptoms like these?
While you may be tempted to replace the radiator cap yourself, it’s best to have a mechanic handle it if you’re unsure. This ensures your safety and your car’s well-being.

That’s where AutoNation Mobile Service can help. We are a mobile auto repair service that can fix your car’s bad radiator cap and other problems right in your driveway.

Contact us and our technicians will get your car up and running in no time.