Blog Car Care Advice 5 Bad Radiator Cap Symptoms You Shouldn’t Ignore (+ the Fix)
Car Care Advice

5 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.

This Article Contains:

Let’s get to it!

5 Common Bad Radiator Cap Symptoms

Here are the five tell-tale signs pointing to a radiator cap that has seen better days:

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 coolant leakage. As the leaking coolant dries, it can form white streaks on the radiator.

Important: A coolant leak will eventually result in a low coolant level, leading to engine overheating and triggering the coolant warning light. This can result in engine damage if not addressed promptly.

2. 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.

3. 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 radiator 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).

4. 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, it can fill up beyond its normal level.

5. Steam from Under the Hood

A faulty radiator cap can cause the liquid coolant to boil and escape as steam through damaged hoses and seals in the coolant system. This can lead to a low coolant level over time. Additionally, a leak from a radiator hose can drip coolant on the hot engine exterior, resulting in vapors rising from the engine bay.

Are you convinced it’s a bad radiator cap behind those cryptic indications?
Let’s help you get it fixed.

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 with the engine running or just after switching it off can result in severe burn injuries from the hot coolant rushing out. 

Here’s what a mechanic will do to address a faulty radiator cap:

  1. Check if there’s a loose radiator cap and tighten it.
  1. Replace the cap if it can’t be adequately tightened or its metal body is damaged.
  1. Check the seals on the radiator cap and replace it if they’re worn out.
  1. Check the spring on the radiator cap for rust. Replace the radiator cap if the spring is rusted or compresses with difficulty.
  1. Pressure test the radiator cap to ensure it holds the pressure specified for your car’s coolant system. For the pressure test:
  1. Check if the radiator cap’s vacuum valve functions properly. 

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

Here’s why: 

If you’re not sure about choosing the right radiator cap for your vehicle, it’d be best to leave it to a professional like the ones at RepairSmith.

Additionally, it’d be a good idea to get the cooling system inspected 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?
The following section will help you understand it better.

4 FAQs about the Radiator Cap on Your Car

Here are the answers to four common questions about the radiator cap.

1. What Does a Radiator Cap Do?

The radiator cap keeps your car’s cooling system “closed”, preventing air and contaminants from entering it. It also 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 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, overheating the engine. However, if you have no option but to drive, keep checking the coolant level frequently and head to a mechanic for inspection. 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 costs something in the range of $10- $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.

Wrapping Up

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 a number of 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, if you’re uncertain, it’s best to have a mechanic handle it to ensure both your safety and your car’s well-being.

Here’s when RepairSmith can help you. 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 qualified technicians will get your car up and running in no time.