Signs Your Radiator Cap Needs to Be Replaced

Stephen Fogel
March 30, 2018


Your radiator cap is a small part that plays a big role in keeping your engine from overheating. There are several warning signs that your radiator cap is not working right:

1. You have a coolant leak: If your coolant is leaking, it could mean your radiator cap is bad. If the cap isn’t releasing excess pressure properly, that pressure can force the coolant to leak out at the point of least resistance. This can be from the cap, a hose, engine gaskets, the water pump or even from the radiator itself, especially if you have an older vehicle with lots of miles on it.

2. Your engine is overheating: If your temperature gauge starts reading high, your radiator cap could be at fault. It could mean the cap is failing to maintain the correct pressure. Inadequate pressure means that the boiling point of your coolant gets much lower and unable to remove sufficient heat from your engine. 

Another possibility: Pockets of air in your cooling system can prevent coolant circulation and lead to overheating. This air can enter the cooling system through a radiator cap with a bad seal. 

To avoid serious engine damage, you must shut off your engine immediately at the first sign of overheating. Do not drive your vehicle home or to your mechanic. To avoid personal injury, do not open the hood to investigate until the car is completely cooled down. Never, ever remove your radiator cap when the engine is warm — the risk of being burned or scalded is extremely high.

3. Your radiator hose collapses: If you open your hood and notice that your radiator hose has collapsed from internal suction, this can be a symptom of a bad radiator cap. Your cap is likely not letting coolant from the expansion tank be drawn back into the cooling system as it cools down. The negative pressure inside your cooling system shows up as a collapsed hose, which can compound your problems quickly.   

4. Your engine boils over: The normal flow of coolant through the radiator cap changes quickly if the liquid begins to boil. Boiling coolant becomes steam. Massive amounts of pressure must be released quickly through the radiator cap. This pressure will be released into the overflow tank, and steam can usually be seen escaping from under the hood. 

If you see steam, don’t open the hood, as it may scald you.  This is another situation where you should shut the engine off right away and let it cool. After it cools, you can open the hood. 

Never drive a car that has lost coolant; you’ll need to add more before the engine is restarted. Call your mechanic for advice before you drive your vehicle.


What is the radiator cap?

The radiator cap sits on top of the radiator and contains a spring-loaded plunger. When the car is cold, the cap can be removed to check the coolant level inside the radiator, or to add coolant.

When working properly, the cap provides a tight seal that helps keep your car’s cooling system pressurized. A properly pressurized cooling system raises the boiling point of the coolant, which allows it to remove more heat and protect your engine at higher operating temperatures. This increases the engine’s efficiency and improves gas mileage. 

Too much pressure can damage components of your cooling system and cause leaks. Before this can happen, your radiator cap releases excess pressure by venting coolant into the expansion tank. Once your engine cools, the resulting drop in cooling system pressure draws the coolant back into the radiator.  

Fixing a bad radiator cap

There’s good news: The radiator cap is easy to check and inexpensive to replace. Be sure to use a replacement radiator cap with the correct pressure rating for your vehicle. 

If you suspect the radiator cap is failing, give it a thorough visual inspection. Look for any damaged, cracked, hardened or worn-out seals. If the seals look bad, replace the cap.  

The spring, plunger and valves inside the radiator cap should have some resistance, but still be easy to move. If there are any issues, replace the cap. 

Your mechanic can use a pressure tester to verify that your radiator cap is opening and closing at the correct cooling system pressure settings. If your cap fails any part of the test, it should be replaced. 

If you see rust inside the cap, or have any other reason to doubt the condition of your radiator cap, replace it. It is such an inexpensive part, and so important to your engine’s health, that there’s no reason not to replace it with a new one.

And if you find oil, foam or anything other than the correct color coolant on the radiator cap, be sure to show your mechanic. This could be a sign of oil mixing with coolant, which could mean you have a failing head gasket or other engine problem. You’ll want to get that addressed right away.

Related repair advice

Some of the issues that accompany a bad radiator cap can cause damage to your coolant expansion tank, too. Check it carefully for cracks and leaks and replace it if it’s damaged. This is another inexpensive component that protects your engine.

Other parts of your cooling system may have been weakened or damaged, as well. It makes sense to have your mechanic check the thermostat, the hoses and their connecting hardware, the water pump, and the gaskets that seal everything. This is a great opportunity to make sure that your entire cooling system can do its job well and keep your vehicle running smoothly and reliably.

If you work on your cooling system yourself, check the service manual closely before adding fluid. Many modern engines have complex cooling systems that require purging all the air from the system through bleeder ports or hoses. These must be removed when filling the system.


Stephen Fogel

About the Author

Stephen has been an automotive enthusiast since childhood, owning some of his vehicles for as long as 40 years, and has raced open-wheel formula cars. He follows and writes about the global automotive industry, with an eye on the latest vehicle technologies.

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