Steam Coming From Radiator Cap [SOLVED]

If you decide to check under the hood of your car and unexpectedly see thick smoke or steam coming from radiator cap– there could be a couple of things that have gone wrong. It’s best to immediately solve the problem; while it’s not the worst thing to happen, it’s also not the best idea to ignore it!

How do You Know the Smoke Coming from your Radiator Cap is Steam? 

You’ll notice that this isn’t foul-smelling pollutant smoke but thick and white steam. It will also be moist (though you definitely should not try to touch it). 

Smoke Coming from your Radiator Cap

Important to note: Do not touch the radiator cap when you see steam coming out of it. If you attempt to pry the cap open whilst steam is escaping, scalding hot water will spurt out, likely scalding you and covering other parts of the car with hot water — which are both undesirable outcomes!

Why is there Steam Coming From my Radiator Cap? 

The bottom line is that you’re probably having steam coming out of your radiator cap because the cap itself or the gasket of the cap is faulty and needs to be replaced. Most of the time, simply replacing the inexpensive radiator cap will solve the problem. 

Steam Coming From my Radiator Cap

However, there are still a few cases where you might have to do some more troubleshooting. Let’s look at the many causes and solutions for steam coming out of your Radiator Cap. 

Causes for Steam Coming From Radiator Cap + Solutions

One of the first things you will notice is that the reservoir is filled with coolant, this is probably because of the blown gasket. If you see steam, make sure to check the reservoir immediately. If it has coolant all over it, it’s clear to see that the gasket is to blame!

Here are a few of the reasons why this could be happening, as well as a solution for how to deal with it!

1. Coolant was spilled

Coolant was spilled

If you refilled coolant in your radiator and happened to spill it around the radiator, that could be one of the causes for unexplained steam.
This is because the radiator has now risen in temperature and, as a result, is causing the surrounding spilled coolant to evaporate. This is why you see thick steam. 

However, please note that in this case, you won’t be seeing steam directly from the radiator cap. Rather, it will be somewhat surrounding the radiator or appear from the radiator itself. If the steam is very clearly coming from the radiator cap and not from any surrounding areas: then you might have a different issue on your hands than this one. Refer to the next points. 

Solution: The solution to this particular problem is the most straightforward and simple of the lot. You simply switch off your car and engine and let it cool down completely to room temperature. Then you perform a careful cleaning of the radiator. You can do this with a slightly damp cloth, being careful not to touch any of the other parts.

Once the car is switched on, you shouldn’t see any more steam.

2. Radiator Cap Needs to be Replaced

Radiator Cap Needs to be Replaced

This is another very straightforward cause and solution on the list. Luckily, it also happens to be the most common problem and can always be fixed on your own without having to consult a mechanic. 

What happens very often is that the radiator cap you are using is no longer tight enough. This happens because the gasket has worn off. You should generally expect a radiator cap or its gasket to last about five years, at around 150–225,000 mileage, but sometimes it can give away before that as well. 

The rubber of the gasket tends to loosen up, rendering the radiator cap insecure. Without a tight cap, the radiator is no longer under pressure, and this allows it to come to a boil, thus releasing thick white steam.

Solution: A replacement of the radiator cap is all you need to do in this situation. You will generally find a replacement very easily. It should be available in any local spare car parts store in your locality.

It’s very important to ensure that you are sourcing the right size radiator cap or gasket for your car. Needless to say, the wrong size is not going to solve the problem and might actually worsen it. You might have to wait for some time if your car model is not a common one and the radiator cap is not available — since it takes a few days to order it online and source it. If the option is available, always opt for the radiator cap from your car manufacturer, even if it slightly more expensive than a generic one. It is likelier that it will be a closer and better fit. 

3. Coolant and Oil Mix

Coolant and Oil Mix

The thermostat being faulty is often a side effect of the radiator cap needing to be replaced. You will notice that the thermostat is bent almost in half on its way to the H. 

Another issue could be that the coolant and oil have somehow gotten mixed. You have to do a check to see if somehow both these liquids have gotten mixed up since that will definitely be the cause for some problems. If this is the case, make sure that you do a compression test. Here’s how to do it. For more information on a compression test, click here.

  • Ensure you remove the fuel-injection fuses and fuel pump. Remove spark plugs as well. 
  • Start the threaded end of your compression gauge. Do this with a spark plug in hand. 
  • Then, turn the ignition switch on, depress the throttle, and then perform four-engine revolutions. Do the same with all cylinders. 

Also Read: Our friends at RodsShop have written a really good article about getting your car ready for a road trip in Summers, you can click here to check it out.


While there could be a few things that have gone wrong when steam is coming out of your Radiator Cap, they are all mostly interconnected. Most of the time, you can easily solve the issue by yourself. In fact, asking a mechanic about steam coming from your radiator cap will almost certainly prompt them to tell you to change the gasket!

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