Missing Adblue Cap? Here’s Everything You Need to Know!

The AdBlue, also known as the DEF reservoir cap, is a vital component of the whole AdBlue system’s functionality. It might surprise you, but it doesn’t just keep the fluid in the tank.

The good news is you can drive without the DEF cap in 90% of vehicles; the car and the AdBlue system will still function as it should.

However, there are some risks to be aware of, especially if driving for long periods without the cap. There are also some temporary solutions you can do yourself at home that will solve the problem and eliminate any concerns covered in this article.

What Happens if You Drive Without an Adblue Cap?

Most cars will still drive and function as normal without the AdBlue cap. There is the odd couple of cars, that won’t function because of vacuum issues, but that really is only the case if the AdBlue level is low. The problem is what happens to the solution inside the tank when you drive without the cap, which can result in more severe problems.

AdBlue is a very fragile fluid compromising a specific mixture of deionized water and urea. When it becomes exposed to the elements i.e., the cap is missing or stored in a bottle without a cap, the potency of the fluid weakens. Weak fluid could damage the SCR (Selective catalytic reduction system). So if you drive without the cap it’s important to be aware of what is happening and the consequences:


AdBlue evaporates at 86 degrees Fahrenheit (30 degrees Celsius), just a typical hot day in the summer. The worst part about that is the AdBlue tank is in the trunk of a car (the location varies from car to car); I’m sure you’ve got into a car on a hot day, and inside, the car is hotter than outside.

If all of the AdBlue in the tank evaporates, which it will do quickly, you could run the risk of running out of AdBlue. Running out of it can be especially bad because certain vehicles will not restart when switched off until the tank is refilled. Which you might think is not so bad, but it always comes at a not-so-convenient time.

Concentration change

This is another result of evaporation, but if the water in the AdBlue evaporates, you will be left with a highly concentrated AdBlue solution. Excessively concentrated AdBlue can harm the SCR system and cause issues such as crystallization or blockages in the injector or catalyst.


This is more specific to AdBlue tanks in the engine compartment. Without the cap, fluid can become contaminated by rainwater, oil, dirt, debris, and anything that can get into the tank.

The AdBlue systems and the SCR are very fragile. Contaminants could cause blockages to the pump and injector. Even worse, if they find their way into the SCR, it could also cause it to get clogged. An SCR is similar to a catalytic converter in that the car could misfire or activate limp mode if it becomes blocked.

Temporarily Replace the Adblue Cap

If the DEF cap is missing, it is much better to temporarily cover the reservoir filler hole while you source a new cap. The best materials to use are aluminum foil to cover the hole and gaffa tape to keep the foil in place.

You should consider which materials you use depending on where the AdBlue filling point is on the car. For example, do not use anything flammable if the tank is located in the engine compartment. However, if it is in the trunk you don’t need to worry about using a potentially flammable material.


If the AdBlue cap is missing, you should temporarily cover the hole while you source a new cap instead of driving without one. If the DEF fluid becomes contaminated or evaporates, it can cause damage to SCR, the pump or injector which are all costly items to replace. But if you need to drive the car and can’t cover the filler, don’t panic the car will run as normal, at least for the short term!

My name is Tom although my friends call me Tommy. Messing around with cars and bikes has always been a hobby of mine even from a young age. So I made it my day job 17 years ago. I am a fully qualified mechanic as you would expect. I've worked in all different areas of the motor trade, valeting, panel beating, engine repairs, I'm sure you get the idea. I enjoy sharing my wealth of knowledge and experience with others, which is the reason I spend a lot of time here writing for this website.

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