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What is diesel exhaust fluid (AdBlue) and why is it used?

Diesel exhaust fluid (DEF) helps heavy vehicle drivers achieve Euro-5 emission control standards by reducing the level of nitrogen oxides emitted. As the spotlight is turned on pollution caused by diesel vehicles, it has gained popularity due to European trucks imported to New Zealand featuring it. Even some SUVs, vans and cars require it. It’s commonly known by trade names, such as AdBlue and Penblue.

AdBlue is made from water and urea and is added to a separate tank next to the diesel tank. This is automatically injected into the exhaust system. It’s important that it is never mixed with diesel because it causes terminal damage to the engine and fuel system which costs tens of thousands of dollars to repair. Contamination would, at minimum, require you to change the fuel filters, high-pressure fuel pump and the injectors. However, it is safe to handle and non-toxic.

AdBlue tank with level indicator

If AdBlue is added to the engine, serious corrosion can occur. As it’s very difficult to clean, usually all parts are replaced to prevent ongoing damage.

This is beyond repair and must be replaced
AdBlue clogs up fuel filters

As the tanks are usually next to one another, the AdBlue tank will usually have a restrictor that makes it difficult for you to put diesel into it. Even as little as 5ml of diesel is enough to clot the AdBlue and cause damage.

AdBlue levels should be checked as part of the pre-trip inspection process.

Tired drivers filling at nighttime in bad light need to take care when tanks are next to one another.
A protective cover with a warning helps avoid confusion between the AdBlue and diesel tank
The AdBlue tank has a smaller opening than the diesel tank.

What should you do if you accidentally add AdBlue to your fuel tank?

It’s important not to start your engine. If you can drain the fuel tank without any of the fuel entering the engine, then you’ll just lose the fuel rather than have a massive repair bill.

How full should you fill your AdBlue tank?

Products like AdBlue contain water and can freeze and expand. For this reason, don’t overfill the AdBlue tank. Some tanks have a level indication tube (see above); if not, only fill until the pump shuts off, or if you’re filling from a small container, ensure that there’s an air gap at the top of the tank.

Don’t allow liquid to enter the tank’s breather tube otherwise it will crystallise and block the breather tube, preventing the system from drawing in AdBlue. At first, you’ll start seeing error codes, and eventually this will cause the engine to not start.

How should you store AdBlue?

Sunlight and dust reduce the life of AdBlue. Store it with the cap tightly on and in a dark cupboard.

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Darren is a member of the Institute of Advanced Motorists and the NZ Motoring Writers' Guild

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