What is diesel exhaust fluid?

More diesel powered vehicles require diesel exhaust fluid (DEF) - otherwise known as, Z DEC, AlliedBLue, GoClear, or AdBLue - but what exactly is it?

DEF is a solution that’s made with 32.5% liquid urea and 67.5% deionised water. It’s used as a consumable in selective catalytic reduction (SCR) in order to lower the concentration of harmful pollutants in the diesel exhaust emissions.

In the United States, the EPA’s (Environmental Protection Agency) rules for diesel vehicle emissions are the most stringent standards in the world. The majority of manufacturers have found the only way to comply with the EPA’s rules, without compromising engine performance and fuel efficiency, is through SCR, which has been common in many truck engines for years.

Diesel engines with SCR systems fitted will have a separate tank for holding the DEF. The fluid is injected into the pipeline of the exhaust, causing a chemical reaction and it’s within the SCR system that the harmful pollutants are then broken down into nitrogen and water, which are released through the exhaust. Mercedes-Benz BlueTec, Volkswagen, Peugeot and Citroen BlueHDi, are just some examples of cars that are equipped with SCR systems.

If you have a vehicle that requires DEF, it’s important to store and use it correctly to avoid contamination and causing damage to your vehicle. A cool, dry, and well-ventilated area that’s out of direct sunlight makes a great storage environment for DEF. While this fluid isn’t toxic, it is corrosive to some metals such as carbon steel, aluminium, copper and zinc. This means if this fluid is ever poured into the fuel tank it can cause serious damage to your vehicle.

Generally, the size of the DEF tank has been designed so that owners don’t have to refill between service intervals, but, if you do need to fill up on DEF, you can buy a small container of the fluid from a conventional service station and vehicle manufacturers. Make sure you remove the correct cap for the DEF tank - it’s generally the blue cap on the vehicle which is usually clearly labelled. If you choose to fill up at a truckstop, make sure that you don’t get the DEF and the diesel fuel pumps mixed up. Some will have completely separate dispensers for DEF and diesel fuel, making identification between the two easier. However, be aware that some stations will dispense DEF at the same pumps as diesel which can be confusing. Always read the signage on fuel pumps carefully before filling up and if you’re ever unsure which pump to use, it’s better to stop and fill up at a regular service station.

If you make the mistake of putting DEF straight into the fuel tank do not turn on the ignition or start the car and get the fluid drained and disposed of correctly. Any contaminated components will need to be flushed out, and they may even require replacement to avoid long-term corrosion.

On average, a litre of DEF should last about 1000km, however, usage can be higher depending on how and in what conditions the vehicle is driven. Some vehicles will have an alert system to warn the driver when the DEF tank level drops below a certain percentage, making maintenance really easy. It’s also important to note that DEF doesn’t last forever and that generally it has a two year lifespan from its production date.

Keeping on top your exhaust fluid levels is crucial because vehicles equipped with an SCR system cannot operate without DEF. Failing to keep your exhaust fluid levels topped up can compromise your vehicle’s fuel economy and can also cause your car to shut down completely. So, don’t let your DEF run out or you could find yourself broken down at the roadside – a situation everyone wants to avoid.

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