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Your need-to-know guide to diesel particulate filters

What are Diesel Particulate Filters?

In 2009, European legislation determined that automotive manufacturers must fit all new diesel vehicles with a Diesel Particulate Filter (DPF), in an attempt to reduce emissions and meet ever-tightening regulations.

Fitted to the exhaust system of all diesel cars produced since 2009, the DPF is designed to catch and trap harmful particles, and prevent them being released into the atmosphere. Detrimental to health, the minute particles emitted from diesel engines have a direct link to respiratory and cardiovascular problems and with sales of diesel vehicles ever-increasing, the DPF is a vital step towards reducing levels of pollution and improving air quality.

Yet, while beneficial for the environment, the DPF is not without problems, with some diesel motorists finding themselves out of pocket when the filter clogs up and requires a costly replacement. Like any filter, the DPF needs emptying on a regular basis, with diesel vehicles now designed to self-clean the filter in a passive process known as ‘regeneration’.

This process occurs automatically when the engine reaches temperatures hot enough to burn off the particulates on the filter, rendering them harmless to the environment. For the engine to reach such temperatures, the vehicle needs to drive at high speeds for an extended period, i.e. a long motorway journey. Problems arise however when the primary use of a vehicle is for slow stop-start about-town driving, with many motorists not using the motorway or A-roads often enough to raise the engine to the optimum temperature for the regeneration process to occur.

Active Regeneration

Owing to this problem, car manufacturers recently built in a new process known as ‘active regeneration’, in which engine software senses a blockage in the filter and initiates the cleaning process. By injecting fuel, or an additive, on to the filter to raise the temperature, the filter can be cleaned effectively even at lower driving speeds. This process is usually initiated, on average, every 300 miles, and takes up to 10 minutes to complete.

Although this seems like an ingenious solution, problems continue to arise when a car is only used for short journeys and the regeneration process is left incomplete. Although a blocked DPF will not stop your car from running immediately, it will restrict performance and cause the engine management light to show, which in turn will cause an MOT failure. This is where it can get expensive, with the DPF costing motorists more than £1000 to replace.

The obvious solution may be to remove the filter completely but with DPFs being fitted to meet EU legislation, it would be harmful to the environment and a criminal offence to drive a vehicle which does not comply with these regulations.

Therefore, with filter removal not a legal option, what else can be done? First, when in the market to purchase or lease a new car, it would be sensible to consider what the primary use of your vehicle will be. If you know that you will mainly use this car for slow-moving around-town driving, it may be pertinent to consider petrol alternatives as opposed to diesel, thus avoiding the issue altogether. Although, it has been estimated that a Diesel Particulate Filter should last upwards of 70,000 miles from new when properly maintained, meaning that opting for petrol vehicles isn’t the only solution.

So, what can you do to avoid needing to replace your DPF prematurely?

Alter your driving habits

Do your research and find out how many miles you need to drive, and at what speed, to initiate the regeneration process and allow it sufficient time to complete. This will prevent any build-up of soot and particulates in the filter and extends the life-expectancy of your DPF.

Use the correct engine oil

Check your vehicle handbook and be sure to use the manufacturer recommended oil for your vehicle. Most manufacturers suggest a low SAPS oil, specifically formulated to be low in Sulphated Ash. This means that your engine produces as little particulate matter as possible, therefore lowering the possibility of a clogged DPF.

Get a regular service

Some cars are now made with an additive tank which needs to be refilled for the active regeneration process to work effectively. Having a low level of additive in the tank can prevent regeneration from occurring. So, keep the levels topped up to achieve optimum performance.

Use a high-quality fuel

As with oil, the quality of the fuel you use can affect the amount of particulate matter that is produced. A low sulphur content will produce less particulates and therefore lower the chance of the DPF being blocked.

Check your Exhaust Gas Recirculation (EGR) valve

A faulty EGR valve that is stuck open will increase the level of particulates that are generated and fed back into the engine, causing the filter to become clogged quicker than usual. A warning light will usually show if there is a problem with the EGR valve, but if you are having problems, a quick check by your local mechanic may be all that is needed.

Don’t ignore a warning light

Putting into practice the above tips should prolong the life of your DPF and prevent any major problems, however, should a warning light show on your dashboard, do not ignore it. Contact a mechanic as soon as possible to obtain advice and guidance on how best to proceed. Doing nothing could ultimately result in a replacement filter and a lot of hassle which could easily be avoided.

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