Looking After Your Diesel Filter

If you own a diesel car, you probably have a diesel particulate filter, however, you may not know exactly what this is or how to maintain it.

Diesel particulate filters have been fitted to diesel-fuel cars for almost two decades now - but if not maintained, or if tampered with there could be serious consequences for your car.

Here we explain exactly what they are, what they do, why you need them and how to look after them.

 

What is a diesel particulate filter?

A diesel particulate filter (DPF) is a filter that captures and stores exhaust soot (some refer to them as soot traps) in order to reduce emissions from diesel cars. But because they only have a finite capacity, this trapped soot periodically has to be emptied or 'burned off' to regenerate the DPF.

This regeneration process cleanly burns off the excess soot deposited in the filter, reducing the harmful exhaust emission and helps to prevent the tell-tale black smoke you used to see from diesel vehicles, particularly when accelerating.

Euro 5 exhaust emissions legislation introduced in 2009 to help lower car CO2 emissions effectively made DPFs mandatory, and since then, around one in two new cars a year have been diesel-powered.

 

What causes a diesel particulate filter blockage?

Short journeys at low speeds are the prime cause of blocked diesel particulate filters.

This is why car makers often go as far as recommending city-bound short-hop drivers choose a petrol car instead of diesel (and it’s why diesels are something of a rarity in the city car sector). However, petrol vehicles in the wheelchair access market is something of a rarity.

Other things that are bad for DPFs include poor servicing. A diesel particulate filter on a poorly serviced car may fail sooner than a well maintained one, generally, they should last for at least 100,000 miles.

It’s important you use the right type of oil as well – some oils contain additives that can actually block filters.

 

How do I maintain a diesel particulate filter?

The best way to maintain a DPF is to make sure it’s fully able to regenerate itself when it’s full of soot (when the warning light appears).

There are two types of regeneration: passive and active.

Passive regeneration 

Passive regeneration occurs when the car is running at speed with revs around 3,000 rpm on long motorway journeys which allows the exhaust temperature to increase to a higher level and cleanly burn off the excess soot in the filter.

So it is advised that drivers regularly give their diesel vehicle a good 30 to 50 minute run at sustained speed on a motorway or A-road to help clear the filter. Alternatively, you can take on a 15 minute run more often, say once a week (this is the most common option).

However, not all drivers do this type of driving regularly and for those that do, it is difficult to maintain high speeds with so many cars on our roads. For this reason, you may consider driving in a lower gear at a slower speed to obtain the 3,000 rpm. E.g 30mph in 2nd gear = 3,000 rpm.

Active regeneration 

Active regeneration means extra fuel is injected automatically, as part of the vehicle's ECU, when a filter reaches a predetermined limit (normally about 45%) to raise the temperature of the exhaust and burn off the stored soot.

Problems can occur, however, if the journey is too short, as the regeneration process may not complete fully.

If this is the case the warning light will continue to show the filter is still partially blocked. 

In which case it should be possible to complete a regeneration cycle and clear the warning light by driving for 10 minutes or so at speeds greater than 40mph.

You will know whether active regeneration is taking place by the following symptoms:

  • Engine note change
  • Cooling fans running
  • A slight increase in fuel consumption
  • Increased idle speed
  • Deactivation of automatic Stop/Start
  • A hot, acrid smell from the exhaust

As seen on rac.co.uk