DPFs explained

Diesels have been fitted with DPF’s as standard since 2009, when Euro 5 came into force. 50% of all new car registrations are diesel. This means more and more DPF related jobs entering your workshop. Replacement is recommended from around 75,000 miles onwards (depending on vehicle model).

What is a DPF?

DPF’s are designed to reduce emissions from diesel fuelled vehicles. They remove soot from the exhaust gases, before they’re emitted into the atmosphere.

How do DPFs work?

The DPF traps the particulate matter (soot) from the exhaust gases.

  • Exhaust gases flow into the DPF but cannot exit down the same channel, because the exit is blocked.
  • This forces the gases to escape through the porous cell walls.
  • The holes within the cells walls are not large enough to allow the particulate matter to pass through, so trapping the matter inside the filter.
  • Clean exhaust gases exit the filter

Maintaining a DPF

Over time soot builds up within the DPF. Unless it’s regularly regenerated (cleaned), the build up of soot will eventually block the DPF and adversely affect the performance of the vehicle.

To avoid the DPF being blocked, a process called regeneration must take place. There are two types of regeneration – passive and active. Passive regeneration takes place automatically on long or high speed journeys, when the exhaust temperature is high. The soot inside the fi lter burns off naturally.

Active regeneration occurs when the soot reaches a pre-determined level. Depending on the vehicle model, the ECU makes small adjustments to the fuel injection timing to increase the exhaust gas temperature, burning off the soot. Some vehicle manufacture’s, notably Peugeot / Citreon / Ford use a fuel-borne catalyst called ‘EOLYS’ fluid, which is added to the diesel during fuelling. This
fluid enables the trapped particles to be burnt at a lower light-off temperature.

Why do DPFs fail?

Typically the DPF will become too blocked and cannot regenerate. Although the MIL light

may show, there could be many reasons for the blockage, including:

  • Continual urban driving
  • EGR valve stuck or failed
  • EGR pipes blocked
  • Turbo failure
  • Injectors – leaking and / or stuck open
  • Incorrect oil
  • Faulty sensors
  • Incorrect oil temperature
  • Fuel additive too low

These should always be investigated, before replacing a DPF, otherwise the new unit is likely to quickly fail because the actual cause of the blockage hasn’t been found.

DPF fitting advice

If the DPF definitely needs replacing, remember that DPF’s are a direct fit item. Follow the same procedure as fitting a catalytic convertor.

  • • Do not use exhaust paste on the joints
  • Ensure all flanges and joints are properly sealed
  • Ensure the fuel borne additive tank is full (if fitted)
  • Ensure that the fuel injection system is working correctly
  • Normal engine working conditions apply
  • Use correct oil during the service change

Is it illegal to remove a DPF?

From February 2014 the MoT test must include a check for the presence of a diesel particulate filter. A missing DPF, where one was fitted when the vehicle was built, will result in an MoT failure.

Whilst a vehicle may still pass the MoT visible smoke emissions test, it’s an offence under the Road Vehicles (Construction and Use) Regulations to use a vehicle that’s been modified in such a way that it no longer complies with the air pollutant standards it was designed to meet. Removing the DPF will almost certainly contravene these requirements, making the vehicle illegal for road use.

Owners could face fines of up £1,000 for a car or £2,500 for a light commercial vehicle, if the DPF has been removed.

Protect DPF’s with low SAPS oils

  • Sulphated ash within the passing exhaust gas can cause the ‘mesh’ structure in a DPF to become irreversibly blocked. For this reason, using normal oil could block the DPF.
  • To protect the DPF you should use Low SAPS oils. These oils have been specially designed to be low in Sulphated Ash.
  • You can identify a Low SAPS oil by looking for the ACEA ‘C’ classifi cation on the bottle. However, as most manufacturers have their own specifi cations for Low SAPS oils, we’d recommend you call your local branch.