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The exhaust emissions standards for new cars have effectively required fitment of a DPF in the exhaust of diesel cars since 2009 when the ‘Euro 5′ standard came into force. In fact, many cars registered before 2009 will have had one fitted too in anticipation of the change in standards.
Standards aim to deliver an 80% reduction in diesel particulate (soot) emissions but the technology’s not without problems – AA patrols are regularly called to cars with the particulate filter warning light on indicating a partial blockage of the filter.
Even if your driving isn’t mainly urban/stop-start, changes to driving style may be required to get maximum benefit from these systems.
If you’re buying a new car and plan to use it mainly for town-based, stop/start driving it would be wise to avoid a diesel car fitted with a Diesel Particulate Filter (DPF) because of the possible hassle of incomplete ‘DPF regeneration’.
How do they work?
Diesel Particulate filters (DPF) or ‘traps’ do just that, they catch bits of soot in the exhaust.
As with any filter (think of the bag in your vacuum cleaner) they have to be emptied regularly to maintain performance. For a DPF this process is called ‘regeneration’ – the collected soot is burnt off at high temperature to leave only a tiny ash residue.
Regeneration is either passive or active
Passive regeneration takes place automatically on motorway-type runs when the exhaust temperature is high. Because many cars don’t get this sort of use car manufacturers have to design-in ‘active’ regeneration where the engine management computer (ECU) takes control of the process.
When the soot loading in the filter reaches a set limit (about 45%) the ECU can make small adjustments to the fuel injection timing to increase the exhaust temperature and initiate regeneration. If the journey is a bit stop/start the regeneration may not complete and the warning light will come on to show that the filter is partially blocked.
It should be possible to start a complete regeneration and clear the warning light by driving for 10 minutes or so at speeds greater than 40mph.
If you ignore the warning light and keep driving in a relatively slow, stop/start pattern soot loading will continue to build up until around 75% when you can expect to see other dashboard warning lights come on too. At this point driving at speed alone will not be enough and you will have to take the car to a dealer for regeneration.
If you continue to ignore warnings and soot loading keeps increasing then the most likely outcome will be that you will have to get a new DPF costing around £1000.