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Particulate Filters (DPF): the thorn in the diesel driver’s side.

2nd May 2013 | Share:

A small city commuting car with the torque and economy of a diesel engine seems like the perfect match and many people have started to realise this. SMMT shows diesel models are accounting for 50% of the overall new car market. The trouble is to meet the strict 2009 Euro 5 emissions guidelines which all car manufacturers must adhere to lowering the harmful gas output from vehicle exhausts.

Among these gasses is Particulate Matter, small molecules of solid or liquid matter suspended in the atmosphere. These can be seen on older diesels as the black foggy cloud of soot that comes from the exhaust under heavy acceleration.

To meet the Euro 5 regulations all new diesel cars are now fitted with a Diesel Particulate Filter (DPF). These filters are a very effective way of removing this particulate matter and can be as much as 85-100% more effective. The DPF filter collects all of the particulate matter, much like a filter would collect dust in your vacuum cleaner. This runs in cycles and when it becomes full needs “emptying”, known as DPF regeneration – this is done by burning off the soot at high temperatures.

There are two forms of regeneration:

  • Passive, done at constant speed over 50mph
  • Active, the ECU increases the fuel entering the engine in order to heat up the exhaust gasses

Active regeneration is more common in cars where the filter does not get as hot, cars with a smaller engine or cars that have the filter further from the engine because of space. Some smaller diesel engines also have a fuel additive in a small tank which is added automatically during the regeneration as the engines do not get hot enough.

So how does this affect you?

Not clearing your filter can cause the soot to build up over 75% and after this the filter cannot be cleared via the standard cyclic regeneration. If caught quickly a dealer can induce forced regeneration costing around £200, but as with the fuel additive based engines, this can leave a residue which slowly builds up meaning in these cases the filter has a limited life.

In the long term, if the filter is left too long without clearing, or the residue builds up too much, the filter will need replacing which can cost upward of £1000. Overloading the filter, ignoring warning lights and not seeing a dealer when the error does not clear can completely overload the filter causing excessive back pressure and may lead to irreparably damaging the engine. This could mean paying out even bigger bills in the multiple thousands, to top it off most manufacturers will not cover DPF issues under warranty.

The popular Nissan Qashqai, which has 3 different diesel engines, offers an 18 page thread with 627 replies on DPF issues on the popular, ranging from members not being told about the DPF when buying the car, to the light coming on signifying a required regeneration after only 3k miles of ownership. In extreme circumstances, taking an extra journey every 2 weeks in order to cycle the filter is the only suitable solution.

But don’t give up hope, this is where you come in!

The first port of call is to refer to your vehicle handbook, as many vehicles will have varying instructions to maximising DPF efficiency and life.

When a filter reaches its pre-set limit, usually around 50%, a light will come on on the dash which indicates the filter should be cleared as soon as possible. To do this generally will involve an un-interrupted 20 minute drive at 40-50mph above 2k rpm; this varies between manufacturers as Nissan ask for above 50mph for 30mins. Because of this, a road where you must slow down for a roundabout will not complete the cycle.

Cars where there is a fuel additive will have a small tank for the additive, very little of this is used and it is topped up during the usual service schedule, this must be completed as without the additive the filter may not be possible to regenerate.

Some cars will suffer from these issues more than others; cars that are used predominantly for town trips will not clear the filter during normal driving. Similarly, small engine diesels which frequently do not get hot enough alongside cars with a very economical top gear keeping the revs extremely low will not reach the right temperature when on the motorway. These may require an extra trip in a lower gear just to clear the filter, whereas some diesel drivers who use the motorway regularly may never see the light or have to clear the filter.

DPF Filters and Diesel Tuning Boxes

It was a common thought that Tuning boxes used to cause harm to the DPF, the poor tune of some boxes and resistor based tuning boxes did, due to the generic extra load on the engine would create increased soot and clog up the filter much faster. However the latest Speedhawk boxes do not suffer with the same issue, as they are mapped for the engine specifically, so do not increase soot output as they are optimised. The boxes are actually if anything good for the DPF, especially with smaller engines as the increased boost pressure and performance means the DPF should be easier to clean, which makes the correct cycling temperatures also easier to reach.

There is a wealth of information available on DPF filters at your local forum, but in summary, following your hand book instructions and never ignoring a warning light will save you a lot of money. If you are in the market for a new car and don’t get out on the motorway much, make sure you take into account DPF cycling before you take the plunge.

Posted by: A. Sturney


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