Diesel Particulate Filters – winter advice
Many technically-interested motorists are aware that the soot traps (DPFs or Diesel Particulate Filters) on modern cars require high temperatures to keep clean. If temperatures are too low, they clog and create a potentially expensive problem.
It is unsurprising, therefore, that many DPF-related issues happen during the winter months. While much is written on the repair and cleaning of DPFs, to keep things simple, here is our top ten guide to reduce the risk of a DPF giving you an expensive headache.
- Keep your car serviced. On many newer models, if the oil has not been changed (or the car’s on-board service monitor has not been reset), this can prevent the DPF from generating the high temperatures itself, in order to keep clean.
- Know the dashboard light meaning. Many drivers think that an illuminated DPF dashboard lamp indicates a fault – it could well do so – but, on some cars, it is a request for the driver to take the car on a longer, faster journey, so that the DPF can self-clean.
- Read the handbook – Linked to our second tip, familiarise yourself with the DPF and its operation, plus any driving techniques that facilitate its cleaning. This information should be in your owners’ manual.
- Use the correct oil – DPFs are poisoned by high levels of sulphated ash and Engine oil, therefore, should contain as low levels of these additives (called ‘low SAPS’) as possible. Be very wary of buying the incorrect oil and check the specifications carefully in your handbook.
- Check the oil level. During normal use, the engine oil becomes contaminated with diesel fuel, which causes the level to rise. Never add oil over the ‘maximum’ mark on the dipstick, because this could lead to severe engine damage. Also look for evidence of the engine oil rising over the maximum limit, which may indicate a problem with either the DPF, or the fuel injectors.
- Never overfill with diesel – This can lead to fuel spilling onto the road, which is especially dangerous for motorcyclists. Yet, it can also cause the fuel-borne additive system that is fitted to Peugeots/Citroens and certain Fords, Mazdas and Volvos, to malfunction and this can cause the DPF to clog. If this additive needs to be replaced, insist on using the genuine fluids, not one-size-fits-all aftermarket additives, because the various generations of these additives are incompatible and no aftermarket manufacturer could provide me with evidence of their products’ performance, compared to that of the original fluids, when I investigated this issue several years ago.
- Use premium (expensive) fuel – This may be a contentious tip but the makers of the expensive versions of diesel claim that their fuels burn cleaner. Presuming that this is true, less unburnt fuel (soot) is likely to be produced, which benefits the DPF, especially in winter. In my own experience of using BP Ultimate and Shell V-Power diesel during the winter months, the engine runs smoother from cold and the DPF has to self-clean (‘regenerate’) itself on fewer occasions.
- Do not ignore problems – Should your DPF develop a problem and become excessively clogged and will not self-clean after a continuous high-speed run (again, consult your owner’s handbook for ideal DPF cleaning driving conditions), do not ignore the problem. A clogged DPF will continue to fill and the longer it is driven, the likelihood increases considerably of a more expensive repair bill. Similarly, the ability of a DPF to regenerate depends on the health of other engine-related components. Therefore, if another part of the car/engine is faulty, or improperly modified, this could impact on the DPF.
- Use an experienced garage – Should you experience problems with the DPF, use a garage with specific experience of these systems. Such outfits should spend extra time diagnosing the problem and testing components, rather than replacing expensive parts alone. DPF pressure differential sensors, for example, tend to be expensive and many problems end-up being caused by defective wiring, or software, and not the sensor. In most cases, blocked DPFs can be cleaned without having to be removed from the car but, once again, ensure that the garage you use is knowledgeable about how to do this, or further damage could result. When a car reaches around 150,000 miles, the DPF may need to be removed to have accumulated ash cleaned-out. Unfortunately, the ash cannot be removed by relatively inexpensive on-car cleaning methods.
- Do not remove the DPF – It is an offence to drive any car that has had its standard-fit DPF removed and we believe that the DVSA is investigating DPF deletion detection methods beyond the current visual-only check. Many garages continue to offer this service – resist the temptation.