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The Motorists Guide

Diesel Particulate Filter (DPF)

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Diesel Particulate Filters (DPF)

Many cars registered after 2007 will have a Diesel Particulate Filter (DPF) factory fitted in an attempt to lower diesel soot emissions. The introduction of the DPF is aimed to deliver an 80% reduction in diesel particulate (soot) emissions. This appears to be a great idea but in reality the system has many problems and the most common one is the particulate filter warning light illuminating, indicating a partial blockage of the filter. Different driving styles are required to keep the DPF system functioning correctly. One of the major issues surrounding this is with urban stop/start driving where the filter cannot regenerate sufficiently and the system becomes blocked.


The DPF is designed to trap the soot generated by the engine and burn it off whilst driving. Once collected, it is burnt off at high temperatures by a process called 'regeneration'. This regeneration is generally carried out passively while driving at sufficient speed (such as a motorway run) which will raise the exhaust gas temperature to burn off the soot emissions. If 'passive' regeneration does not work sufficiently to keep the DPF clear then 'active' regeneration will be required. This is when the engine electronics take over to run a programmed running strategy to clear out the DPF by introducing a post-combustion process to raise the temperature. Sometimes, the DPF active regeneration gets interrupted and cannot complete the process under normal urban driving conditions. This may bring on the DPF warning lamp on the dashboard indicating that the filter is partially blocked. Instructions are usually included in the drivers' handbook to instruct on how to drive and for how long to fully clear the DPF.


Generally, a blocked DPF can mean very expensive repair bills. If the filter system block ups to an unacceptably high level, then the vehicle will need to be taken to a garage for a forced regeneration. This can result in a very expensive replacement DPF being fitted. Another problem that can arise is that of 'washdown' of the cylinder bores. This is when fuel being introduced in the active regeneration stage washes down the cylinder bore and mixes with the engine oil, therefore diluting it. Two further problems can arise from this....poor compression, which can prevent the engine from starting and running efficiently. Also, the engine can become a 'runaway' which quite simply means it will suck up the oil and fuel mix from within the sump and burn it, which in a diesel engine will run the engine (even with the ignition switched off) to an ungoverned speed by consuming the oil and ultimately run to destruction!


In the long run, it may be worth having the DPF removed from your vehicle if you only drive the car locally or for short distances. Removal involves cutting out the DPF and replacing it with a straight through pipe. Additionally, the engine management computer will need remapping to enable the car to run with new settings to bypass the DPF. This generally improves performance and also improve the overall fuel efficiency.

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