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The cost of replacing a diesel particulate filter (DPF) can exceed the value of your car. We look at potential problems and how to avoid them.
If you own or plan to buy a modern diesel vehicle, you've probably heard of diesel particulate filters. But what are they - and why are they a problem?
A diesel particulate filter (DPF) is designed to reduce emissions from diesel vehicles by capturing and storing exhaust soot. It prevents harmful particulate matter from being released into the atmosphere.
DPFs have been a requirement for all new diesel vehicles since the "Euro 5" emissions standard came into effect in 2009. However, particulate filters date back to 1985, when Mercedes-Benz became the first car manufacturer to introduce them on the 300D (sold in California).
Today, there are two main types of DPFs: passive regeneration and active regeneration.
Passive regeneration uses normal exhaust temperatures and nitrogen oxides (NO2) as catalysts to oxidize particulate matter in the DPF. This tends to occur at high speeds, usually on freeways or A-roads, when the engine is running for 30 minutes or more.
The main advantage of passive regeneration is that it requires no input from the driver, the process is automatic and there are fewer components.
However, it relies on the driver driving regularly on faster-moving roads - otherwise, problems can occur. It is for this reason that manufacturers have introduced the so-called active regeneration.
Active regeneration uses the car's ECU to sense when the filter is clogged with soot and injects additional fuel into the engine to raise the exhaust temperature, which triggers regeneration.
This tends to happen every 250 to 300 miles and can take up to 10 minutes to complete. In theory, this should not cause any problems, but if the system is unable to complete the regeneration, problems can occur.
If the process is interrupted too many times, the DPF warning light will come on and it is recommended that you take your car to a highway, dual carriageway or A-road for 15 minutes to regenerate. If you continue to ignore the warning light, the car will go into "limp home" mode.
There is a third form of DPF that uses a fuel additive to reduce the ignition temperature of soot particles so that regeneration can occur at lower temperatures. The additive is stored in a separate tank or "air bag" and should be changed every 70,000 to 100,000 miles.
Be sure to refill the tank when the warning light appears on the instrument panel, as the DPF will be clogged if there is no fluid.
A significant disadvantage associated with active regeneration is engine oil dilution caused by small amounts of diesel fuel during the post-injection cycle, where the fuel is injected into the cylinders after normal combustion. A thin layer of fuel can accumulate on the cylinder walls, which can lead to premature engine wear, so drivers are warned to consider shorter oil change intervals.
Various studies have been conducted on engine oil dilution, with the scale of the problem varying depending on the make and model of the diesel vehicle in question. There is evidence that the problem is exacerbated when the regeneration process is stopped prematurely or when the car is used for short trips.
Modern systems should alert drivers via dashboard messages when oil dilution reaches a certain level, but regular maintenance remains critical to the long-term health of the engine. A number of high-profile problems have emerged with some major car brands.
If regeneration is not carried out, the DPF will need to be cleaned or replaced and your bill will be over £1,000. A quick search of some forums and discussion threads suggests that you could be charged up to £5,000, which could be more than the value of the car.
This means that if you are buying a high mileage diesel car, it is important to look after your DPF and keep a close eye on it. A well maintained DPF should have a life expectancy of 100,000 miles, which can be significantly reduced if the car is used for short journeys and not regularly regenerated.
To avoid a blocked DPF, you can start by not buying an unsuitable car. Typically, if you drive less than 12,000 miles per year, a gasoline, hybrid or electric vehicle will be better suited to your needs.
Diesel vehicles tend to be more economical on long trips, so they are not suitable for short trips or city driving.
If you drive a DPF-equipped diesel vehicle, read the manual to find out if your vehicle uses passive regeneration or active regeneration and make sure you know how to maintain the filter. It's important to use the right oil.
RAC says performance modifications can damage DPFs, as can using poor quality fuel. Even running a car on low diesel can cause problems, as the car may avoid regeneration to save fuel.
It is illegal to use a vehicle that has been modified in such a way that it no longer meets the emission standards it was designed to meet. For this reason, removing a DPF will subject you to a £1,000 car fine or a £2,500 light goods vehicle fine.
Further issues include the immediate invalidation of the MOT if the DPF has been removed and the invalidation of insurance as the car has been modified from its original specification.
Clearly, you need to be vigilant if you are buying a high mileage diesel vehicle. Insist on checking the latest MOT certificate - details are stored online - and check that the DPF is present.
Shindary Automotive Parts Co., Ltd.
