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Car Repair - Exhausts

Diesel Particulate Filters: What you need to know

At Fergies we’re keen to ensure you have trouble free motoring in a car you can trust. We receive a fair number of enquiries for DPF faults so thought it’d be a good idea to give you the information you need to keep yours healthy.

Modern diesel cars are now fitted with Diesel Particulate Filters (DPFs) to cut pollution. Unfortunately, when they become blocked repairing any ensuing damage can be very expensive indeed.

DPFs were brought in to meet European emissions regulations and it is now an offence to drive a car that does not comply with prescribed emissions standards. Removing a DPF can also invalidate your insurance cover.

Since February 2014, any car that has had a DPF fitted is bound to fail its MOT if the DPF has either been tampered with or removed.

What is a DPF?

Diesel engines generate a lot of soot particulates. The reason these are discouraged by the regulations is that they cause respiratory problems and increase the risk of heart disease. DPFs have been fitted to all diesel cars coming off the production line since 2009 with the aim of preventing this soot from billowing into the atmosphere.

The Euro legislation’s aim is to reduce as much as 80% of a diesel engine’s particle emissions. However, there is a significant downside to this DPF technology and as a result, garages often come across diesel cars where the DPFs have become blocked.

For a car’s performance to be maintained the DPF needs to be emptied on a regular basis every 300 miles or so. This process is known in the trade as ‘regeneration’ and is usually carried out in a passive manner. This means the regeneration of the DPF is achieved when the car’s exhaust temperature is high enough to burn off the soot.

Most manufacturers recommend that this needs the car to be driven on a smooth-running road like a motorway, a dual-carriageway, or another fast A-road. This action should burn the soot off leaving just a small amount of ash residue. You’ll find the specific instructions for your vehicle in the driver’s handbook.

The only alternative way of removing this ash is by dismantling the DPF and carrying out a specialist cleaning process, which can cost a four-figure sum with labour.

A car fitted with a DPF and used correctly with lots of open road motoring so regeneration can take place on a regular basis should be good for at least 100,000 miles.

Common Causes of Failure?

However, many cars are used mainly in towns with lots of stop-start motoring in halting traffic. This makes it almost impossible for passive regeneration to take place. As a result, a lot of manufacturers have taken to fitting ‘active’ regeneration systems to their models.

Active regeneration enables the engine to control regeneration with software that keeps track of DPFs and spots when they are becoming blocked. In cases like this, the computer instructs extra fuel to be injected this increasing the exhaust gas temperature and triggering the required regeneration.

This active regeneration can take anywhere between 5 and 10 minutes to complete. If the journey is too short when this process is triggered the regeneration won’t complete. This may cause:

  • The cooling fans coming on
  • The engine idling at a faster speed
  • Any Stop/Start mechanism failing
  • Increased fuel consumption
  • An acrid smell
  • The engine sounding odd

It also should illuminate a warning light that could be indicating the DPF is blocked. This is a sign that you need to instigate and enable the active regeneration process by driving the car for over 15 minutes at more than 40mph on an open road.

It would be dangerous to continue driving in a slow, halting pattern in traffic, which would likely cause soot to increase and your car to go into its ‘limp mode’ in a bid to halt the risk of additional engine damage.

When a warning light comes on it’s advisable to visit a garage, who may advise that manual filter regeneration is required. This may involve replacing the filter, which can cost more than £1,000 plus labour. In most cases, there is not a lot of time between a DPF becoming partially blocked and emergency manual regeneration being needed.

How To Avoid A Big Bill?

So… if your DPF light is on and remains lit after you’ve followed the instructions in your handbook for regeneration (driving on a dual carriageway etc), what next?

We’re keen to keep any remedial work to a minimum which is why an accurate diagnosis is essential.  There are a number of subsystems that are required for efficient regeneration (glow plugs, additive injectors etc), it’s very common for one of these to have caused the fault. Once we’ve found the root cause we can then assess if cleaning the DPF is suitable.

At Fergies we have a lot of experience in DPF repairs. You’ll be guaranteed a high level of workmanship and great value.

Call today with you DPF faults. We’re here to help.

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