BMW introduced DPF – Diesel particulate filters around 2004 and eventually phased them in to the entire diesel range. If you drive a post 2006 BMW diesel then you probably have a DPF fitted!
A Diesel particulate filter gathers, diesel particulates. These particles (carbon) get caught in the filter and begin to build up, eventually getting to a point at which the car will attempt a regeneration. A DPF regeneration involves the car altering the fuel injector timings which results in the exhaust temperature increasing to a point at which the build up of carbon should burn off.
For the car to regenerate the DPF, certain criteria need to be hit. These are:
- at least 10L of fuel – ideally above 1/4 of a tank of fuel.
- A journey of 20-30 minutes of driving at speeds of above 40mph.
- Engine must be up to temperature.
The problem arises when the car fails to regenerate the DPF. The exhaust becomes blocked and this triggers fault codes to store in the DDE ( engine control unit ) At this point it is highly unlikely that the DPF will “naturally” regenerate.
So why would the DPF block?
There are multiple reasons why a DPF can blocked up, but the main two are
- The car only gets used for short journeys, or stop-start city like driving, where the car fails to get up to speed for any length of time. – We also see cars that are used for short journeys driven on the fuel light.
- There is an underlying fault with the engine, there may be additional faults in the DDE that have prevented regeneration from taking place. – We always carry out a diagnostic check for this first.
When the DPF filter blocks the car will show a warning symbol on the dashboard or monitor if fitted. The car at this point may also start to feel like it is down on power.
So my DPF is blocked, what are my options?
Some garages don’t understand the function of DPF filters, and they don’t understand why they are blocking up and they certainly don’t know how to regenerate them. When this is the case they usually do 1 of 3 things in order to “fix” the problem.
- They suggest removing the DPF completely – This is effectively an illegal practice, as of January 2014 the removal of a DPF is an instant MOT FAILURE
- They suggest removing the DPF and cleaning it out with a jet wash or chemicals. – This results in often large labour bills for the removal and refitting of the filter, this will also only result in a temporary solution if any underlying faults have not been resolved.
- They replace the DPF – This can be hugely expensive, a DPF can cost between £1000 and £2000. Again if any underlying faults have not be resolved this can result in a very expensive and temporary solution.
Now you have seen the wrong ways of dealing with the fault we will explain how we correctly diagnose and rectify the blocked DPF
- Firstly we discuss with the customer the circumstances under which the DPF appears to have become blocked. – In order to determine if driving style has contributed to the blocking of the filter.
- We carry out a full vehicle diagnostic check. This will give us 2 important pieces of information. We will be able to read off the back pressure in the exhaust which will confirm if the filter is blocked or not. As well as highlighting any under lying faults that may have also prevented regeneration.
- We work through any faults that may have prevent the regeneration process – This is essential for future regenerations to work correctly
- Once the above has been carried out we can “force regenerate” the DPF by using our Autologic diagnostic software and giving the regeneration order to the DDE (ECU)
- Once a Regeneration has taken place we can compare the before and after backpressure readings, and clear all fault codes.
We do not need to replace the DPF, saving ££££.
We do not remove the DPF which is an instant MOT FAIL.
We do not waste time removing the DPF filter.
A recent example of a before and after backpressure test on a 120d
As you can see the back pressure has been dramatically reduced, and is now well under the maximum permissible values.
We also recommend an interim oil change if this has not been carried out in the previous 12 months. Excess fuel can enter the oil during multiple Regeneration attempts.
Important DPF news, MOT changes 2018:
As of May 2018 a removed DPF is now an instant MOT Fail, the check is not only visual but the emissions test limits have been drastically lowered. If your vehicle has had the DPF removed, contact us for solutions to get your car legal.