IMPORTANT NOTE: If you are replacing your Diesel Particulate Filter it is highly likely that either a fault occurring elsewhere on the vehicle or the driving style has caused it to fail. A full fault diagnosis must be carried out by a fully trained technician with the appropriate diagnostic equipment to establish if the DPF needs to be replaced. The checks listed in this guide should be carried out prior to replacement to ensure the new DPF operates correctly when fitted. By not carrying out these checks the new DPF could fail shortly after it is replaced.
How do DPFs work?
Unlike a catalytic converter a DPF is not a flow through device, it is a soot trap that collects particulates from exhaust emissions. The exhaust gasses flow into the DPF but cannot exit down the same channel as the exit is blocked. The gasses escape through the porous cell walls, but the particulate matter is too large to escape so is trapped within the DPF. The DPF disposes of this soot during a process called “regeneration” when the particulates are burnt off. On a properly functioning vehicle regeneration will occur approximately every 300-500 miles (this is very substantially dependent on the vehicle and driving conditions).
Why do DPFs Fail
The most common reason for the failure of a DPF is that it has become too “clogged” with soot to be able to regenerate. It is easy in this instance to simply assume the DPF is at fault and replace it, but if no diagnostic work is carried out it is likely the new DPF will fail quickly (within 300 miles)
Initial Investigation
The best source of information is the vehicle owner/driver. The information they provide can be invaluable in identifying the fault.
What type of driving does the car do?
If the car does mostly town or city driving it is likely that the DPF has not reached the optimum temperature to regenerate and has become “clogged”. If the car does a lot of motorway driving and the car has 6 gears and the engine revs are sometimes so low that the exhaust temperature does not get high enough for regeneration to occur. Occasional harder driving in low gears should be enough to burn off the soot in these cases.
How long has the DPF warning light been illuminated
Many DPF problems are caused by simply ignoring the warning lights that advise that the DPF needs to be regenerated. Once the DPF is around 45% full the vehicles Electronic Control Unit makes changes to the fuel injection timing to increase the exhaust temperature to burn off the particulates. If the journeys completed at this point are stop/start the conditions required for regeneration may not be met and the warning light will be illuminated indicating that the DPF is starting to become blocked. It should be possible to clear this warning by driving at speeds greater than 50mph for around 20 minutes (the engine must be running at a minimum of 2500 rpm).
If the warning light continues to be ignored or conditions for regeneration are not met the level of soot in the DPF will continue to increase, and at around 75% full the DPF will have to be regenerated by a dealer or specialist garage. You can expect other dashboard lights to be illuminated as well. If the warnings are still ignored and the soot levels increase to around 95% you can expect the vehicle to be put into “limp mode” by the ECU in order to protect the engine from damage due to the increased back pressure caused by the blocked DPF. At this point the DPF cannot be regenerated and will need to be replaced.
Has the vehicle been serviced recently or has the oil been changed?
If the vehicle has recently been serviced it is worth checking that the correct type of oil has been used. Most DPF vehicles require a special type of low ash oil, and using the incorrect oil can lead to problems during the regeneration process. If the vehicle uses a fuel additive system to aid regeneration it should be checked that the additive tank has been filled. The additive tank is normally located near the fuel tank.
Diagnostic Checks & Installation
Before fitting the DPF diagnostic checks should be carried out with an appropriate fault code reader to establish any codes held within the ECU. Comprehensive testing of the components listed below should also be carried out as a fault with one of them either alone or in combination could lead to DPF failure. Be aware that an unresolved fault with the vehicle that illuminates the engine management light will usually prevent the regeneration process from initiating and therefore the DPF will become blocked.
DPF Pressure Pipes & Sensors – checks should be made to ensure all pipes are free from damage or blockages and the differential pressure sensor is working correctly. It is best practice to replace the differential pressure sensor when fitting a new DPF.
Oil Level – if the oil level is high this is a sign that it has been contaminated with fuel from failed regeneration attempts. The extra fuel intended to increase the exhaust temperature can find its way into the engine sump, contaminating the lube oil and sometimes leading to a breakdown as the engine can start to run uncontrollably on its sump oil. An oil change will be necessary.
Oil Specification – check that the correct low ash oil has been used.
Fuel Additive (where applicable) – check the level of the fuel additive, and fill the additive tank as required. There is normally a manufacturer’s procedure that must be followed to reprogram the ECU to the new additive level.
Sensor Checks – check all sensors (exhaust gas temperature & oxygen sensors) to ensure that they are operating correctly.
EGR System – check the Exhaust Gas Recirculation valve is working correctly and is not blocked. As a minimum the EGR valve must be removed, checked and cleaned before replacing the DPF.
Glow Plug System – check all glow plugs and the glow plug controller/relay are working correctly. It is very common that when one or more glow plug fail the controller/relay also fails possibly due to a surge in voltage. Alternatively the controller/relay may have failed first allowing the glow plugs to run continuously which causes them to burn out. Either way when you have a glow plug fault you should replace all the glow plugs and the glow plug controller/relay. A failed glow plug controller/relay very often does not trigger a fault code. On a BMW the regeneration system will not initiate if the glow plug system is not working correctly and the glow/plug controller/relay has failed leading to the DPF becoming blocked.
DPF Installation & Set Up
Once the fault that caused the previous DPF to fail has been rectified the new DPF can be fitted. The new DPF should be fitted loosely to line it up correctly with the rest of the exhaust system before tightening it up. An appropriate diagnostic tool will be required to reset the ECU in accordance with the manufacturer’s guideline. This includes telling the car that a new DPF has been fitted, resetting the ash counter and calibrating the differential pressure sensor. Failure to complete the setup procedure correctly will cause the new DPF to become blocked as the vehicle is unaware that a new DPF has been fitted and therefore the regeneration system will not have been reset. A forced regeneration may be part of the reset procedure and if all guidelines have followed a new aftermarket DPF will work in the same way as the O.E. specification unit did prior to failure.
***A new DPF must be fitted by a competent mechanic/garage with DPF knowledge/experience that has the required level of and diagnostic equipment necessary to complete the set up procedure after installation - failure to adhere to this requirement may invalidate the warranty***
If the procedures set out in this guide have not been followed the new DPF could become blocked within a very short space of time and may not be able to be regenerated.
