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 gases flow into the DPF, but cannot exit down the same channel as the exit is blocked. The gases 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 the soot during a process called ‘regeneration’, whereby the particulates are burnt off (oxidised). On a properly functioning vehicle, regeneration will occur approximately every 500 miles (this can vary 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 a short period of time (less than 200 miles).
The best source of information is the vehicle owner. The information they provide can be invaluable in identifying the fault:
What type of driving does the customer mostly do?
If the customer mostly does town or city driving, it is likely that the DPF has not reached the optimum temperature required to regenerate and has become ‘clogged’. If the customer does a lot of motorway driving and the vehicle has six gears, the revs are sometimes so low that the exhaust temperature does enough for regeneration to occur. Occasional harder driving in lower 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 the DPF needs to be regenerated. Once the DPF is around 45% full, the ECU makes changes to the fuel injection timing to increase the exhaust temperature and burn off the particulates. If the journey at this point is stop/start, the conditions for regeneration may not be met and the warning light will be illuminated indicating a partial blockage. It should be possible to clear this warning light by driving at speeds greater then 40mph for around 10 minutes, although this can take up to 25 minutes if there are lots of downhill descents etc. 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 warning 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. At this point, the DPF cannot be regenerated and needs 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.
DPF Pressure Pipes and Sensors
DPF pressure pipes and sensors checks should be made to ensure all pipes are free from blockages and the pressure sensors are operating correctly.
Check that the correct low ash oil has been used.
If the oil level is high, this is a sign 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.
Fuel Additive (where applicable)
Check the level of 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.
Check all sensors (such as the oxygen sensor) to ensure they are operating correctly.
Check the EGR valve is working correctly and the EGR pipe is free from blockages.
The following should also be checked: air flow meter, engine and turbo wear, injectors (in case of leaking), glow plug condition, air filter condition and the ECU should also be checked for any malfunctions.