We have been doing this job a long time now and learnt a lot over the years, we know exactly how these systems work and how to test them and how to repair them. We have some questions that need answers and answers that need to be questioned though.
The problem is many car drivers assume the DPF is blocked, when in reality the car only thinks the dpf is blocked, the DPF is basically a filter, just like an oil filter or an air filter, an air filter you can look at and a DPF you can’t,it’s just not possible to see inside. These can be quite easy to fix and may only need a forced regeneration to clear the soot and reset the counters. There are many methods of monitoring the condition of the DPF by the engine management system, the most common are theoretical calculations based on mileage, frequency and length of regenerations, time and fuel amount used. Then the most accurate is the monitored back pressure reading from the DPF pressure sensor, which is more accurate. If the blockage is reported and logged by the pressure sensor with a fault code for excessive pressure, then it truly is blocked.
We will never just clean or replace a DPF without a thorough diagnostic evaluation and a questions and answers session. We need to to know the service history, your driving style (short journeys, stop-start driving) etc. We must gather as much information as possible, otherwise, it will just happen again and again if the problem is not rectified.
So what causes a DPF to block, well there are lots of reasons, an engine management or mechanical fault, sticking EGR valves, Air leaks, turbo faults, poor quality fuel, poor quality engine oils, poor servicing, poor quality service parts used, short journeys, frequent stop-start journeys, the list is endless.
While ever the engine is running, the DPF is storing soot (particulates) which is partially burnt fuel, in reality, the engine shouldn’t be producing soot, all the fuel should be burnt during combustion, so an efficient engine running at its best should really never have any issues. But, nothing is ever perfect and particulates will be produced, (more so if the engine or management has problems or faults), it is the job of the DPF to store these particulates and burn them off during the regeneration process, here lies the problem with the stop-start, short journeys, it will never get the opportunity to enter regeneration mode, this normally happens when the engine and exhaust temperatures are at a determined level to burn off the soot, and will only happen at a steady cruise speed. So, if you are one of the many that only do stop start short journeys, then you are likely to always have DPF problems. Another reason for a blocked DPF is ash, ash is produced when anything burns, the fuel in your car, the engine oil also, which should be a good quality low ash type (the importance of good servicing) and the soot burnt off in the DPF during regenerations. Ash will always be left behind and can never pass through, the only way to get this out is by cleaning, and it can’t be done on the car, it has to be removed and backflushed out, no matter what anyone says, on the car cleaning solutions do not work, trust us, we have tested many and learnt the hard way. It’s also worth mentioning and I hear it all the time, the man in the pub said “just thrash it down the MOTorway and it will clear”, well this just isn’t true, it may go some way to get the temperature up and burn some soot off, but it will never enter regeneration mode while you are demanding power from the vehicle.
So as you can see, there are many variables to consider, why is it blocked? What is it blocked with? How did it become blocked? How do we restore it? How do we stop it happening again?
This is not a quick job, neither is it a cheap job, but we are thorough! The average cost for our DPF cleaning service is between £150 and £450 and can take up to three days for a full deep clean. We guarantee that the DPF is clean and clear at the time of refitting, but if you have read all the above, we just can not guarantee it will not block again, at the end of the day it is just a filter, hopefully, I have explained enough for you to understand what happens and how it can be avoided or kept to a minimum.
- An EGR (Exhaust Gas Recirculation) valve not working correctly, may not produce a fault code.
- A leak in the intake system may not produce a fault code.
- An underperforming turbo may not produce a fault code.
- A split boost pipe may not produce a fault code.
- A problem with fuel supply.
- Faulty glow plugs
- Engine not at correct operating temperature
- Engine oil of incorrect specification.
So, what do we do?
- A Full diagnostic test.
- Further oscilloscope testing to verify sensor outputs.
- A smoke/pressure test of the intake system.
- A pre back pressure test of the DPF to check the extent of the blockage.
- A forced regeneration of the DPF
- Cleaning of the DPF
- A further back pressure test of the DPF to confirm the results of cleaning.
- A road test to verify condition of turbo boost control, (this can only be tested once the DPF is clear, as low exhaust gas flow will affect turbo performance).
In some extreme cases or cases of improper diagnostics, cleaning of the DPF is unsuccessful, due to the soot and ash so densely packed in, in this case, replacement of the DPF is needed. Please note, we no longer fit the cheaper aftermarket DPF’s that are available, as we have found them unreliable.