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Having issues with your emissions system? From regeneration, cleaning, and software adjustments we can help you get your car back on the road. 

DPF and EGR 

Common problems and solutions

Cleaning and Regens

Car feeling a lot more sluggish than it used to? You're probably in the same boat as a lot of other UK motorists who aren't doing the long journeys that they were a few years ago. Your emissions system is a clever and complicated system which requires maintenance and attention to make sure you're getting the life span of your DPF and EGR. 

So let's start with the basics, how much do you know about your emissions system? 

What is a DPF?


A diesel particulate filter (or DPF) is a device designed to remove diesel particulate matter or soot from the exhaust gas of a diesel engine.


Located within the exhaust system, a DPF captures small soot particles, preventing them from being expelled into the atmosphere. As with any filter it needs emptying periodically and this is done automatically, by burning up the particulates in a process called regeneration.


To meet or exceed European-dictated emissions standards, DPFs have been fitted to most diesel cars for almost a decade and, because they need high temperatures to regenerate, low mileage stop-start motoring can cause them to block.


Common Reasons a DPF can Fail


There are many reasons a DPF can fail, here are some of them:


  • The vehicle is high mileage and the DPF is compacted with fine soot

  • Failed sensors which prevent DPF regeneration

  • Lack of additive in fuel (where fitted) causing incomplete burn off

  • Engine issues i.e. leaking injector, valve stem oil seals, worn piston rings

  • DPF unable to regenerate due to driving conditions i.e. City centre

  • Poor performance tuning

  • Driving in too low of a gear on a motorway disallowing the regeneration to commence


So with all that in mind it's always best to start with a regen and cleaning. If you're booked in for custom tuning then we will always check your DPF soot levels whilst it's on the dyno. We do this for a few reasons. Firstly if your DPF is full there are some telltale signs such as poor or restricted performance and codes in the ECU which can be an early warning sign. If your vehicle needs a regen to proceed then rest assured we'll attempt this for you whilst your vehicle is in our care at no additional charge. You need at least half a tank of fuel in there for the vehicle to meet its regen parameters so make sure that you come prepared to your appointment. 


If a regen won't cut it, i.e. your DPF is too full and a regen alone can't clear the blockage then you should go for a particulate filter clean. You can buy products like Forte and Redex from most garages or fuelling stations of a particular size but failing that it's always good maintenance to run your vehicle on premium fuels and we'd recommend 1 in 4 tanks be premium whether your vehicle is tuned or not. This is because premium fuels have additives in them which are good for fuelling and emissions. 

When in tank cleaners won't work it's time to move on to the professional level cleaning fluids which are stronger. We can perform a particulate filter clean for you on site with our pro-level cleaning solutions which are non-invasive for the vehicle and can be completed within the day with before and after results proven with reports from our suite of diagnostic equipment. 


Motorsport Solutions

At Big Fish we offer two services regarding DPF removal. We can either do the full works of removal and tuning or simply delete the DPF software from your vehicle post removal. For full DPF removal the filter hardware needs to be removed from the exhaust system and this can be achieved via two methods:


1) A replacement pipe can be fitted (only available on some vehicles)

2) The current pipe can be modified by removing the DPF brick from the inside.


Both of these methods are permanent and tested. At Big Fish, we usually remove the DPF brick from the existing pipe, and refit. Following removal, the software needs to be removed from the ECU of the car so that the active and passive regeneration cycles, and any other DPF functions are no longer in operation. This can be done via removing the ECU and programming it on the bench or flashing it through the OBD socket inside the car. For many people, removing the DPF from the exhaust system will be because they have experienced problems with the unit itself. Many filters become clogged as a result of them failing to regenerate and therefore display warning lights on the dashboard and in some cases limiting engine performance. Due to a Diesel Particle Filter being described by dealerships as a ‘wear and tear’ item, vehicle manufacturers will not replace them under warranty and can be costly (usually around £1,500) to replace. We can usually remove the DPF from your vehicle for a fraction of that price.


Removing the DPF from your vehicle is almost certainly going to cause you issues with your MOT. For this reason we would only ever recommend removing the DPF from your vehicle if your vehicle is not used on UK roads (i.e. track cars) or you are exporting your vehicle from the UK


How will removing a DPF effect the emissions testing:


A DPF is designed to filter out soot from diesel exhaust gasses, therefore a DPF will reduce the opacity of the smoke. So why does a car not fail the smoke test if the DPF is removed? Removing the DPF will not cause the opacity level to rise high enough to fail, in fact, it is very difficult to get a diesel vehicle to fail an MOT smoke test, the fail mark is very high.


It is worth remembering that DPFs were not designed to help cars pass the MOT smoke test, they are a result of recent European legislation and the continued pressure on car manufacturers to clean up their emissions. In fact the diesel smoke test was implemented before DPF’s were even applied to cars.


Why is the fail mark so high? Well this is quite simple, the fail mark is not on a per vehicle basis, i.e. every turbocharged diesel vehicle manufactured before July 2008 has the same fail mark. Therefore modern turbo diesel cars with DPFs fitted from factory have the same fail mark as turbo diesels manufactured in the late 90s without DPFs.


What About MOT Legislation?


There is a very simple visual inspection of the DPF unit to ensure it is still in place, because of this legislation we do not fit DPF bypass pipes. Instead, we modify the original DPF unit, discreetly cutting a small window in the top of the chamber, removing the internal filter before rewelding the window and refitting to the vehicle. This way the vehicle still appears to have a DPF fitted and appears unmodified in anyway.

Why is Emissions Removal Motorsport Only?

Quite simply, tampering with your emissions system in any way may result in MOT failure because its not a road legal modification. You are required by law to have a working emissions system on a car manufactured after 2008 and it's not legal to knowing sell a vehicle without a working DPF. 


*Removal of a DPF will almost invariably make your vehicle illegal for use on a public road. This is not legal advice and if you are unsure if this applies to your vehicle, please seek further advice. Please call us to talk in more detail about this.


We are happy to only do the software removal if you have a mechanic who is willing to remove the DPF and the DPF brick from the exhaust. We will not do a DPF delete on a vehicle with the unit still full and fitted. This can cause very serious engine failure. 


For more information on DPFs and how they work, please follow this link for Wikipedia –

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