Diesel Particulate Filters – What’s all the fuss about?

DPF removal with Coby Autos
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Firstly – I don’t understand what a DPF is – What’s all the fuss about?

Simply put, the DPF (or Diesel Particle Filter) does exactly what it says on the tin – that is it removes particulates of soot from the exhaust gasses. These particulates, when released into the atmostphere in high volumes can and do cause respiratory (and other environmental) problems, especially in cities where you have many thousands of cars on the go at any one time.

This sounds like a good idea, and generally speaking it is.

When euro5* was pending, car manufacturers came under increased pressure to clean up their act. To develop newer, more efficient engines that produce less emissions would be hugely time consuming, not to mention vastly expensive. And so all the main motor manufacturers added a filter (the DPF) to catch the offending particulates. These particulates would then (in theory) be burnt up within the filter as the car hits higher speeds.

The problem is most manifest when such a car is used primarily for city driving or driving to and from work. In rush hour a car will never get a chance to hit the speeds required to burn up the particulates caught by the filter. As a result the filter becomes utterly clogged up to the point when it needs to be replaced.

* euro5 regulations explained: The Introduction of Euro 5 and Euro 6 Emissions Regulations for Light Passenger and Commercial Vehicles

What the dealers are advising

The instructions mooted by the main dealer workshops when presented with a prematurely blocked DPF is to take the car on the motorway at very high revs for 40 minutes.

The idea is that this will enable the the car to regenerate or in other words, to complete its own cleaning cycle.

“So just to be clear, the garages are telling you to buy unnecessary diesel, go on an unnecessary journey, and drive at high speeds in order to save the planet!!!”

It is no surprise then that no manufacturer will quote any emission figures for when the regeneration process is happening.

It is our experience that the amount of diesel consumed in this regeneration mode is very high, and as a consequence the emissions are very very high indeed.

An example: A 2014 Mazda 6 presented recently in our workshop, with 70,000 Km on the clock and over 500 recorded regenerations.

Remember that a single regeneration – i.e. trying to clear a blocked DPF – will produce a high quantity of both CO2 (Carbon Dioxide) and NOX (Nitrus Oxide). Consider then how much of both is released to the atmosphere when you do that 500 times. This on a car that’s only 2 years old. Finally, take a moment to imagine the number of cars with DPFs that are doing this every single day.

It is common knowledge among those in the remapping circles that software updates provided by unsuspecting main dealers are opening up the parameters of the DPF systems, to try and stop them blocking up.

This is pushing up emissions of both C02 and NOX, by making changes to the function of the ERG valve. We can only guess as to whether this is a cynical quick fix or not, but it’s hard to say as it can’t be checked.

How widespread is this problem?

Most diesel engines manufactured from 2007 onwards have been fitted with a DPF. The mere mention of DPF problems strikes fear into those who are even vaguely familiar with this issue, as the expense of replacing the DPF can run into thousands of euros.

One of the strange things we have noticed is that you can have two cars of the same year, make, model and engine, and one model will have a DPF while the other one won’t.

Both cars will have the same C02 emissions on the documentation – the same tax bracket, and yet one has been manufactured with a DPF and the other manufactured without.

If you find this very strange, you’re not alone. We do too! If you can achieve the same end emission result without a DPF then why have it?
Onto the more recent problems surrounding Diesel Particulate Filters (or DPFs as they’re more commonly known). Here we will take a look at the DPF and try and understand why it blocks up prematurely – and why replacing it is not always a solution.

It may surprise you that car manufacturers do not include the DPF as an item covered under warranty. This will give you some idea how much trouble DPF systems are causing in modern cars.

Recently we had a 2014 Skoda Octavia 1.6 diesel with a blocked DPF in the workshop. Our customer had booked it into the dealer he bought the car from – and they informed him that the DPF was blocked. He quite reasonably suggested that the car was under warranty so do whatever was necessary. Unfortunately he was then informed that this was not covered under the warranty and the cost of replacement would be €2000!

Our customer was shocked as you can imagine, especially given that the car had only 65000km on it. He mentioned that he was thinking of having it removed and remapped as a more affordable solution, only to be told that if he did this, his complete warranty would be void. A definite opt out for the Dealer.

The flip side of all this

So what’s happening more and more is that people are trying to find clever (if somewhat questionable) ways of getting around the issue. Here’s one such example:

Recently we had a Porsche 911 in the workshop for an emissions problem and a tune up. When the costumer phoned to book the car in he told us that the car had been tuned to Californian emission regulations. This made some sense at the time as some imports often would be setup for different regions – some would be tuned for hotter or colder climates, with different engine oil and coolant to suit.

On closer inspection of the Porsche in our workshop, we found that the emissions were incredibly low for a system with no catalytic converter or lambda sensors. When we examined the engine and exhaust system we discovered a pump driven from the fan belt was pumping fresh air into the exhaust system at the exhaust manifold – a nice trick – as the California state emission regulations only stipulate that the emissions levels need to be at a certain level when tested at the tail pipe.

Is this cheating – or clever engineering? Just because you can’t sample it at the tail pipe doesn’t mean it isn’t there!

What can be done to try and avoid DPF issues?

People often ask us for advice on how to help keep their diesel car running for as many miles as possible without their filter blocking up. Here’s a few tips:

  • Firstly, some common sense. Don’t drive around with the engine warning light on, and / or the DPF blocked light on as well.
  • Regular servicing helps a lot, as this may show up issues that can be dealt with BEFORE they become bigger isues.
  • Quality of your engine oil. The engine oil used on a vehicle with a DPF fitted must be a low sulphur oil. This is one of the main contributors to a DPF prematurely blocking up, and it’s easy to see why, as follows:
  • When most people who need their oil topped up go into a petrol station they are presented with a range of seemingly very similar oil products at varying prices. The staff behind the counter are not generally qualified to advice on the right type oil for specific cars, and what happens as a result is that most people buy the cheapest oil available. This is a false economy and greatly increases the odds of problems in this area.
  • Working in the garage trade we are well acquainted with many petrol / service station owners. Many of them will confirm that the sale of the more expensive oil in their stations is virtually nil – while ironically pointing out that [of their customers that] “They don’t buy the cheapest wine!” This all too often – when repeated over time – causes major problems.
  • Oil contamination or diesel getting into the engine oil is another big problem on these systems. When the regeneration is taking place so often – as there is so much fuel being injected into the engine, inevitably some gets past the piston rings. From here it travels into the sump where the engine oil is, very often causing catastrophic engine failures on many cars and vans. This has long been a huge problem for manufacturers while they are still under warranty.
  • Some manufacturers – mainly the French – have gone down the road of fuel additives. This involves fitting a small tank under the vehicle that injects a small amount of a special liquid into the fuel tank every time the fuel filler cap is removed and fuel is put in.
  • Again there is a warning on the dash when this additive runs low, but it gets ignored mostly, until the DPF gets blocked up. Usually by then it’s too late.
  • Using a good brand of Diesel fuel additive that you yourself put into the tank every so often is another good option, but this needs to be a routine from early in the cars life, and not waiting until a dash light comes on and the DPF is full.
  • DPF cleaning services is something else that is getting some good results, but we have had many costumers for whom the cleaning process has not worked.
  • The success of the ‘cleaning’ will depend on the level of blockage in the first instance, and again, very often by the time such cleaning takes place the damage has already been done.
  • Manufacturers do not advise resetting the ash accumulation levels in the cars engine computer unless you are fitting a new one. This is because it can only be reset back to 0 and no assumed amount can be inputed. Therefore if cleaning has been carried out, it’s difficult to know how much has come out and how much is still in there.
  • Similarly, if someone fits a second-hand DPF the same problem occurs; how do you know how much ash is remaining in it? This is often over looked by garages.
  • Something else that’s not commonly known is the problems that can occur where you have a duel mass flywheel type clutch that is causing a vibration in the engine. Very often this results in the engine computer or management system applying a fuel strategy to constantly try and keep the engine running even and smoothly, also resulting in additional fuel being used, blocking up the DPF.

Should car sales people be more open and honest in their approach?

One of the questions asked by service department personnel when trying to explain to a costumer why the DPF is blocked on their 2 year old car with 20,000 on the clock, is:
“Why did you buy a diesel? When clearly you don’t do any mileage, you almost never drive on a motorway, and its only used to go to the shops or mass once a week?”

The customer will invariably will tell you they were sold it on the basis of cheap tax, great resale value, cheap to run, etc etc. There has to be a responsibility on the sales person to advise on the best solution to a person’s needs, i.e. what will the car be used for, how many miles a year would you do etc.

This would be much better all round, both for the costumer and for the environment. Think of how much less pollutants’ come from a small petrol engine doing small mileage, with no need to do crazy regenerations causing extra fuel to be pumped into the exhaust to try and clean it out.

With the ever increasing number of electric vehicles being developed by all car makers, and a resurgence of the petrol engines, we believe the diesel engines and the DPF are firmly on the way out. With London talking about banning Diesels out of the city, and euro7 now looming, who can say with any certainty what is coming next.

Hopefully it doesn’t turn out to be another stealth tax on the motorist like the DPFs are. We were all promised cheap tax for a cleaner emitting car, only to have to pay dearly for it further down the line, when the filter under the car that nobody knew about fills up and bites you in the pocket.

It’s not a very level playing field when the airlines are given a maximum amount of pollutants they can emit each year, only to have to pay a small fine when permitted pollutant levels are (routinely) vastly exceeded.

Even more so when you consider the huge additional revenue the airlines gain from the extra flights. Dublin Airport is about to build a second runway that will produce an additional 3 Mega tons of CO2 a year.

Where is the furore about this? Who is thinking of our environment or the quality of our shared air supply here?
Perhaps, like our friends with the Porsche solution, they will start pumping oxygen into the air around Dublin airport so it simply can’t be measured.