10 reasons you'll come to hate your diesel's particulate filter

If you have a diesel car fitted with a particulate filter and it has never given you any trouble, you’re in luck. But be warned: if it starts to fail or you don’t look after it properly, you may come to hate it.

Since 2006, many manufacturers have started fitting their diesel-engined cars with special anti-pollution devices so they can meet emissions regulations. This device, which complements the oxidation catalyst, is the increasingly well-known particulate filter, known by its acronyms FAP or DPF.

If you have a diesel car fitted with a particulate filter and it has given you some trouble, you’ll know what I mean. There are also many drivers who, without knowing whether their car has a particulate filter or not, have no idea how this device works, or the faults it can develop if it isn’t looked after carefully.

How the particulate filter works in a diesel

We won’t go into too much detail. Basically, these filters trap the solid particles present in the exhaust gases. These particles are harmful, as they consist of hydrocarbons that have not been fully burnt during the combustion process. This is simply part of how a diesel engine works and of the characteristics of this fuel.

For this reason, petrol engines, which burn more “cleanly” as far as particles are concerned, have not needed these devices to meet emissions standards. Even so, brands such as Audi, BMW and Mercedes already announce the use of particulate filters in their new generations of petrol engines. It is too early to say for sure, but everything suggests they should not cause anywhere near the number of problems these devices cause in diesel cars.

So what does the filter do? It “traps” the particles so they don’t escape into the atmosphere, and when it reaches a certain level of saturation, it burns them off. To do this, before it becomes completely full, a pressure sensor sends the corresponding signal to the control unit so that the so-called filter regeneration process begins. There is no fixed mileage at which the filter fills up and needs to regenerate; it depends on driving style and the particles generated. Under normal use, it can be estimated at intervals of around 500 km.

Regeneration occurs by raising the temperature of the exhaust gases to around 600 degrees Celsius, which is achieved through specific engine management that injects small additional amounts of fuel – which is why consumption rises momentarily – after the main injection that produces combustion. Once incinerated, the filter is clean and can continue trapping particles.

In a very schematic and summarised, yet illustrative, way, you can see the process in the following video.

https://www.youtube.com/watch?v=4dpb2-x8ESE&feature=youtu.be

Particulate filters with or without AdBlue

There are two types of particulate filter: those that work without an additive, and those that need an additive to function. Until recently, small engines (under 2 litres) did not need filters with additives. But lately, and ahead of future anti-pollution standards, we are also seeing small cars with low-displacement engines using particulate filters with additives, such as the new Volkswagen Polo.

What the additive, known as AdBlue – a urea-based solution – achieves is lowering the temperature needed for the particles accumulated in the filter to burn off. To do this, AdBlue is injected into the exhaust gas stream before it enters the filter. The additive is stored in a dedicated auxiliary tank, which needs topping up every so often. It is not very expensive, around 3 euros per litre. Tanks typically range from 12 to 25 litres depending on the car, and depending on use they can last between 10,000 and 20,000 km.

You will normally have it topped up at each annual service at the dealership, but be careful. A warning light on the instrument panel will alert you when a few thousand kilometres remain before the tank runs dry; if it runs out completely, the next time you try to start the vehicle, depending on the car, you may not be able to, or it will start with the engine in protection mode. It is as easy as stopping at any petrol station, buying a canister of AdBlue and topping it up yourself, although it would always be easier not to have to worry about it.

Particulate filter faults

Now that we understand how the filter works, it is much easier to grasp the possible faults it tends to cause… and what needs to be done to prevent them. The first and most common is a loss of engine power caused by a clogged filter that does not regenerate properly.

This can happen because the filters reach the end of their useful life: regenerations are not 100% effective and residues remain in the filter, which gradually loses its properties. There is usually no mileage specified in the service schedules; it is replaced when it fails. Depending on usage, this can happen anywhere between 100,000 and 150,000 km, although some brands set a lifespan for their particulate filters of up to 250,000 km. If you do a lot of city driving, which is when the most particles are generated and where constant stops often prevent regeneration from being completed, the filter will last much less.

Be careful, because replacing the filter is very expensive: between 1,000 and 3,000 euros on average depending on the type of car. Depending on the fault, it is sometimes possible to remove it and carry out a particulate filter cleaning at specialist workshops using ultrasonic procedures, which reduces the bill to less than half. And if you are thinking that a solution might be to disable it, be very careful. Technically it is possible to empty the filter and reprogram the ECU so the car keeps running. But it is illegal, since the car will pollute more. And although until now this has been a more common practice than you might think, with the new MOT procedure introduced this year, 2018, that is over.

Another fault that is causing a great deal of headaches for many drivers is the continuous rise in engine oil level. It is caused by some of the additional fuel injected to carry out regeneration passing into the engine oil without burning off, which raises the oil level… and reduces its lubricating capacity, which can end up affecting engine longevity.

Tips for keeping the particulate filter working well

Looking after the particulate filter therefore requires quite a bit of attention. To begin with, you must scrupulously respect maintenance intervals and check the oil level frequently. It is advisable not to use additives, and it is not recommended to fill the fuel tank to the brim. It is also wise not to switch off the engine if the regeneration process has started, which is not always possible, as there is no way of knowing whether the filter is regenerating.

But the main problem for particulate filters is using the car in town or on short journeys in which the engine does not reach its optimal operating temperature without getting onto a main road. If this is how you use your car, and it is fitted with a particulate filter, consider that you will need to take it out onto the open road from time to time and drive for at least 15 or 20 minutes.

The 10 reasons you’ll end up hating the particulate filter

1. They increase the price of the vehicle compared with engines that don’t need filters: around 600 euros on small or medium cars, more than double on larger ones.
2. To work, they need post-injections that increase consumption. It is not an alarming increase, but it is there. A car without a filter uses less fuel.
3. If you drive a lot in town, the filter does not regenerate properly and faults occur, so you need to get out onto the open road from time to time… even if it’s not in your plans.
4. If the filter becomes full because regeneration has not occurred properly, the engine loses power and may go into fault mode, and you have to visit the workshop for a forced regeneration. It will cost you around 100 euros.
5. On filters with AdBlue, you need to monitor the additive level, topping up the tank at official services or doing it yourself when necessary. A 10-litre canister costs 30 euros and will last you roughly 15,000 km.
6. If AdBlue runs out while driving, you can continue to the next stop, but you won’t be able to use the car normally again until you visit the workshop.
7. Engines with particulate filters need specific engine oils with low ash content, designed to withstand the effects of the additional fuel injections needed for the filter to regenerate. They are somewhat more expensive than standard oils.
8. On some brands, filters are causing oil level rises in the engine above the maximum, as excess fuel from the post-injections that does not burn inside the cylinders passes into the sump.
9. Replacing it when it fails is very expensive: between 1,000 and 3,000 euros, depending on the type of vehicle. As a cheaper alternative, they can sometimes be cleaned.
10. In the case of continuous faults, for example due to the type of use the car gets (short journeys and always in town), you cannot consider disabling it, as it is illegal, and you will not pass the MOT.

If you have read this far and you have a diesel with a particulate filter or are thinking of buying one, you may have become a little worried. No, I am in no way suggesting that you should run away from diesels. They remain unbeatable in terms of consumption, especially for those who cover many kilometres a year. What I am saying is that you need to consider the characteristics of each technology, with its pros and cons.

And at present, given the current regulations and the additional emissions-control systems that diesels use, if you are going to use the car mainly in town and you are not prepared to accept that, to avoid faults, you must take it out onto the open road from time to time, it is better to opt for a petrol. Or a hybrid car.

Related information
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