diesel Particulate Filter on new cars from 2008-

Jun 10, 2011
Hi everyone,
Just thought i would share this very important information about cars which are diesels! I have a new diesel car and its a worry for me but luckly enough i do long enough journeys. Have a read of this article if you are deciding to get a new diesel! :dry: :eek:hmy: :(

We get quite a few enquiries about a dash warning light on some vehicles which are fitted with diesel particulate filters.

Several vehicle manufacturers are a bit concerned over the way some drivers are ignoring the warning light – for a long time.

As you will be well aware there are health issues over the sooty particles produced by diesel engines (and petrol engines, to some extent) which penetrate deep into the lungs. These particles are blamed for an increase in heart and respiratory disease.

As part of the increasingly tight exhaust emission regulations, particulate filters are being fitted to more and more new diesel cars to trap these sooty bits.

Two types of particulate filter are in use - those with additive and those without. To enable a vehicle to use a system without additive the particulate filter must be fitted close to the engine. Because the exhaust gases will not have travelled far from the engine they will still be hot enough to burn off the carbon soot particles. In these systems an oxidising catalytic converter will be integrated into the particulate filter.

In other systems the particulate filter is fitted some distance from the engine and as the exhaust gases travel along the exhaust they cool. The temperatures required for ignition of the exhaust gas can only be achieved by the use of an additive.

The particulate filter is made from a honeycomb ceramic monolith, like a catalytic converter, of silicon carbide. The monolith is contained in a metal casing and consists of many tiny channels which run parallel to each other and are interconnected.

Manufacturers of particulate filters favour silicon carbide because it is very thermally strong, has good conductivity and is resistant to wear.

The particulate traps do just that – they catch bits of soot – until they are full – and then they can’t trap any more. At this point the DPF needs to go through the ‘Regeneration Process’. During regeneration the soot is converted to a very small volume of ash which remains in the filter.

There are two types of regeneration – passive and active. Passive regeneration takes place, automatically, on motorway-type runs in which the exhaust temperature is high (somewhere between 350ºC and 500ºC). As the exhaust is so hot (hot enough to ignite the soot particles) the regeneration process can carry on continuously and steadily across the platinum coated catalytic converter.

A surprisingly large number of people don’t take their cars anywhere near a motorway so passive regeneration is only going to be possible once in a blue moon. Soot builds up in the DPF and intervention from the ECU is needed to shift it. In the case of a filter without additive when the soot loading reaches about 45 per cent the ECU switches off the EGR and increases the fuel injection period so there is a small injection after the main injection. These measures help to raise the engine exhaust temperature to over 600ºC which is high enough to burn off the soot particles.

This isn’t a very straight forward process if the journey is a bit stop/start. If the driver suddenly has to stop or slow down, the ECU can’t complete the regeneration and it has to temporarily abort the operation. When the vehicle’s speed reaches a neck-snapping 38mph, the ECU can have another go at regeneration. The ECU will keep trying for 15 minutes, but after two 15 minute attempts the soot levels (loading) in the DPF will have increased. At a soot loading of 50 per cent the ECU will have another go at clearing it. It raises the maximum exhaust temperature to somewhere between 600ºC and 650ºC to try and trigger a regeneration process. The system will try to create a regeneration for another 15 minutes and if this doesn’t work it will have one more go for 15 minutes. After this the DPF light on the dash will be illuminated.

If anyone opens the driver’s handbook to see what this light might mean, they will see that the DPF has been partially blocked with bits of soot because the car has only done short journeys. The driver then needs to drive at a speed of at least 40 mph for around 10 minutes to clear it. This type of drive should allow the temperature in the DPF to become high enough to burn off the soot.

If this doesn’t work then it’s a dealer job.

So, the light is triggered at a 55 per cent soot loading. If the driver ignores the light and continues with his normal driving pattern, with no chance of DPF regeneration, the soot loading will continue to build until it reaches 75 per cent. Then some other dash warning lights will illuminate [DPF light, engine management warning light and also glow plug light]. At this point the driver should have noticed that his car isn’t going as well as it did.

It’s now too late to hope to clear the DPF by driving and the vehicle needs to go to a dealer straight away. If the driver insists on driving and allows DPF loading to reach 95 per cent then the vehicle will need a new DPF – at about £1000 a time.

If the filter has become blocked because the driver ignored the warning light(s) then he’ll be paying for the new filter himself. It will be covered under warranty if filter blockage occurred as a result of a system fault.

If a car has a DPF then it isn’t suitable for all roads and driving conditions. They aren’t recommended for cars which only trundle into town and back and places like the Channel Isles are deemed wholly unsuitable.

Sometimes a fault code is stored (18434) – which indicates that there is a problem in filter bank 1. However, this doesn’t mean necessarily mean there is a fault in the DPF – it relates to the whole exhaust gas handling system – from a dodgy temperature sensor, pressure sensor, wiring/connector fault.

If you or any of you attend a vehicle on which the DPF light is illuminated then it would be worth spending a few minutes educating the driver on how the DPF system works and why it needs to regenerate. This is particularly worrying if the car only does tiny journeys and wouldn’t recognise a motorway or dual carriageway.

The system described above is of the type used by the VW group, but we are also having a few issues with those used on Peugeot/Citroen cars. This is not because there is anything wrong with the set up, but the nature of the driving again means the FAP isn’t able to regenerate.

The PSA Group use a FAP filter which uses an additive; Ford use a similar system.

Use of an additive lowers the ignition temperature of the soot particles and the engine management ECU raises the temperature of the exhaust gas so the filter can be regenerated.

The engine management ECU initiates the whole regeneration process – it receives information from the exhaust gas pressure sensor (a full particulate filter gives a high resistance to gas flow), the air mass meter and the temperature sender which is fitted just before the particulate filter.

Regeneration is usually necessary after between 300 and 450 miles, depending on how the vehicle is driven. The process takes about 5 – 10 minutes and the driver shouldn’t notice it is occurring, although sometimes there may be a puff of white smoke from the exhaust during regeneration.

The Additive
The additive is stored in a separate tank. It is difficult to establish it’s precise chemical composition, but it is described as an iron-rich substance dissolved in a hydrocarbon mixture.

It works by allowing the carbon particles trapped in the particulate filter to burn at a significantly lower temperature than would usually be required (500ºC rather than 600 - 650ºC). This means that the particulate filter can be regenerated at part throttle.

The additive automatically mixes with the fuel at every fuel tank fill up. The fuel tank sender sends information to the engine ECU about how much fuel has been added.

The engine ECU then actuates the particulate filter additive pump. Additive is supplied to the main fuel tank so that the concentration of iron molecules in the fuel is 10 parts per million. In practical terms this means you need 1 litre of additive to 2800 litres (about 620 gallons).

We are finding that in these systems, again filters are sometimes failing to regenerate. In this case it seems to be just as bad in cars used predominantly for motorway use – travelling in sixth gear. In this gear the revs are very low – often well below 2000 rpm. Taller gearing in cars designed for extra economy is causing similar problems. In such circumstances the car needs to be driven hard in a lower gear so the temperature in the particulate filter will be sufficient to burn off the soot.
Jan 15, 2011
Thanks for posting that Mr Bell. I'd heard of the filters and the need for re-generation and because I travel often on the motorways I felt confident that I would be ok and so far touch wood the issue hasn't arisen. However I'm usually in 6th gear ASAP. and your comment about revs probably not being sufficient in top gear to complete regeneration has got me thinking.
Can I assume that if I occasionally remain in fifth gear for about 15 minutes whilst driving on the motorway then I'm doing the best for the car but other than the warning lights coming on which is I presume a little late how can I tell when it's the best time to carry out the higher rev run?
Again thanks for posting I think it's very interesting.
Best regards Brian
Jun 10, 2011
Hi Brian,

The question you are asking is hard to answer as it depends on the car you drive! DPF's only work at high temperatures when the system needs to clean! I have a vauxhall Antara and it's does a clean between 450 to 500 miles! I'm guilty of using 6th as much as possible to keep the economy good but after reading the information on the net I would say try not to use 6th too much! When my car needs to regenerate or you could say do a clean I will notice the car judder at lower revs and tick over be higher! also the mpg gets hit hard!
When the car goes into the clean mode it's best to use 5th gear and keep the revs high around 2-3000 rpm which generates enough heat to do the clean, the clean can take around 20 minutes for mine!
I use additives which I mix in the tank of diesel so it's keeping everything clean.
If you stop before the clean has finished you will notice your car fans will kick in to cool the engine, due to the amount of heat needed. If you can try to keep an eye on the amount of time the car is doing the clean, I usually use my second trip, I will clear it after the clean to see how many miles I have done between the cleans. it's worth doing this as you then can plan you journeys better!
I have found when the caravan is being towed the cleaning process is quicker as the car is working harder so the temperatures are higher!
Aug 11, 2010
nice article thank you. although i disagree about the need to use 5th gear on motorways rather than 6th. unless you only use the motorway for a few miles to trigger the burn off.. the systems are designed to work perfectly well once a car is running at proper operating temp and 6th can give more temp heat than 5th anyway on any light incline on a motorway due to it running low revs and the need for extra torque to keep the car pulling, ie more internal heat generated...... go for a push bike ride hit an incline in whatever is the highest gear and see how much more effort is needed to get up even a slight incline than to change down a few cogs. totally agree the system seems incomparable with shopping trip driving..
Dec 7, 2010
I do lots of short journeys in my car and in 2 years never had a dpf light on or any problems. It does a programmed regen every 400-600 miles, normally you do not know it is happening unless stood still or in slow moving traffic when the engine revs will increase to 1,000 rpm and the radiator fans kick in to clear the build up of heat in the engine compartment. Sometimes when turning the engine off the fans keep running, the regen has been stopped and that is what you need to understand, so next time the car is used the regen re starts so you have to make sure it then completes to keep it clear. I also think it helps that the dpf on the end of the turbo.
May 7, 2012
The DPF light only came on on the last car after almost five years and we had driven from Chester to near Glasgow on the motorway. Had it checked out and it was a false alarm. The garage did say do not ignore the light though as the engine may leak water into the engine oil with potential for serious damage.
Oct 28, 2006
people must remember half of this is self inflicted.Being honest and im as bad is there really any need for a 240hp diesel to do the school run or go shopping in?Euro 6 has seen many changes in the automotive/construction sector.By this i mean a decease in engine power and the correct sized engine in terms of cc for the specced job.This leads to engines running at 100% loads and operating hotter and cleaner.Engines are becoming smaller again.


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