Diesel particulate filter DPF FAQ


This page describes how the DPF filter works on VW Jetta TDI, Golf TDI, and Audi A3 TDI, common DPF problems like DPF filter clogging, how to manually activate an active DPF regeneration cycle, why you can't remove or bypass it with a DPF delete kit, and more FAQ.

For the VW Touareg TDI, Audi Q7 TDI, or 2012+ VW Passat TDI, see 1000q: Adblue-DPF FAQ page 1.  There are substantial differences from the non-adblue system.

Earlier diesels only had an oxidation catalytic converter for emissions.  With the introduction of cleaner diesel fuel in North America, diesel particulate filters (DPF) are now standard.  Larger engines or heavier cars also use Adblue fluid sprayed into the exhaust to cut NOx emissions by up to 80%.  The current generation of diesel cars with DPF are so clean they'll pass the white handkerchief at the exhaust pipe test.

The first North American VW or Audi TDI that got a DPF was the 2006-2008 VW Touareg TDI V10.  The 2004 Touareg TDI had no DPF and there was no 2005 because it wouldn't pass emissions without one.  All TDI 2009 and newer have a DPF.  The 2009 and newer VW Touareg TDI V6 (Touareg2) and Audi Q7 TDI V6 also have an Adblue system.  Again, see the link above for specifics on the Adblue system.  For general info on the 2009-2010 VW Touareg, see 1000q: 09-10 Touareg TDI buying guide.  For info on the new style 2011 Touareg, see 1000: 2011+ Touareg TDI buying guide.  (For buying guides on other TDI and more FAQ, go to the home page).  Fitting the Adblue system in a smaller body is one of the delays in any new Audi A4 TDI in the US and North America.  Audi does sell their Q7 with the TDI engine, see 1000q: Q7 TDI guide for details on that model. The new 2012+ VW Passat TDI also uses Adblue but with the 2.0L engine. See 1000q: mk6 Passat TDI buying guide for info on that car.

Except for the pumpe duse Touareg V10, DPF followed the introduction of common rail injection in North America.  See 1000 answered questions: pumpe duse vs. common rail for an explanation of the differences in the fuel injection system.

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Ultra low sulfur diesel (ULSD) required for new common rail clean diesel engines

The first step in allowing clean diesel technology was the change in North America from low sulfur diesel to ultra low sulfur diesel (ULSD).  This is why you don't see stinky dark smoke clouds coming out of 18 wheelers anymore.  The reduction in sulfur was from 500 parts per million to 15 parts per million, a reduction of 97%.  The new emissions technologies wouldn't work with low sulfur diesel which caused a delay in their adoption - the automakers couldn't use the technology without cleaner fuel and the refineries didn't want to make cleaner fuel without justification, so the government required ULSD by the end of 2010.  All refineries have made only ULSD for road fuel for a while now so you won't find non-ULSD at a retail pump even if it doesn't have the ULSD sticker.

Not using ULSD will result in DPF clogging from particulates and catalyst failure from sulfur.  See below for a detailed explanation why.  The only place you'll find the older LSD fuel in North America is in Mexico or for off road applications.

While the engine will also run completely fine on up to 100% biodiesel B100, the problem is that it's not compatible with the emissions system or engine tuning.  A full explanation requires a basic understanding of the emissions system so read the sections below for more detailed information.

The DPF or Adblue equipped exhaust system on VW TDI and Audi TDI

There are 2 basic types of modern clean diesel exhaust systems on TDI.  Those that use Adblue (2009+ VW Touareg TDI and Audi Q7 TDI) and those without (2006-2008 Touareg TDI and all other 2009+ TDI, some Euro engines).  The larger and heavier cars with V6 engines need Adblue to meet 50 state emissions.  The older V10 Touareg only had DPF because it was sold during a time of looser emissions standards.  See 1000q: 3.0L engine DPF FAQ for the Adblue type systems.

All modern clean diesels have 3 major emissions components: the oxidation catalytic converter, DPF, and NOx catalyst.  Each of these are described in further detail below.  There are also numerous O2, temperature, and backpressure sensors.  Because of these sensors, if you delete the DPF or do a DPF bypass without a rewrite of the engine management computer, it will set the check engine light and the car may not run correctly. In one case, a DPF delete before the replacement ECU flash melted the reflector because of the excess fuel during an attempted regen cycle.

The 2009+ VW Jetta DPF system is shown above right.  The Golf DPF and Audi A3 TDI exhaust are basically the same.  The difference is that it combines the oxidation catalyst and DPF into 1 unit and has an exhaust valve to increase backpressure for proper low pressure EGR flow during a NOx regen.  Because there is no Adblue, it also uses an H2S converter. Sometime during 2010, the Jetta exhaust split the long cat/DPF-NOx cat (VW# 1k0 254 706 L) into 2 separate pieces (cat/DPF: VW# 1k0 254 705 g  and NOx filter: VW# 1k0 254 401 t).  The 2 piece unit should be easier to remove and cheaper to replace if needed.  This is shown below.

See the video below for how the cat and DPF work.  The last section about Adblue is different on non-adblue engines - see 1000q: Passat, Touareg, Q7, and Cayenne Adblue DPF FAQ for more info on those engines.

Oxidation Catalytic Converter on TDI engine

The oxidation catalytic converter is the first step in post engine emissions control.  It converts and oxidizes hydrocarbons and CO into water and CO2 using platinum and palladium as catalysts.  It has to be close to the engine to use the energy from the hot exhaust and for quick light up to around 250oC.  This is what older TDI had.  In Jetta/Golf/Audi A3 with DPF, the converter and DPF are a single piece.

The diesel particulate filter (DPF) on VW Jetta TDI and Audi A3 TDI engines

The DPF is a catalyst filter on all North American TDI 2009 and newer to meet the strict emissions standards.  Particulates are what makes diesel exhaust black and sooty.  The DPF is made from a silicon carbide body coated with platinum and aluminum oxide as catalysts to make NOx and carbon into NO2 and CO2.  The filter traps particulates and burns them off passively during normal driving or actively during a "self clean" regeneration cycle.  The oxidation catalytic converter and DPF can be seen below on the back of a demo engine.  On the 2.0L TDI engine both are combined into one unit.  The DPF is always downstream of the first cat.

Why the DPF can become clogged and how to clean a VW or Audi DPF filter

Unlike a regular catalytic converter which has a pass-through honeycomb-like structure, a DPF really is a blocked off filter.  If you hold a cat converter up to a light you can see through it - you can't see through a DPF because each honeycomb is blocked off at alternating ends (shown below).  Its construction and the fact that it traps particulates between self clean cycles means that it also increases backpressure vs. older systems.  This slightly reduces the energy differential that the turbo uses to spin and fuel economy at the cost of better emissions.  The filter traps solid particulates at the far end while still letting exhaust gasses pass through the filter upstream of the particulate blockage.  During the automatic filter regenerations, the particulates oxidize into ash which build up in the DPF and fill it over the life of the system.  VW and Audi suggest inspection of the DPF at around 120,000 miles.  While average lifespan before failure is not yet well known, it's possible to remove it if you have the car's computer reflashed. As far as I know, the only TDI chip tuner who does DPF specific programs is Malone tuning in Canada. Other companies can do power flashes but only Malone tuning does both power and DPF removal tunes.

To inspect filter loading, you must use a VCDS cable and software or dealership tools.  Once the filter reaches 95% clogged, it must be removed from the car and replaced or cleaned using a DPF cleaning machine.  Heavy duty truck services do DPF cleaning but as far as I know, this is not possible for the DPF/cat units used on 2.0L TDI engines because the cat/DPF is combined into one piece. Truck services require the DPF to be separate for effective cleaning.

While it's possible to clean out a DPF you have to cut and weld the case back. Here are some details on how to clean it. It's very important that it's seated correctly because the sensors are looking for a certain pressure differential before and after the filter.

Below are some thumbnails (click to enlarge) that Greg Roles took of what's inside a failed DPF. This member is in Australia so his unit was the DPF only, no cat. He found that there was little black soot but lots of ash. This is because the ash accumulates while the soot gets burned off. There are precious metals inside which are worth money at a recycler so if you have a failed one, take it to a junkyard for cash.

According to the service manual, DPF removal in Audi A3 TDI, VW Jetta TDI, or VW Golf TDI is difficult because it says the front subframe must be lowered for access.  According to people who've actually removed the DPF, you can just disconnect the passenger side driveaxle and twist it out.

If you choose to lower the subframe, you must support the suspension, engine, and subframe.  For 2010 Jetta and Golf TDI, they separated the DPF/cat and NOx cat into 2 pieces - 2009 and Audi A3 TDI have a 1 piece assembly all the way back to the exhaust valve.  Removal of the 2 piece assembly should be much easier. The early 1 piece unit is (VW# 1k0254706L) and the later 2 piece units are (cat/DPF: VW# 1k0254705g and NOx filter: VW# 1k0254401t).  The DPF/cat is connected to the turbo with a v-band clamp on the 2.0L engines.

Below are some thumbnails of the DPF and the back of an 2.0L engine removed from the car. As you can see, it's really tucked in there. Also are some other pics of the system.  Unlike the demo engine shown earlier, the actual cat/DPF unit has a heat shield and sensors all over it.  The demo engine also had the EGR filter removed.
audi tdi dpf systemvw tdi dpf systemvw tdi dpfaudi a3 tdi dpfjsw tdi dpf

To the right is another view of the system. It's taken from the front instead of the rear as shown in the thumbnail above.  You can only see the bottom of the DPF with the engine and front subframe in place but you can see the NOx and H2S catalysts.

If your DPF light comes on (shown at top and below) and it's not clogged, the exhaust back pressure sensor or exhaust gas temperature sensor could be faulty.

European TDI brochures include the warning: "Please note, driving conditions within inner-cities and the Channel Islands may not provide optimum conditions for the use of DPF technology. Therefore, it is advised that you consider this before ordering your vehicle with DPF".  This note isn't in North American brochures.  The Channel Islands are about 5 miles long so my interpretation of VW's warning is that being restricted to 4-5 mile trips may cause a malfunction due to the DPF not being able to warm up sufficiently.  Catalytic converters generally take around 5 minutes to fully warm up so I don't see why the DPF would be much different.  As a side note, this is why most modern turbo engines usually have a pre-cat close to the turbo.  The turbo and exhaust manifold are a pretty big cast iron heat absorbing lump so it increases main cat light up time.  Since most emissions are created on cold engines, the pre-cat significantly reduces overall emissions levels.  On the TDI, the main cat/DPF is right up against the engine where a pre-cat on other engines would be.

Very short trips in general are not optimal for any engine because moisture in the engine and exhaust won't have time to fully evaporate.  This is why some cars have a white oil-water mixture on the underside of the plastic oil filler cap.  As long as you drive more than 15-20 minutes every once in a while, my best guess is that the DPF will have enough time being heated to regenerate. Many TDI drivers in the US who regularly take only short trips have reported no unusual malfunctions.

507.00 engine oil required for VW TDI and Audi TDI with DPF filters

DPF clogging is why VW Audi (VAG) 507.00 engine oil is recommended for cars with DPF.  If for no other reason to make sure the engine warranty is upheld if they say the problem was cause "because you didn't use the right oil", VW 507.00 oil is low ash and will maximize the life of the DPF.  While the regeneration cleans out almost all of the soot buildup, there are things like metals from normal engine wear and additives in the engine oil which cannot be burned out of the filter.  Over time, this will build up.  While it's possible to remove and clean a clogged DPF filter, it's not easily accessed on Audi A3, VW Golf or Jetta.  504.00 was used on Touareg V10 with DPF.

The part number for 507.00 engine oil is VW# gvw 052 195 m2.  Some brands that meet this spec and some places you can buy it are: Castrol SLX Professional LL03, Motul specific VW 507.00, Total Quartz Ineo 5w30 507.00, and Mobil 1 ESP Formula 5W-30 (spec sheet linked).  The part numbers for 5w-30 507.00 engine oil are:
VW# g v52 195 a1 (gv52195a1) for .5 liter 
VW# g vw 052 195 m2 (gvw052195 m2) for 1 liter
VW# g 052 195 m4 (gvw052195 m4)for 5 liters 

It's also possible that there is something about 507.00 engine oil vs. others that is designed to deal with all the fuel that ends up in the oil from post combustion injection.  The post combustion injection is also known as an active regeneration "self clean" cycle.

Continued on page 2: with passive vs. active regeneration symptoms, using biodiesel with DPF, how to check soot loading, and manual regeneration initiation



If you have more questions or comments about Adblue or why a DPF delete or DPF bypass is illegal, feel free to ask in the myturbodiesel.com forums