The Complete Guide to DPFs – Deletes, Bushfires & Regeneration

The underside of a car on a hoist showing the DPF

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The Rough As Guts mandate is that we must always tell it like it is, regardless of popular opinion. Sometimes it may seem like we’re trying to gate keep the word “adventure” when we say things like “real four wheel driving” or “real camping”. That’s not our intent, but what we damn-sure are hell-bent on, is to make sure people are never putting their limitations on others, advising against reasonable risk and lowering the bar for people who just might have gone and done something incredible if they hadn’t been talked out of it.

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Table of Contents

Everything to Know About a DPF – Bushfire Risk, DPF Removal?, Cleaning & Regeneration

I don’t care about you getting better performance from a DPF delete or where you weigh-in on the debate about their necessity. I’m only writing this article, because DPFs have caused bushfires in the past, they will again in the future and it’s a subject that requires more illumination.

But, while I’m here, I may as well cover everything about them. There’s a lot of misconceptions getting around the place these days when it comes to DPFs.


What is a Diesel Particulate Filter and How Does it Work

Diesel combustion causes small particulate matter in the emissions from the combustion. In our case, it’s the exhaust smoke from a diesel car. These particles are harmful. Many people believe that without a diesel particulate filter, that exhaust emissions will be worse for the environment. It’s not so much a greenhouse gas problem as it is a human health problem. The particles being contained by a diesel particulate filter are carcinogenic and are a respiratory irritant. Diesel smoke also causes ground level ozone, which is harmful to breathe.

This is a problem in busy areas. Most notable is London where they have zones where diesel vehicles either aren’t allowed, or have to pay a tariff. This is in the response to the alarmingly high diesel particulate matter found in ongoing air quality test.

Is it a problem in the bush? I don’t really think so. The particles don’t stay airborne forever, but the problem is that these vehicles don’t stay in the bush and eventually make their way back to down. This is probably why the regulations are so sweeping and all-inclusive.

A DPF is essentially just a filter that mounts in-line near the end of your exhaust that traps the particles as the exhaust gases leave. They look a little bit like mufflers. They’re simple in theory, but their problems come from the fact that they start to fill and clog with the particles, requiring burn-off or cleaning as we’ll discuss below.


Bushfires Caused by DPFs – What’s the Risk?

I should admit, that an exhaust (with a DPF) is pretty unlikely to start a fire. The main problem is if you’ve got trapped grass coinciding with it going through an automatic regeneration cycle, making it much hotter than normal. Those two aligning makes for a higher risk and if you’re doing enough outback four wheel driving, it’s bound to happen at some point.

Spear grass and spinifex are the main culprits for causing a build up of flammable material in any part of your 4WD, so it’s areas with this kind of growth to be wary of in summer. Tracks that see infrequent travel are also a bit more likely to leave a bunch of material in your car, as the plants between the wheel ruts have more time to grow and recover in between getting beaten down by a car’s undercarriage.

DPFs are often located near cross members of the chassis. I believe this is just coincidental, but it leads to one of the likely buildup areas for tinder like organic matter, being right where the DPF is.


How to Prevent Bushfires Being Caused By Your DPF

There’s always risks in moving vehicles and equipment in summer. You should know when you’re in a total fire ban or a movement restriction, but it’s still possible outside of these times that a vehicle’s engine or exhaust can cause a fire. The risk goes beyond just setting your own car on fire, but no one wants to live the weight of having accidentally starts a bushfire that destroys property or takes lives. People have killed themselves over the guilt of accidentally starting fires that become catastrophes.

The DPF specific risk here, is that it may go through an automatic cycle while you’re driving through somewhere that presents a fire risk.

If you have a vehicle that has DPF regeneration, here’s how to reduce the risk. You can routinely check for built up grass etc or you can make sure you go through a regeneration cycle before entering these areas. Preferably, do both.

If your vehicle doesn’t have a manual active regeneration function, you may just need to try and cause a passive bur-off in the DPF. We cover that below.


DPFs a Diesel Killer? How They Impact Reliability

DPFs suck. If the particulate matter wasn’t such a problem and therefore not regulated around, there’s no way that manufacturers would ever include them. They’re only made to meet new emissions standards.

DPFs constrict exhaust gas egress and they require a whole bunch of functions that don’t benefit the engine. As we see regulations become more and more stringent and DPFs becoming more complicated (and therefore temperamental), we’re going to see more and more problems with diesel vehicles. Will DPFs kill diesel faster than electric cars? I hope not.


Burn Off & Regeneration

Often referred to together, burn off and regeneration are two different things. Burn off simply refers to the exhaust gases being naturally hot enough to burn the buildup as it passes through the DPF.

Regeneration is a process that’s initiated when there’s a certain amount of buildup in the DPF, to avoid it becoming clogged and creating too much back pressure against the engine. Additional fuel is injected after combustion causing it to burn in the exhaust gases and rapidly increase the heat in the DPF.

Some models of car, say from the ~2008 mark, came with DPFs due to newer regulations, but they’re not the type that have a regeneration function. These rely solely on burn off.


Automatic Regeneration

Automatic regeneration is common in late model diesels. It’s simply when the ECU initiates a regeneration cycle once the DPF reaches a certain threshold buildup.

When DPF Regeneration Won’t Take Place

Some errors may put your car into limp mode to protect the engine, in which case automatic regeneration won’t take place. Ironically, a DPF that hasn’t been able to go through a regeneration cycle, may trigger limp mode.

Problems with the sensors that detect levels in the DPF or anything to do with exhaust gases can also prevent a cycle from occurring.

Some models need a minimum level of fuel before starting the regeneration cycle, because it shouldn’t be interrupted.


Manual Regeneration

It’s much less common than automatic regeneration, but some newer models have the option where you can manually initiate a DPF regeneration cycle. This is what we recommend prior to entering the sort of areas we discussed that present a bushfire risk.


Passive DPF Burn

Passive DPF burn is what we’ve been referring to as burn off. It requires certain driving conditions, such as highway driving when the engine is already at operating temperature. Frequent, short trips with a lot of stop/start, don’t offer any passive DPF burn and are also the conditions that create the most buildup.

Avoid the Need for Regeneration

If you’ve got a 4WD that’s always needing DPF regeneration, here’s what you do: Pick up a rock, pass it to someone and get them to throw it at your head, because it’s your fault and you probably don’t deserve a four wheel drive.

A four wheel drive that gets relegated to commuting, school drop-offs and shopping trips without getting a good flogging once in a while is the victim of a crime. Drive your car like a man and you can avoid DPF issues.

If you’re getting the light on your dash and it wont go away, kick your car down a gear for 10 or 20 minutes on the highway.


DPF Cleaning Fuel Additives

Fuel additives for cleaning DPFs do seem to work somewhat reasonably. While they’re not particularly bad for your vehicle, the engine isn’t designed for having these types of additives in the fuel. They should be used as a fix for a clogged DPF, rather than as maintenance.

Prioritise your engine over your DPF.


Do DPF Additives reduce the Lifespan of the DPF?

Who cares.

Sometimes, there’s going to be a problem regardless of how you’re driving. High ash oils and contaminated fuel are just some of the things that may cause clogging.

People surmise that it does, but my thought is that things perish more due to cycling, than to consistent exposure. What I mean by that, is that going through relatively rapid temperature fluctuations is probably going to cause more premature wear than regular exposure to slightly higher temperatures. Therefore, fuel additives shouldn’t be a problem for DPFs. However, I still think it’s better to take risks with the DPF rather than your engine, so it’s probably best not to always have DPF additives in your fuel.


Legalities of DPFs and Modifying a DPF

DPF Delete

The main reason for people removing their DPF is for performance gains, but these are usually totally overestimated. The fact is, most DPF removals occur at the same time as getting a tune or installing other mods, leading to over interpretation of the benefits of the DPF removal. Now, if you’re changing a tune, adding a bigger turbo, adding oversized injectors or any of that sort of thing, the removal can make a big difference.

However, it’s probably one of the worst modifications you can ever get caught for.

Individuals face a $300 on the spot fine and up to $22,000 in court. Corporations can face up to $44,000 in court. This is for removing their own DPF. Offering it as a business service is an entirely different kettle of fish. Companies can be hit with a $1,500 on the spot fine and up to $1,000,000.00 (1 million dollars) in court. That’s crazy consequences, so not something I’d recommend.


Cleaning a DPF

Ash content in your DPF won’t burn off with a regeneration cycle or passively.

As ash is already burned, it won’t reduce from further burning. Studies have shown that after 50,000km, more than half of the contents in a DPF will be ash. If you consider the amount of passive burn and regeneration the DPF has had by this stage, it shows that it isn’t going anywhere.

Ash accumulation is the primary factor in reducing longevity of DPFs and is one of the big problems that car manufacturers have been working on.

Ash accumulation can only be dealt with via removal and manual cleaning. Depending on the model of DPF that you have and how mechanically inclined you are, it might be something you have to take to a professional.


Low Ash Oil

Low ash oil is often marketed for diesels as having the ability to reduce ash buildup in a DPF.

I haven’t found anyone who can answer, how so much of the engine oil would be making it into exhaust gases, to the point where that would affect the DPF to such a degree. I’m tempted to write it off as just marketing. If anyone does know the answer, please leave a comment, I’d rather be corrected than falsely assume I’m right.

The fact is that these days, basically no normal engine oil could be described as high ash, therefore “low ash”/”low SAPS” (sulphated ash, phosphorous and sulfur) oils are essentially extra low ash oils.

High levels of sulphated ash in engine oils is a problem because it can create buildup on valve and piston surfaces, creating hotspots that can lead to pre-ignition, however this doesn’t occur with diesels. The other problem high ash causes is that it can leave a small buildup between the valve face and the seat, causing deformities and valve stem stretch. But again, we’re not going to find a conventional engine oil that has that much ash.

The bigger problem is that ash actually acts as a mechanical lubricant of sorts. Small amounts of ash between a valve face and seat is a good thing. If you can imagine what leaded fuels used to do to top-end components, that’s what ash is achieving on a smaller scale.

Coming back to our philosophy of prioritising the engine over the DPF, I think low ash oils may be more problematic than they are helpful.

I will concede that there are a lot of publications noting the issues caused by ash accumulation in DPFs and discussing the benefits of low ash oils, but they all seem to be working on the assumption that we’ve all collectively made, which is that this ash comes from the engine oil. I believe this assumption needs to be checked.

Yes, a small amount of oil will make it into the combustion chamber or exhaust gases, that’s a fact. But, so much that the minute amount of ash in it can block a DPF in 50,000km? I just don’t see how they could be consuming that much oil.

Diesel is known to contain sulphated ash and biodiesel can contain up to nine times as much. I’m not a chemical engineer – my wife is but she’s asleep on the lounge as I write this – but it does seem like we don’t have to worry about seeking out low ash oils. There may be plenty of good reasons to, depending on your vehicle and uses, but not for the sake of the DPF.


DPF Failure – Causes and Risk of Engine Damage

Ash accumulation is one of the main offenders for DPF failure.

It’s not always just buildup however. It’s not too uncommon to see the filter mechanically wear out and start allowing some particulate matter through. This is bad for air quality and health, but not a problem for your engine as it restricts the exhaust flow less.

Most of this sort of damage is caused by driving conditions. Frequent, short or stop/start trips, as we have mentioned, cause rapid clogging within the DPF. If it’s really blocked and exhaust gases are being pushed through, this could maybe cause some damage, but not what we’re really looking at here. What happens with this sort of driving is that the additional buildup requires more automatic cycles to regenerate the DPF which wears it faster. It may also mean that the exhaust isn’t often hot enough for an automatic cycle to start, making the cycles more erratic.

There’s a lot of talk about clogged DPFs causing too much back pressure and damaging engines. Sounds plausible, but any vehicle new enough to have a DPF will probably also have a limp mode, that will prevent you from causing any damage.


Get the Most Performance Out of a DPF Legally – DPF-Back, Cat-Back, Turbo-Back Exhaust Systems

If you’re not familiar with the terms, back is just used as a suffix indicating afterwards. I.e. cat-back is everything after the catalytic converter and DPF-back is everything after the DPF.

Not only do the emissions laws require that the DPF stays, they also prevent changing the exhaust upstream from the DPF. This means that for DPF vehicles, a DPF-back exhaust is required if you want to upgrade your exhaust system.

The problem with this, is that no matter what size exhaust and muffler you run, the DPF is still the main point of constriction. While you will see some improvement, a lot of the benefit will just be that you can get a better exhaust note. Which, for some, will still be worth it.

If you’re 4WD is pre-DPF, you still require a catalytic converter though you have a lot more flexibility. Some manufacturers offer turbo-back exhausts that come with a different catalytic converter that still meets emissions standards while allowing for more exhaust gas flow.

If you can find one that’s genuinely compliant, a turbo-back exhaust is the best option for seeing the best increase to performance. This of course depends on having other modifications as well such as a tune, bigger turbo or different injectors. A turbo-back exhaust on a totally stock engine will largely just be an expensive way to gain more sound.

A cat-back exhaust is still a great option and much cheaper than turbo-back, for pre-DPF 4WDs.



Freedom does not come automatically, it is achieved. And it is not gained in a single bound; it must be achieved each day”

– Rollo May, Man’s Search for Himself

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Rough As Guts offers guided tag-along tours through WA’s rugged Outback.

two four wheel drives driving on the holland track

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Challenge yourself just enough, as we take you through the Golden Outback and on the Holland Track.

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