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How to know if DPF (Diesel Particulate Filter) is blocked

How to know if DPF (Diesel Particulate Filter) is blocked

03/11/2022

If you drive a diesel vehicle, there is a good chance it is fitted with a DPF, especially if it was made after 2009. The diesel particulate filter reduces harmful emissions of soot by collecting them inside. 

When the filter reaches a certain temperature, the soot burns off into ash, clearing the DPF’s inner filters and allowing more soot to be contained, continuing the cycle. 

The only problem is that the car needs to be driven for a certain amount of time to reach that temperature, if it’s not, and is only driven short distances, the soot doesn’t get burnt off, and remains inside the filter. In this post, you will learn how to check for any DPF issues to ensure it doesn’t cause any more serious car problems

Symptoms of a blocked DPF

Because the exhaust system is integral to an internal combustion engine, blocking part of it off is not recommended. There will be telltale signs of a blocked DPF, these will get worse the more blocked the filter is.  

The main signs of blocked DPF are: 

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Can a blocked DPF cause power loss?

Because the engine needs to get exhaust gases out as quickly as possible to allow air and fuel in, blocking the exhaust can restrict the amount of fuel and air that is burned.  

Further to this, almost all modern diesel engines have a turbocharger, which takes otherwise wasted energy from the flow of exhaust gases, and uses it to force higher volumes of air into the engine, improving efficiency. 

If the DPF is blocking the flow of air through the exhaust, the turbo will not be spinning fast enough to create sufficient pressure, further reducing the engine's power. 

Low engine RPM

Alongside the engine not producing much power, the engine may not be able to rev very high, sometimes reaching a top RPM of 3,000. This is a consequence of both the exhaust gases restricting the engine's usual cycle, and because of the lack of turbo pressure. 

As engines make the most power at a specific RPM range, all three of these issues compound to make the car feel extremely sluggish on acceleration. 

Doesn’t DPF regeneration solve blocking issues?

As we explained in our DPF article, the car’s ECU will detect when a DPF regen is required, and start one when the conditions are met.  

Driving the car incorrectly whilst a regen is in progress can cause the DPF even more problems, here's how to tell if a regen is in progress and what to do: 

How to tell your car is regenerating its DPF

  • High Idle speed - the engine will remain at 1500-2000 RPM whilst stationary 
  • Excess heat from under the car  
  • Very poor fuel economy - instant economy readout will likely halve 
  • Warning message on the dashboard

If any one or more of these symptoms are present, you should consider how you continue your journey. 

What to do if your DPF is being regenerated

It’s wise to read your owner's manual regarding this before it is necessary but the general advice is as follows: 

  • Continue to drive on roads that permit at least 40mph 
  • Drive for at least 30 minutes or until the DPF light has gone away 
  • Do not turn off the car or allow it to idle for a long time 
  • Be cautious of your fuel level, Regens use more fuel than normal running 

Some manufacturers are different, so it’s best to check the specific signs and dashboard lights that your car will display, especially if you feel your DPF is becoming clogged.  

If your DPF Regen fails

If you are unable to continue driving or get stuck in traffic, the regen may be cancelled by the ECU. This will mean all of the soot buildups will still mostly remain, and the DPF will need to be regenerated soon. 

Try to schedule a drive at high enough speeds to clear the DPF, continual driving at slow speeds will make the problem worse and may require manual DPF cleaning, that is removing and chemically dissolving the soot buildup. 

What if your DPF cannot be regenerated?

If the regen process has failed multiple times, especially at the ideal conditions, your DPF may need to be cleaned by hand or replaced altogether. Usually, this involves removing the DPF and soaking the built-up soot with a solvent. 

Diesel Particulate Filter Replacement Cost

There are some garages that specialise in replacement DPFs, but generally, most normal garages will be able to remove and replace a DPF, they can cost anywhere between £800 to £2,000.  

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