29/06/2008
If you're the proud owner of a BMW diesel, particularly one from around 2006 onwards, you're likely to be familiar with the term DPF. But what exactly is a Diesel Particulate Filter, and what happens when it decides to play up? This guide will delve into the intricacies of DPF systems in BMW diesels, covering their function, common problems, and the most effective solutions, helping you keep your vehicle running smoothly and legally.
What is a Diesel Particulate Filter (DPF)?
BMW began introducing Diesel Particulate Filters (DPFs) to their diesel vehicle range around 2004, gradually phasing them into their entire diesel line-up. Essentially, a DPF is an exhaust after-treatment device designed to reduce harmful particulate emissions, commonly known as soot, from diesel engines. It's a crucial component for environmental compliance and vehicle health. The filter works by trapping these carbon particles. As these particles accumulate, the DPF begins to fill up, eventually reaching a point where the vehicle's engine control unit (ECU), known as the DDE (Digital Diesel Electronics) in BMWs, will initiate a process called regeneration.
How Does DPF Regeneration Work?
DPF regeneration is an automated process where the trapped soot is burned off, clearing the filter and allowing it to continue its function. For a successful regeneration to occur, several key criteria must be met:
- Sufficient Fuel: The fuel tank should ideally have at least 10 litres of fuel, preferably above a quarter of a tank.
- Driving Conditions: The vehicle needs to undertake a sustained journey of approximately 20-30 minutes at speeds exceeding 40mph. This allows the exhaust gases to reach a sufficiently high temperature.
- Engine Temperature: The engine must be at its normal operating temperature.
During regeneration, the DDE alters the fuel injector timings. This adjustment causes a slight increase in exhaust gas temperature, which is sufficient to ignite and burn off the accumulated carbon deposits. It’s a clever system designed to keep the filter clean and functioning optimally with minimal driver intervention.
When Regeneration Fails: The Common Problems
The issues arise when the DPF fails to complete its regeneration cycle. This can happen for various reasons, leading to a blocked exhaust system and triggering fault codes within the DDE. Once this occurs, it becomes highly unlikely that the DPF will regenerate naturally. So, what causes a DPF to block?
Reasons for DPF Blockage:
There are several common culprits behind a blocked DPF:
- Short Journeys and Stop-Start Traffic: This is perhaps the most frequent cause. If a vehicle is primarily used for short trips or in urban environments with frequent stop-start driving, it rarely reaches the sustained speeds and temperatures required for a successful regeneration. The DPF never gets the chance to 'clean' itself.
- Low Fuel Levels: Repeatedly driving with very little fuel can sometimes interfere with the regeneration process.
- Underlying Engine Faults: Any existing engine issues, or faults stored within the DDE, can prevent the regeneration process from initiating or completing. It's vital to address these underlying problems first.
When a DPF becomes significantly blocked, you'll typically notice a warning light illuminated on the dashboard or the vehicle's information display. The car might also feel sluggish and noticeably down on power, indicating that the exhaust flow is restricted.
Blocked DPF: Your Options Explained
Dealing with a blocked DPF can be a confusing and potentially expensive affair if not handled correctly. Unfortunately, some garages may not fully understand the DPF system's operation or the reasons for its blockage. In such cases, they might resort to one of the following, often ill-advised, 'solutions':
Common (and Incorrect) DPF 'Fixes':
| Incorrect Method | Consequences |
|---|---|
| DPF Removal | This is an illegal practice. As of January 2014, the removal of a DPF is an instant MOT failure. It also results in significantly higher emissions and can lead to further vehicle issues. |
| DPF Removal and Cleaning (Jet Wash/Chemicals) | While seemingly a cheaper option than replacement, this often incurs substantial labour costs for removal and refitting. Crucially, it's usually only a temporary fix, especially if the underlying cause of the blockage isn't addressed. The effectiveness of these cleaning methods can also be questionable. |
| DPF Replacement | This is the most expensive option, with DPF units often costing between £1000 and £2000. Without rectifying any underlying engine or DDE faults, a new DPF can quickly become blocked again, making this a costly and potentially futile exercise. |
The Correct Way to Diagnose and Rectify a Blocked DPF
At our workshop, we adopt a thorough and professional approach to DPF issues, ensuring a cost-effective and lasting solution. Here’s our tried-and-tested method:
- Customer Consultation: We begin by discussing the driving habits and circumstances that may have led to the DPF blockage. Understanding how the vehicle is used is key to identifying potential contributing factors like short journeys.
- Full Diagnostic Check: We perform a comprehensive diagnostic scan of the vehicle. This allows us to:
- Read the exhaust back pressure. High back pressure is a clear indicator that the DPF is blocked.
- Identify any underlying fault codes stored in the DDE that may have prevented regeneration or contributed to the blockage.
- Address Underlying Faults: We meticulously work through and resolve any identified engine or DDE faults. This step is absolutely critical. If these issues aren't fixed, any subsequent regeneration attempts will likely fail, and the DPF could block again.
- Forced Regeneration: Once we are confident that the underlying issues are resolved, we use advanced diagnostic software (such as Autologic) to initiate a 'forced regeneration'. This process commands the DDE to undertake a regeneration cycle, even if the usual criteria haven't been met.
- Verification: After the forced regeneration is complete, we re-check the exhaust back pressure readings. A successful regeneration will show a significant reduction in back pressure, bringing it well within the manufacturer's specified limits. For example, on a BMW 120d, we've seen back pressure dramatically reduced post-regeneration, moving from critical levels to well within acceptable ranges. We then clear all stored fault codes.
The Benefits of Our Approach:
- Cost Savings: We avoid the need for expensive DPF replacement, saving you potentially thousands of pounds.
- Legality: We do not remove the DPF, ensuring your vehicle remains legal and will pass its MOT test.
- Efficiency: We don't waste time and money on unnecessary DPF removal or cleaning procedures.
Example: Before and After Back Pressure Readings on a BMW 120d
[Note: In a real article, this would ideally be accompanied by an image or a table showing the specific back pressure readings before and after the regeneration process. The text here describes the outcome.]
As demonstrated by our tests, the back pressure was drastically reduced, falling well below the maximum permissible values after our intervention.
Important DPF Maintenance and MOT Updates
MOT Changes (May 2018 Onwards): It's crucial to be aware of recent changes to the MOT test. Since May 2018, any vehicle found to have had its DPF removed will automatically fail its MOT. The inspection is no longer just visual; emissions test limits have been significantly tightened. If your vehicle has had its DPF removed and you're concerned about its legality or future MOT tests, please contact us for solutions to get your car compliant.
Interim Oil Changes Recommended:
We strongly recommend an interim oil change if your vehicle hasn't had one in the past 12 months, especially after a DPF regeneration. During multiple regeneration attempts, small amounts of unburnt fuel can sometimes enter the engine oil, diluting it and potentially affecting its lubricating properties. Fresh oil ensures optimal engine protection.
Conclusion
Understanding your BMW diesel's DPF system is essential for maintaining its performance, longevity, and legality. While DPF issues can seem daunting, a correct diagnosis and appropriate action can save you considerable expense and hassle. By avoiding illegal modifications and opting for professional, thorough servicing, you can ensure your BMW diesel continues to provide reliable and clean service for years to come.
If you suspect your BMW diesel has a blocked DPF or are experiencing any related symptoms, don't hesitate to contact us to book your vehicle in for a comprehensive diagnosis and expert service.
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