Disclaimer:
This post is based on my personal experience and intended for informational and educational purposes only. Procedures, tools, and parts may vary depending on the vehicle and situation. Always consult your service manual or a professional before performing any maintenance.For more information, please read the full disclaimer here.
DPF Regeneration Lessons
Diesel engines equipped with a Diesel Particulate Filter (DPF) require periodic regeneration to burn off accumulated soot. When regenerations are interrupted, the filter cannot reach optimal conditions, leaving behind residual soot mass and potentially leading to performance issues.
In this article, I share detailed data collected via BimmerLink from my BMW 420d F36, covering the values before, during, and after a successful regeneration. I also explain why previous attempts were unsuccessful and what finally allowed the regeneration to complete properly.
What I’ve Learned So Far About DPF Regenerations
When I looked into my own car’s data and monitored the DPF regeneration, I found some important lessons:
-
⛽ Fuel Tank Before Regeneration:
Before heading to the highway for today’s regeneration, I made sure the fuel tank was 3/4+, so there was no risk of interruption due to low fuel. -
🧮 Soot Mass Reduction:
A completed regeneration reduced soot mass from 37.67 g to 12.27 g. Values between 10–15 g are normal for BMW engines and do not indicate a problem. -
⚠️ Differential Pressure:
The most important indicator of a healthy DPF is low differential pressure; mine dropped to 0.002 bar at idle after regeneration, confirming the filter is clear. -
🚗 Driving Conditions:
Driving with a stable load (three passengers and luggage) at a constant speed of 135 km/h and fixed RPM (~3,000 RPM) helps maintain high exhaust temperatures and improves regeneration efficiency. Increasing or changing RPMs causes the temperature to drop, slowing the process. Therefore, it’s best not to frequently change RPMs during regeneration. -
🔄 Interrupted Regenerations:
If a previous regeneration was interrupted, the next cycle may not clean the filter completely, leaving residual soot mass. Patience and proper highway driving are required. -
⏱️ Next Regeneration Advice:
A steady 15–20 minute highway drive during regeneration may lower soot mass below 10 g, but values slightly above 10 g are normal if pressures remain low.
Analysis Before Regeneration
After approximately 486 km since the last regeneration, the vehicle reached the following values:
| Parameter | Value |
|---|---|
| Soot Mass in DPF | 37.67 g |
| Oil Ash Mass in DPF | 25.97 g |
| Last Regeneration | 486 km ago |
| Fuel Consumed Since Last Regen | 22.17 l |
| Engine Runtime | 07h:06m:06s |
At this point, the soot mass had crossed the 30 g threshold, typically a trigger for regeneration. However, due to a previously interrupted cycle in the garage (details here), the car delayed the next regeneration attempt.
During Regeneration
The regeneration finally started after about 40 minutes of highway driving. To keep the engine at higher RPMs, I used Sport mode and shifted manually.
| Parameter | Value |
|---|---|
| Exhaust Gas Temperature (DPF) | 646 °C |
| Coolant Temperature | 84 °C |
| Engine Speed | 3000 RPM |
| Vehicle Speed | 135 km/h |
| Differential Pressure Across DPF | 0.075 bar (0.075bar×1000=75hPa) |
The regeneration process lasted between 5–10 minutes, which is within the expected range for BMW diesel engines under optimal conditions.
Other Conditions During Regeneration
Before heading out to the highway, the car had a full fuel tank, ensuring there was no risk of the regeneration being interrupted due to low fuel levels. The outside temperature was around 20°C, providing ideal conditions for the engine and exhaust system to reach and maintain the required operating temperatures. In addition, there were three passengers in the car along with luggage in the trunk, meaning the vehicle was under a slightly heavier load than usual compared to when driving alone. This extra load may have contributed positively to maintaining higher exhaust temperatures during regeneration.
Why Soot Mass is not Lower Than 10g?
Even though the soot mass did not drop below 10 g, the reduction from 37.67 g to 12.27 g shows that the regeneration cycle was effective. In practice, the DPF is never completely empty—BMW’s ECU typically leaves a residual value (5–15 g) to reflect realistic conditions. Since my previous regeneration attempt was interrupted, the system likely did not reset to its lowest soot levels.
The most important confirmation of a successful regeneration is the differential pressure, which fell to 0.002 bar at idle. This indicates that the filter is free-flowing and that the remaining soot mass is within acceptable limits.
Post-Regeneration State
After the regeneration completed, I let the engine idle before shutting it off. The readings were as follows:
| Parameter | Value |
|---|---|
| Differential Pressure Across DPF | 0.002 bar (0.002bar×1000=2hPa) |
| Soot Mass in DPF | 12.27 g |
| Oil Ash Mass in DPF | 25.97 g |
The differential pressure dropped significantly, confirming that the filter was cleared. However, the soot mass did not reach near-zero values, indicating that not all soot was burned off. This is most likely due to the previously interrupted regeneration attempt, which prevented the filter from resetting fully.
Conclusion
This case demonstrates the importance of allowing the BMW DPF regeneration process to complete without interruptions. When I previously tried to let the car regenerate at idle in the garage, the cycle failed, leaving behind soot mass. Later, even with soot levels above 30 g, the car initially refused to regenerate (more details here).
Ultimately, a long highway drive at steady RPMs and high temperatures allowed the system to complete a proper regeneration, reducing soot mass from 37.67 g to 12.27 g and restoring low differential pressure.
The key takeaway:
- Always ensure regenerations are completed in full.
- Avoid frequent short trips where the DPF cannot reach the required conditions.
- If regeneration is interrupted, the vehicle may delay or resist starting the next cycle, requiring extended highway driving to resolve.
