DPF “knock firing” sounds in BMW – how to fix it?

BMW 1 E87 driver’s question regarding DPF and “knock burning” sound

I have a problem that is recurring. During the DPF firing procedure, there is a loud engine noise, it is a “knock combustion” sound, so stated the mechanic to whom I presented it. The sound intensifies in the rev range of about 2000-2500.This occurs during load, when adding gas. On the other hand, it does not occur at a standstill. Engine temperature or ambient temperature does not matter.

How do I know that while the DPF is burning out? I couldn’t tell at what times this sound was occurring because it was at random times. I purchased the bimmer-tool app. With this I was able to arrive at the fact that the knocking combustion sound appears during the active firing procedure.

Activities performed so far

To begin with, different fuel from BP, Orlen, Shell, it did not change anything. Fuel additive to clean the injectors AR6900 and Millers (that’s before I determined that it was during the procedure). I manually burned out the DPF from the application. On average, the procedure runs every 200-250 kilometers. Driving mostly in the city. Although recently in September the car was in Croatia. DPF burned itself out a few times route about 3000 kilometers, but continues to be the same.

The EGR was taken apart and cleaned, but there was no carbon build-up. There are no errors in the computer. Backpressure on the DPF after firing is values of about 6-10 hPa, before firing I do not remember exactly, probably about 35. Ash 39 grams. The mechanic suggests cutting out the DPF. Seemingly the problem will be solved, but I still do not know the cause. I can also replace the DPF with a new one or “wash” it, but I don’t know if it will change anything. When the procedure kicks in, won’t it backfire. I also have some screen shots of the injection correction if needed from a warm and cold engine, but I have no way to attach them here.

Example values:

• temp 88°C injection 1 0.65, injection 2 -0.52, injection 3 0.09, injection 4 -0.22,
• temp 22°C injection 1 0.89, injection 2 -0.18, injection 3 -0.81, injection 4 0.08

Based on this description, are you able to deduce something, advise something, indicate the cause or solution to the problem?

The problem affected a BMW 118 E87, but it may also occur in other models.

Response from BM Cars service regarding DPF and knocking combustion sound

The described problem of noisy engine operation during the firing of the diesel particulate filter is related to the afterburning procedure of this filter itself. Cars equipped with DPFs must periodically perform its so-called burnout, which is the removal of accumulated soot and particles on the filter meshes.

Without this procedure, clogging of the DPF could occur within a few hundred kilometers of mileage. The afterburning procedure itself is unsignaled to the user in BMW cars. It involves extending the injection phase of the fuel and increasing its dosage, so that some of the unburned fuel is expelled with the exhaust gases into the exhaust system. When the DPF is warmed up, such small amounts of fuel cause a significant increase in the temperature of the filter (up to 600°C) and burn off the accumulated particulates.

The procedure described above may cause slightly increased engine operation sounds and may be experienced with a different than normal exhaust odor. Since this procedure requires the achievement of appropriate assumed engine operating parameters, i.e. a temperature above 70°C, a vehicle speed above 80 km/h and a sufficiently warmed-up exhaust system, to start, it is usually not perceptible to the vehicle user. If the startup of this procedure does not interfere with the vehicle’s ability to operate normally or cause any diagnostic errors, we suggest you continue to observe the situation.

We suggest performing engine diagnostics at each service visit and observing any engine-related sounds that appear. The appearance of louder knocks or other abnormal engine noises should be immediately consulted with a service center experienced in servicing BMW vehicles. We strongly advise against cutting out the DPF, as this may result in the immobilization of the vehicle for periodic technical inspections.

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