VW Sharan 2.0 TDI - Yet Another DPF Problem

aquaplex

New member
Joined
Sep 2, 2024
Location
Switzerland
TDI
VW Sharan 2.0 TDI
Hello, I'm new here and hope this collective mind can help me solve my puzzle.

My story:
I bought a used VW Sharan (2013) 2.0 TDI 177 kW a year ago. It was running fine, but then one day, the amber engine light appeared. I checked the internet for possible reasons and found something about AdBlue. At that point, I didn't even know my car had this system because the AdBlue filler cap was hidden in the trunk. I topped up the AdBlue, and the error disappeared. Maybe it was a coincidence.

A couple of months later, the warning light appeared again. Once more, I topped up the AdBlue and decided to investigate further. I bought an OBD dongle and cleared the errors. Unfortunately, I don't remember the specific error codes, but I do recall that after clearing them, one error remained: P203B. So, I bought a new DEF tank and replaced it. After that, the P203B error disappeared, and for the first time, I saw the red warning on the dashboard telling me to top up the AdBlue tank. I filled the tank, and all the errors were gone.

Some time later, I got a warning that I needed to top up the AdBlue again, or the engine would not start after 2400 km. At that moment, I was on a long trip to another country (about 1800 km away), so I figured it would be fine to top up the AdBlue when I arrived at my destination. However, the AdBlue range was decreasing faster than the distance I was traveling. When the range showed around 500 km remaining, I still had 800 km to go. So, I topped up the AdBlue again, but the counter didn't reset. I tried clearing the error with my OBD dongle, but it didn't help.

At that point, I probably made a bad decision. Since it was late on a Sunday evening and I couldn't find any service stations on the way, I drove the last 400 km with the red AdBlue warning light on, without stopping. In the last 100 km, the DPF warning light appeared. In the final 50 km, the amber engine light came on, and the car went into limp mode. The codes were something about insufficient flow and turbocharger underboost, but I don't remember the exact details. When I finally reached my destination, resetting the errors didn't help, and I couldn't get out of limp mode. The only thing that happened after resetting was that I got an additional 50 km added to the AdBlue range.

At that point, I decided to get VCDS to better understand the problem. I performed the following actions:
  • Reset SCR tank level detection
  • Reset SCR lock
I noticed that the soot levels, both calculated and measured, were quite high, preventing regeneration from triggering while driving. I believe the car was in limp mode because of that. After resetting some other parameters (probably differential pressure), I managed to exit limp mode, and after some driving, regeneration occurred. All the errors disappeared.

Initially, it seemed like the issue was resolved, but after a while, I noticed that every time I came home and turned off the car, the fan continued running for 5 more minutes. That seemed odd, so I started monitoring some metrics while driving. The problem was that whenever the car met the conditions for regeneration, it would start regenerating but wouldn't finish. I observed that the calculated soot level was fine, but the measured soot level was accumulating too quickly. While driving, regeneration wasn't reducing the soot level until I stopped and idled.

After checking forums, I decided to monitor the differential pressure values. My sensor has only one pipe attached before the DPF, with the other pipe measuring ambient pressure. An important note: I did this test on a cold engine while standing still.
  • With the engine off, the pressure was around 4 hPa.
  • On idle, it was around 20 hPa.
  • At 2000 RPM, it was around 80 hPa.
These values didn't seem too critical, but I decided to replace the differential pressure sensor with a new one. After changing the sensor and performing the adaptation, the results were similar, so the sensor seemed to be fine. I was puzzled about what could be causing the rapid accumulation of measured soot levels.

Then, I decided to monitor what was happening while driving. Here's where things got strange: when I started driving, the pressure was quite low. For example, at 1800 RPM, the pressure was around 70 hPa. However, after 5 minutes, at the same 1800 RPM, the pressure increased to more than 300 hPa. At 3000 RPM, the pressure could reach 900 hPa, which sounded alarming.

So here I am now, trying to figure this out.

My thoughts:
  • Regeneration isn't completing (even after driving 50 km on the motorway at 120 km/h) because the measured soot accumulates faster than regeneration can reduce it.
  • The measured soot accumulates quickly due to high differential pressure values while driving (on the motorway, it averages between 200-400 hPa).
  • The high differential pressure could be due to something being clogged.
I'm considering that the pipe leading to the DPF might be clogged, although I'm unsure if that would result in such high pressure values. The pressure seems to react quite quickly to changes in RPM. Alternatively, something after the sensor could be clogged. I have the DPF itself, the SCR tank (which contains the AdBlue injector), and another unit close to the SCR. Or is this part of the SCR system? These two units are close together and have almost the same part number (except for the last letter). One or more of these units might be clogged.

I would like to avoid spending too much money and solve the issue with minimal investment.

So my questions are:
  1. Does this immediately suggest that the DPF or SCR is clogged?
  2. Since I had issues with AdBlue, could the problem be with the SCR system?
  3. How can I determine whether it's the DPF or the SCR that's clogged? Could I remove the AdBlue injector (which is attached before the SCR) and monitor the pressure? If gases flow through the injector hole and the pressure returns to normal, would that indicate the SCR is clogged?
  4. When I had a bad DEF sensor, does that mean AdBlue wasn't being injected at all?
  5. Why is the pressure relatively low on a cold car but significantly higher after 10 minutes of driving?
  6. Could the pressure changes after driving be related to something else, like the EGR?
  7. Could the insufficient flow and turbocharger underboost be connected to backpressure, and if I solve the high-pressure issue, will it resolve automatically?
  8. Could I be looking in the wrong place? Is the DPF clogging due to other issues, such as oil in the fuel, a faulty EGR valve, a clogged manifold, vacuum leaks, glow plugs, injectors, NOx sensors, or something else? Do I need to solve the core issue first so that the DPF can regenerate normally?
  9. What metrics should I monitor to get more information? I can record them while driving.
  10. In this situation, could DPF cleaners help? I'm referring to those sprays, foams, and liquids that are injected into the DPF after removing the NOx sensor. Do you recommend them?
  11. In general, what do you recommend as the next steps for me to check or do?

Some chart to better understand the metrics:

1. RPM and soot level.


2. RPM and differential pressure.


3. EGR mass flow and differential pressure.
 
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