Fluctuating idle and jerking - Passat 3B 1.9 TDI ATJ

Hello,

For a long time, I haven't had any problems with my car (mileage at least 250,000 km), but recently, I've been experiencing the following phenomenon:

Especially after a cold start, the idle speed begins to fluctuate after a few seconds. The engine speed drops, then there's a surge of gas, the speed increases to about 1000 RPM, then it drops again to about 800 RPM, and this pattern continues regularly. See also the log file below.


There are black smoke clouds coming from the exhaust. Additionally, the engine jerks when accelerating at low RPMs, around 1200, both when it's warm and cold. At higher speeds, nothing unusual is noticeable.
"For the past several thousand kilometers, I've had an HJS aftermarket DPF installed. From the log data, it seems the exhaust sensors are fine, so I suspect the problem might be related to the air intake..." Can anyone give me a tip on which tools would be helpful, or perhaps someone knows what might be causing the fluctuations in the speed?

Thank you in advance, and have a wonderful Easter!

Regards,

Jan.

I once had the exact same problem, after I mistakenly installed an aftermarket mass airflow sensor. It was an automatic vehicle, and it suddenly started moving while at a standstill when a slight pressure was applied to the brake.

Therefore, the first one. Is the mass airflow sensor (MAF sensor) an original Bosch part?
If possible, replace the component you suspect is faulty with a known working one before you start troubleshooting to find the root cause of the problem. I also had unremarkable results back then.

Are the hydraulic components and camshafts okay? Are the Pleul bearings at the top in good condition?
You know this from the AJM; it's probably a bit more susceptible to it.

Hello,

It's so trivial that it's almost embarrassing. Anyway, thanks for the tip about the LMM (lambda sensor). It was new and an original Bosch part, but I still searched the entire area again. Besides various signs of weasel activity, I also discovered a loose vacuum hose connected to the air filter housing.
Quickly renewed and reconnected - problems solved.

It's incredible how much of an impact something like that can have. Thank you for the comments, in any case.

Regards,

Jan.

The hose on the air filter housing is not a vacuum hose.
This is 'just' the atmospheric venting. Whether it's true or not, it probably won't make much of a difference.

What did I not understand there?

Thank you for the feedback .

Were the cables from the mass airflow sensor also affected?

Hmm,

On the occasion, I installed a used but functional mass airflow sensor (MAF) from the parts inventory. Then the mass airflow sensor must have been faulty after all?!

Regards,

Jan.

Hello,

Other possible causes for an increase in idle speed include:
- Low on-board voltage (depending on the vehicle...)
- Implausible/too low values from the coolant temperature sensor for the engine control unit.

Best regards, Rainer.

janmac wrote:

Hmm, On the occasion, I installed a used but functional mass airflow sensor (MAF) from the parts inventory. Then the mass airflow sensor must have been faulty after all?! Regards, Jan

It was probably something like that, or maybe the plug wasn't making a good connection, or there was a cable problem. The hose has absolutely nothing to do with it.

Thank you for clarifying the venting issue.

I had actually ruled out the mass airflow sensor (MAF sensor) because it was a new part from a VW dealer. The connector was also properly connected... No matter what, it runs smoothly again with the old LMM!

Thank you and best regards!

Jan.