Problem with idle during regeneration

Hello,

I'm Andy, and I'm new here.
The reason, as one might expect, was a problem with the car.

Recently (since just before Christmas), our Caddy 2K (year 2012, 2.0 TDI, 140 hp, 140,000 km) has been experiencing a somewhat annoying problem.

As soon as the engine control unit (ECU) determines that it's time for a DPF regeneration, it has trouble maintaining the idle speed, and the engine simply stalls.
No error message displayed on the device.
It can also happen that the speed fluctuates only as the amplitude increases, before the motor shuts down.
It's annoying when you have to fight your way through traffic, from one traffic light to the next, during the morning commute.
The issue lasts for a few minutes, depending on the driving conditions, after which everything returns to normal.

That's the short version.

Okay, please provide the German text you would like me to translate.

Normally, everything is fine. I haven't noticed any loss of performance either.

The idle speed is perfectly normal and stable, both when the engine is cold (approximately 1000 RPM) and when it is warm (rolling: approximately 900 RPM, stationary: approximately 800 RPM). Even if you briefly accelerate, the engine speed will return to the idle speed without any overshoots or undershoots.


It looks completely different when the motor is running in regeneration mode (or at least, what I believe is regeneration mode).
Even with a warm engine, the idle speed doesn't return to 800 RPM, but remains at 1000 RPM even when the car is stationary. I first noticed this shortly after the great diesel update in 2018 (at that time without any other accompanying symptoms), and when I asked the workshop about it, they said that it's related to the DPF regeneration process. So far, so good.


So, if the engine is running in this state, and you briefly press the accelerator, it will then fail to maintain the idle speed, and the RPM will drop until the engine simply stalls.
(Without stuttering, just like turning a key.)
The same thing happens when you release the engine brake and press the clutch.
And it's quite likely to happen when the speed was previously above 1500 RPM. Typically, the speed then picks up.

There is another effect that primarily occurs when the engine is cold.

You're using engine braking – for example, approaching a traffic light – you disengage the clutch, the engine speed drops to idle speed, and then suddenly, the control unit reacts, briefly raising the speed to over 1100 RPM. The speed then drops again – this time to a slightly lower level than before – causing the control unit to briefly increase the speed to 1200 RPM. This cycle repeats, with the speed fluctuations increasing 3-4 times, until the speed reaches a low point below a certain threshold, and the engine stalls.

When going uphill or downhill, it doesn't help to lightly press the accelerator. It seems to amplify the effect. I need to bring the engine speed up to over 1600 RPM and then slowly release the accelerator. Then it works again. And then the idle speed remains stable (unless you intentionally press the accelerator).

It seems like there's some kind of feedback loop that's too slow. However, this is only the case if a higher RPM is desired, presumably due to the higher combustion temperature.

The sensor readings (speed, airflow, temperature, etc.) shouldn't be affected by that.

But besides the amount of fuel injected, is there another way to control the engine speed?

The engine doesn't have an (electric) supercharger, and the turbocharger isn't active in that RPM range yet, so the variable turbine geometry (VTG) can't have any influence either.

As far as I know, the Caddymotor has an electrically adjustable central intake manifold valve.

Does it have an effect on regeneration?

Or are these the first signs of a problem with the EGR or DPF?

I haven't noticed any increased consumption (at least not significantly).

For a distance of 25 km, with 50% city driving and 50% highway/motorway driving, the fuel consumption of 5.4 liters per 100 km in the summer and 5.9 liters per 100 km in the winter seems reasonable for the vehicle.
I currently experience this issue approximately every 300 kilometers.

Thank you for reading this far .

So, what's (almost) broken?

Should the throttle valve and/or EGR valve be replaced as electromechanical components based on suspicion? (Suggestion from the workshop)


Regards,
Andy.

Hello,

Without VCDS, you'll hardly get very far here.

Best regards, Rainer.

Hello,

I'm currently reading data using an ELM327 car scanner.

I don't have any "Oil Ash Residue," but only...
"[ECM] DPF: oil ash volume" - the latter value is 0.08 liters. It seems to be due to the 2011 TDI model.
I read that the DPF is typically considered "full" at 185 ml.
Does this fit a CCFH 2.0 TDI?
Where can I read more about this?

What about the "differential pressure" at the DPF?
Is there a good/bad distinction to be made there?
I'm reading 6 hPa and 65 hPa at idle and 2500 RPM, respectively, with a current soot loading of 10 g.

That's a shame.

Diagnostic assistance for non-customers:

https://shop.dieselschrauber.org/en/obd-support-key-p-112.php