Diesel Engine Stuttering: Causes and Fixes

Hi.

I recently started a thread about the air filter, and in that context, I mentioned that I installed a Pierburg mass airflow sensor. Since then, my Bora has been sporadically misfiring, especially when it's cold (because it's parked in a garage), which never happened before.
It might be related to the cold weather, as it didn't occur on the first two days.
Dieses Verschlucken läßt sich schlecht beschreiben - als würde er dreimal hintereinander kurz in den Notlauf gehen wollen. Ein Stottern beim Beschleunigen eben. Schlecht reproduzierbar, vornehmlich bei untertouriger Fahrweise (80km/h, 5. gang), aber auch bei gehaltenem Gas (4. Gang, 3000 RPM)

Since the last LMM (mass airflow meter) change, I'll put my old Bosch one back in and drive. There was an approval certificate (ABE) for the Pierburg, and the AJM model was listed. By the way, the grilles are already installed.

What could be the reason for this? Especially since it's not a DC LMM, but explicitly designed for VW?

Fluctuations in the mass airflow sensor!

I once had this issue with an ASV (presumably a type of vehicle). Every time I pressed the accelerator, it would suddenly jerk violently before accelerating again.
A test run showed mass flow rate (MFR) values that suddenly dropped to 0 for a single measurement, before returning to normal.

Plug in, plug out, everything's okay. So, the plug wasn't properly connected.

Otherwise, it could be any connector involved in the injection process, including the control unit.

m;

PS 80 km/h in 5th gear is NOT under-revving, at most it's low-revving .

Hello,

I might be familiar with that, but only with a functioning AGR (Exhaust Gas Recirculation system).

The car doesn't really cough or sputter, but a violent shaking runs through the entire vehicle (I once compared it to chicken-sized birds being squeezed through the exhaust system at regular intervals).

So, if it's only happening at partial load or when the engine is idling, it could also be related to the weather in conjunction with the AGR (Exhaust Gas Recirculation) system.

Try disconnecting the pressure hose from the membrane pump before a potential cold start.

and just drive for a few kilometers.

Thank you for the responses. Unfortunately, it wasn't the plug. The box also doesn't turn on at such low outside temperatures. Okay, let's keep it simple...

Yesterday, I cleaned the Bosch mass airflow sensor with a parts cleaner in a very cold garage and reinstalled it. The problem with the engine still persisted. Okay, so the Pierburg sensor can go back in.

I then consulted several VW workshops (with brothers, you never quite know...), and they more or less came to the following conclusion:

It's quite possible that some water in the diesel filter has frozen due to the mild temperatures of -10°C in our region, and occasionally blocks the outlet, which naturally restricts the diesel supply. I was advised to wait for a change in the weather towards positive temperatures to see if the problem persists.

Yesterday, I had exactly one instance where the system stopped working on the way to the service, and then it didn't happen again. It's possible that the ice has now melted. Interestingly, the D-filter was replaced two weeks ago. Are there any additives that can bind water?

"Well, that water shouldn't freeze that quickly. I regularly drain mine, and I even installed a new one before winter. My car has never acted up like this before. Also, in the past few days, it's been noticeably colder than -10 degrees, and the car is parked in a carport."
A relatively simple solution would be to open the filter at the white drain plug (after attaching a hose to the nipple that leads into a diesel-safe collection container) and drain the filter. However, I'm not entirely familiar with the internal workings, but it should be designed to last for several thousand kilometers and be able to store any water that accumulates. In my case, very little comes out after 10,000 kilometers.

I don't believe that about the ice cream.

At least, my various VP diesel engines reacted to clogged filters with a significant loss of power, never with a sudden stall.

You can easily disable the LMM (Lambda Measuring Module) by driving with the connector unplugged. In this case, the electronics will assume a "default value" that is small but fixed. This means you might only have 80-90 horsepower, but you'll always have a consistent value, so there will definitely be no fluctuations.

If it's still not working after that, then it's not the mass airflow sensor.

m;

"Could it be that some lines were opened during the diesel filter replacement and are now not completely sealed?"

Regarding 'water binding,' there's a very simple method: add some rubbing alcohol/isopropyl alcohol to the tank – I used about a quarter of a liter per tankful. Unlike diesel, the alcohol mixes with the water and the diesel. Everything then goes through the engine, and the water is gone. I tried this experimentally once, and I didn't notice any negative or positive effects; it probably wasn't any water in there to begin with. This trick is 'very old' and was mainly interesting for people/vehicles with metal tanks to prevent long-term corrosion. In our diesels, this can be effective if water gets into the ESP (Electronic Stability Program) and disrupts the regulation – because it doesn't mix easily, it doesn't simply disappear.
'A tip from someone who knows: it costs nothing and achieves nothing' - I do it once a year, but you have to believe in it.

Okay, please provide the German text you would like me to translate into English. I will only provide the translation.

The DiFi has a special drain screw at the bottom.
You can drain the water, which is heavier than diesel, from there.

And precisely because it is HARMFUL to the ESP, it should not extend any further than the filter! That was at least what the developers of the fuel injection pumps had in mind .

m;

Yes, then I will go let some water out .

Here's something else that's not directly related, but I noticed because I'm paying close attention to everything right now. Is it normal that when the car comes out of the cold (8 hours, -6.5°C), the turbocharger takes 3-4 minutes to 'lounge around' and not build any boost? My LDA (likely referring to a pressure sensor or gauge) didn't show 0.5 bar of boost until 3 minutes into the drive at 80 km/h in 4th gear. Initially, nothing seemed to happen at all, it remained at a constant 0.

Answer to your question: No. Definitely not. What did you do before parking? Did you accelerate to the maximum speed and then turn the key?
Something like that, although I can't imagine it lasting that long. It could happen if there's excessive carbon buildup.
Otherwise, check your LDA (likely referring to a lambda sensor or similar) and look in your rearview mirror. It should be producing a lot of black smoke when you give it "a little" more gas.

Hmm. The LDA is a mechanical device. Maybe it's just too cold.

He shook a bit more than usual when starting, but it's normal for diesel engines to be a little harder to start in the winter. Before and after the short stretch of highway I drive, I have 12 km of country road and another 6 km of city driving. So, there's plenty of time for the engine to cool down.

Okay, then you don't have much choice. Connect your VAG Com and start logging data... However, I think it looks bad if you have zero boost pressure for so long. I always try to keep the RPM low before reaching operating temperature and keep the boost pressure below 0.5 bar. I always have a slight fluctuation in the LDA (Lambda sensor), though.

After sleeping on it, it seems likely to me that the LDA is malfunctioning.
The turbo's whistling could be heard. Furthermore, without boost pressure, the car should have experienced a significant loss in driving dynamics. However, it didn't.

What kind of LDA do you have? Is it one of those really cheap ones? It's possible that moisture or some kind of lubricant is blocking the rather delicate mechanism.

Yes... cheap... I don't know. 40 mm diameter, with a rear outlet for 12,- from the company Riegler.
The turbo definitely delivers.

I read the error codes today:

Vacuum switching valve for throttle control - N239.
Interruption/Short circuit according to mass.
sporadically

The mechanic, however, said that this was not the cause. Instead, the sealing rings (O-rings) of the pump-injector elements are likely the problem, as the issue only occurs when the engine is cold. These would typically wear out at around 120,000 kilometers.

I drained the diesel filter, and the intercooler system is airtight. Nevertheless, it stalled again.

Does anyone have an idea how much it costs to replace the O-rings?