Engine speed fluctuations when accelerating, both in neutral and while driving.

[bart2k]

noone in germany can help me: VW Passat B8 2.0 TDI 150 hp, 2018 model year, with 7-speed DSG transmission. Purchased 2.5 years ago with 100,000 km. Now at 155,000 km. A pre-purchase inspection at TÜV (German vehicle inspection agency) was performed, and everything was fine. All services were performed at VW dealerships. The test drive and cold start were both normal.

Approximately two weeks after purchase, the idle became extremely noisy. The tensioner pulley and serpentine belt were vibrating. They were replaced, and the noisy noise disappeared. Two weeks later, the idle pulley failed and was also replaced. Two weeks after that, a squeaking noise started coming from the timing belt. This was also replaced, and the noise disappeared.

Oil changes are performed twice a year, every 10,000 km, using 5W-30 brand-name oil. The transmission fluid was changed at 120,000 km.


Now to my problems, which I've been experiencing for about two years:

1) When I'm driving in a gear (preferably manual mode, regardless of whether it's city, country roads, or highway) and I release the accelerator pedal or turn off cruise control, the RPM immediately drops by about 100-200. When I press the accelerator pedal again, the RPM immediately rises by 100-200. This happens regardless of whether I press the accelerator pedal fully or just lightly. See video: https://www.youtube.com/shorts/-4P7WsAeDc4

2) Idle: It's very rough, loud, and rumbling for a diesel. When the engine is cold, the whole car vibrates. When the engine is warm, the vibrations are significantly reduced. The engine sounds like it's about to stall. And recently, when I rev the engine while cold, in neutral and park, the RPM drops below 700, then the engine recovers and the RPM starts to fluctuate: https://www.youtube.com/shorts/uFN2A0EfLFg

I logged this with an OBD dongle: After releasing the accelerator pedal, the voltage rises from 12.x to 13-14.x. And at the highest voltage point, the RPM drops below 700 and starts to fluctuate. Could the alternator be causing these fluctuations?


Things get interesting when the DPF regenerates: With a cold engine, the idle speed is around 1,000 rpm, and the engine is quieter and vibrates absolutely no longer. Totally pleasant. Quite different from when the engine is cold and the temperature is low: After starting the engine, the idle speed is around 1,000 rpm and


3) During deceleration (without accelerating, while coasting) at around 10-20 km/h, the tachometer needle vibrates: https://www.youtube.com/shorts/37ot46psZio


I suspect the two problems are related. The engine doesn't seem to run properly at idle/deceleration. However, as soon as you press the accelerator, the car has full power and runs smoothly. We tow a 1.7-ton caravan with it 4-5 times a year without any issues. I have very little oil consumption, maybe 0.5 liters per 10,000 km. No coolant loss.

I've already taken it to VW, four different garages (two regular independent shops, two DSG transmission specialists), a mechanic friend, and my brother-in-law (who worked as a VW mechanic for 10 years).

Now to my attempts to solve the problem:
- Replaced the air/diesel filter (bled the system with VCDS)
- Used Shell V-Power for a year. Tried adding two-stroke oil and diesel additives repeatedly.
- For the last six months, I've only used HVO100, and it's made the knocking noise much better.
- Replaced the serpentine belt, including the tensioner, freewheel, and pulley.
- Replaced all three engine mounts.
- Replaced the DSG mechatronics unit because it was registering an error code and only engaging odd-numbered gears (at approximately 140,000 km, which also resulted in a new transmission fluid change). Everything has been fine since then.
- Replaced the dual-mass flywheel and clutch; the dual-mass flywheel was indeed defective, but the clutch was still in very good condition.
- Replaced the timing belt at 110,000 km because a tensioner was squeaking.
- Had the timing checked by another garage at 140,000 km; everything was fine!
- Rail pressure/camshaft/boost pressure and crankshaft position sensors replaced
- Pressure regulating valve on the high-pressure pump and the rail pressure regulating valve at the end of the rail replaced
- EGR deactivated for several weeks, no change
- All four injectors sent to Bosch. The result was that the pre-injection quantity was three times higher than the permitted value for all four. Therefore, all four injectors were replaced with brand new ones, including seals, and programmed using VCDS.
- Fuel return flow after the injectors was checked. Both at idle and under load. All cups were equally full.
- At 150,000 km, three glow plugs were replaced (fault code for the fourth glow plug) and the glow plug control unit was replaced (due to a stored fault code).
- Mass airflow sensor was replaced.
- EFB battery was replaced with an AGM battery and programmed.
- The PVC valve in the crankcase ventilation system was replaced. An oil catch can was also installed.
- Both drive shafts were inspected, as the joints were suspected to be worn.
- Oxygen sensor was replaced.
- Accelerator pedal was replaced.
- Checked for rodent damage and leaks. Everything was sealed.


The following was checked using a diagnostic tool/VCDS:
- Throttle body. Perfectly fine. I disconnected the charge air hose and watched to ensure the throttle body moved freely. It was completely clean.
- Injection quantity deviation: maximum ±0.2.
- EGR checked. Air mass changes by half when the EGR is 50% open.
- Transmission values such as engine speed, clutch 1 & 2 pressures, etc., were measured. Multiple DSG basic settings and adaptation drives were also performed. Everything was normal. The pressures are within spec, and the transmission and engine speeds fluctuate accordingly. The clutch is not slipping.
- DPF regenerates every 500-700 km, depending on driving style. Differential pressure appears normal.
- MAF sensor, rail pressure sensor, and rail quantity valve on the high-pressure pump were disconnected: No changes.
- Throttle body disconnected: Except for the engine stalling poorly, no changes.


I'm currently troubleshooting the problem, focusing on the air and air temperatures. I've been logging the lambda sensor readings. During engine braking, the fuel/air mixture is at 16 (which is perfectly normal, as no diesel is being injected). At full throttle, the value is between 1.1 and 1.2, also normal! With a cold engine, the fuel/air mixture is around 2.x. This suggests a high diesel consumption, possibly due to a lack of air. When the engine is warm and no longer vibrating as much, the value is around 3.8, almost 4. This could explain why the old injectors injected "three times as much" fuel, or why the new ones do the same (I haven't verified this yet). This would also explain the knocking noise.

Fun fact: I sold the four old injectors, and the buyer was aware of this. They worked perfectly for him.

Here's an excerpt from the VCDS log values for the mass airflow sensor (temperature and mass) and boost pressure. The interesting thing is the load changes and accelerator pedal position, and the associated fluctuations in intake temperature. Boost pressure sometimes lags behind the target value, but that could also be due to the sampling rate. In the next excerpt, the target boost pressure equals the actual value.

I even tried simply connecting the old MAF sensor next to the intake to test the engine's running. Even at 0 g/sec, the engine continues to run just as rough and loud.

If I disconnect the lambda sensor, the engine seems to run better when cold.

So, the workshops and I are stumped. This is like looking for a needle in a haystack. Something is affecting the idle/overrun. But what?