A6 2.5 TDI: Misfire & Rough Idle

Hi everyone,
I drive an A6 with the engine and identification code mentioned above, and I need your advice. My car has been in the workshop for almost 3 weeks now, and I'm really fed up with the situation.

The following items were replaced/renewed:
Turbo.
Timing belt with water pump, etc.
Hydraulic lifters and roller rocker arms on the right cylinder bank due to the onset of ticking in the hydraulic lifters.
Diesel filters.

So far, so good. The mechanic (who was trained at Audi and, coincidentally, also owns a 2.5 TDI) has installed everything. During the test run, a very loud and constant knocking sound was immediately noticeable.

It was suspected that the fuel injectors were the problem. All 6 injectors were removed and sent to Motoren Kindt in Lübeck. The result: 2 injectors were defective. Therefore, 2 new injectors were installed, and the other 4 were inspected and cleaned according to the workshop. Unfortunately, I do not have a test report regarding the spray pattern and opening pressure.

Today, I went to pick up the car and noticed a sporadic, loud diesel knocking sound when the engine was cold. Additionally, the entire engine was visibly jerking, and this was also audible inside the car.

As soon as you engage a gear and move out of idle, the knocking becomes more pronounced and occurs more frequently.
In addition, the car emitted a slight amount of smoke.

The mechanic, the boss, and I were at a loss.

There are no air bubbles in the fuel.
It's noteworthy that the mechanic mentioned over the phone that a vacuum hose in the engine compartment had been sealed, and he found that odd. However, I don't know where it was reconnected.

Could it be that more nozzles are defective?

"Until the repair, the A6 was running perfectly fine. The only issue was that it usually took a second longer to start, and it would emit some smoke for a few seconds, which I suspect was due to a leaky fuel injector."

Tomorrow, all the nozzles will be removed and inspected again.
Error memory is empty.

What else could it be?

"Could the closed vacuum hose have been connected to the EGR valve, and was the valve sealed because it was defective? I bought the car like that, and I didn't seal the hose."

Best regards from the north.

Benni.

Quote:

Until the repair, the A6 was working perfectly fine,...

Why the repair then? It looks more like a case of "fixing" something and making it worse.
Error codes, car scan?
hg
Herbert.

Hydraulic noise and clicking sounds when starting?

That makes me think of the instruction in the repair manual to wait for a certain amount of time (over 30 minutes, I think) after installing the hydraulic components, so that the valves don't hit the pistons when starting. Was that done correctly?

Thanks for the tip, I had that thought too before I heard it. Unfortunately, I forgot to record a short video of the noise. It's not a clicking or ticking sound, but definitely a diesel knock, and the engine shakes briefly as well. It reminds me of old LT vans and similar vehicles. The car is now at a specialist engine repair shop, hoping they can quickly find the problem.

Update: The workshop said that this morning the car wouldn't start because there was air in the diesel line. The whole situation is getting stranger and stranger...

I would also attribute the jerking and smoking to air in the fuel injection pump.
There are very few 2.5 TDI engines that don't eventually develop air leaks. Often, these air bubbles only become apparent when the engine is revved up. If there are air bubbles in the fuel pump, the engine will start to misfire severely because the injection timing is constantly changing, and the error code "injection timing regulation difference" will be displayed.

The vacuum hose might have been connected to the EGR valve.
When it switches, the engine noise also changes.

Hello everyone, the story continues. The workshop had installed incorrect (too long) hydraulic lifters. That's why the engine wasn't running properly and was producing a lot of smoke. Now, new, correctly sized long lifters (33.7mm) have been installed. When starting the car in the workshop's yard, it initially didn't run well again. It was misfiring and shaking a bit. I showed it to the master mechanic. Then, the famous demonstration effect occurred, and the misfiring and shaking suddenly disappeared. On the way home (highway and country road), the engine started misfiring again, especially under partial load. I took the A6 back to my local workshop, and after a test drive, the mechanic said that it is almost certain that it is still an injector problem. It also sounds like the engine is only running on 5 cylinders at times.
The nozzles are reportedly checked multiple times, and no defects have been detected recently.
Tomorrow, the car will come back to the engine repair shop. I'm a little surprised, because the problem should have been noticed immediately during a test drive .
Questions upon questions...