Audi B7 Engine: Hard Starting / Diesel in Oil

Hello community, I need help because I'm stuck and can't progress further with my current knowledge.
Problem: Intermittently (9 out of 10 times), the engine only starts after a long cranking period of 20-30 seconds. It briefly tries to start, and then nothing happens for 20-30 seconds. When the engine does start, there is NO white, blue, or black smoke coming from the exhaust. The engine runs smoothly at idle, accelerates cleanly throughout the entire RPM range, and has no loss of power. The problem does not occur when the vehicle is parked with the fuel tank uphill.
The pre-lift pump can be activated audibly using VCDS/ignition.
The errors occurred after I replaced the vacuum pump gasket and the diesel filter.
The oil change is performed once a year, and until recently, I haven't noticed any significant increase in oil consumption. However, this started happening after the replacement of the seal/diesel filter.

Since then, I have (unsuccessfully) tried the following:
- new tandem pump
-PDE sealed and installed with the mounting brackets from 07eins.com.
-The PDE (Positive Differential Pressure) is set to 225°.
- Traded a "Knackfrosch" toy.
-Yesterday, I measured the pressure of the tandem pump (around 8-8.5 bar at 4000 RPM).
- No error codes stored.
-The deviations of the PDEs are all within the acceptable range.

Mileage is approximately 265,000 km (with a new engine installed at 192,000 km due to a broken crankshaft).
Verbrauch und Fahrverhalten sind wie immer. Ich weiß nicht mehr weiter was ich noch prüfen/ersetzen soll und hoffe das der eine oder andere hier noch einen "heißen"Tip für mich hat. DanSure, here's the translation:

"no"

Best regards, Alex.

AlexVR6 wrote:

The problem does not occur when the vehicle is parked uphill with the tank. (...) The errors occurred after I replaced the vacuum pump gasket and the diesel filter.

The first quoted sentence clearly indicates the presence of air in the fuel system.
The second one suggests that it's related to those works. Unfortunately, you also modified the partial differential equations (PDEs), which could potentially introduce additional sources of error. However, let's initially assume that the first issue is the cause. Please carefully check everything you did with the vacuum pump and the diesel filter. Even a tiny crack in a hose, which might have occurred during the work, would be consistent with the observed problem. If the hoses are brittle or otherwise damaged, I would recommend replacing them all.

Otherwise, review the technical article on air in the fuel system. For example, install clear hoses and/or set up an external fuel supply, feeding it directly after the tandem pump. This will allow you to gradually narrow down the sections of the system that are relevant to the problem.

Regards,
Guste, who has been putting off the same thing for a year...

Hi.

When does the error occur? Is it always, or only after a certain period of inactivity, like "overnight"?

guste100 wrote:

The first quoted sentence clearly indicates the presence of air in the fuel system. The second one suggests that it is related to these works.

Let's put it this way: the seal of the tandem pump should be tight; if air is being drawn in, fuel should also be leaking out at that point.
I had already reinstalled the fuel filter, but it didn't work.

guste100 wrote:

Otherwise, please review the technical article about air in the fuel system. For example, install clear hoses and/or set up an external fuel supply, feeding it externally from the tandem pump. This will allow you to gradually narrow down the specific components that are relevant to the problem.

That will be my next action.

Rüdi wrote:

Hello. When does the error occur, always, after certain periods of inactivity, or "overnight"?

"Not always; it seems to be related to the engine temperature as well. I'm currently on vacation and don't have to drive far. This week, I had two trips that were limited to the local area, and the car started immediately the next day. Yesterday, I went grocery shopping, and the engine reached operating temperature. When I tried to start the car again in the evening (about 4-5 hours later), I had to let it run for another 20-30 seconds before it started."

Is it reasonable to use compressed air to depressurize parts of the fuel system? I was thinking about the area around the tandem pump, with a maximum pressure of 8 bar, for example, to see if any air is leaking out at the injectors.

Best regards, Alex.

edit: I checked the tandem pump...
https://youtu.be/jLKbyk_1NR8
Wherever air comes out, air must also go in.

It reminds me of warm-start problems from AFN (presumably a specific system or model) and also from various PD (presumably a specific system or model) models.

Quote:

BPW starts poorly / diesel in the oil

It's good that the elements in the ZK (likely referring to a specific component) have been sealed, but is the ZK also okay without a burr on the sealing rings of the PD elements?
Otherwise, the seals will likely fail right after installation...

The tandem pump for the ZK (likely referring to a specific vehicle or system) has also been known to leak in this forum before.

Okay, I've now installed a new cylinder head. It seems the valve seats for the PDE were worn out too much.

dieselschrauber wrote:

It's good that the elements in the ZK (likely referring to a specific component) have been sealed, but is the ZK also okay without a burr on the sealing rings of the PD elements? Otherwise, the seals will be useless right after installation...

Of course, I had already deburred the edges beforehand.

dieselschrauber wrote:

The tandem pump for the ZK has also leaked before in this forum.

I've heard that before, but I have a new one installed in mine. I think I can rule out that being the problem.

Right now, it's starting up normally again, and regarding the oil dilution, I still need to drive it a bit more to determine if that's still an issue.

Thank you for your suggestions; I will let you know what happens with the oil dilution.

Best regards, Alex.

Just a quick update after 3000 km: everything is back to normal, no more oil consumption. The injectors were too worn, I replaced the head, and everything is fine now.

Best regards, Alex.

Thank you for the feedback .