1Z fails to start/shuts down during operation (not relay 109)

Hello!
It all started when the engine simply stopped working while I was driving on the highway (at 100 km/h). It wouldn't start, even after trying repeatedly. I checked the fuel lines (inlet and outlet) -> they seem to be okay. Finally, the engine started immediately with the help of starting fluid.
The next day, it started without any problems, but after about 5 kilometers on the country road, it stopped working again. Again, the engine could only be started with starting spray. I had to repeat this procedure six times within a 30km distance. (The engine kept stalling while I was driving.)
On the way back, he easily covered another 30km.
I replaced relay 109 as a precaution. (There was an entry in the error memory related to this.)
Then there was peace and quiet for the first four days.
Yesterday, the engine didn't stall, but it wouldn't start after being left idle for a day. Again, using starting fluid helped -> the engine started immediately. No error codes found in the fault memory.
It makes me uneasy because I don't want to be stuck somewhere again.
The VP37 unit installed in my system is slightly leaking (it doesn't drip, but the leakage is visible under pressure).
I thought the pump might not be building up enough pressure. But somehow, it doesn't make sense to me because the engine usually starts, but sometimes it's impossible to get it running at all, except with the spray.
If the trolling motor isn't receiving power, it shouldn't work even if you use starting fluid, right?
Do you have any ideas?
Otherwise, I will be replacing the seals on the VP37 in the next few days.
Update: Seals have been purchased. However, I will only install them once I'm sure that the problem isn't with the shut-off valve on the VP37. So, during the next shut-off, I'll measure the voltage. Let's see.

Is the idle speed too low for starting?

"No, it doesn't sound like that. It wouldn't explain the sudden failure while the engine is running. It usually starts up. Only when it doesn't start does it require starting fluid to get it running." Later, you can turn the engine off and start it up normally.

Hello.
Does he already have a WFS? A friend of mine had a similar problem where the WFS control unit was defective.
Sincerely, Sure, here is the translation of the text from German to English:

"Dirk"

Make sure to check the central crankshaft bolt.

Best regards, efistho.

Hi Dirk!
No, he hasn't gotten a new WFS yet. However, I removed the ELAB (parking brake) and installed a new, empty one. -> That is to say, Elab is always open.
If the WFS fails, the engine should still start due to the (permanently) open ELAB. Does the WFS do anything else?

Since I had a break of 1 1/2 weeks from dealing with the problem, the car completely broke down on the highway today. He also couldn't be started with jump leads or any other method.
ÖAMTC loosened a fuel injection line. -> No fuel was being injected during the starting process (only a slight dripping). The pump was adjusted to increase the fuel injection volume. -> Diesel fuel started flowing properly again. It started again, but the engine was running very unevenly. He eventually made his way home.
Once I got home, I couldn't figure out how to adjust the flow meter so that it wouldn't either stop working or spin out of control.
I have no idea what's broken.
Control knob? What does WFS stand for?
Any other ideas?

@efistho

If the central bolt of the crankshaft has come loose, the valve timing is misaligned. This should result in a harder start and, under normal operating conditions (because TDI engines have electronic adjustment), it should run normally.
In most cases, my car started without any problems (without any cranking or other issues), but it occasionally hesitated. If it was being temperamental, the car would completely refuse to start.
Now, as I said, it doesn't start at all. (Not even with starting spray.)

If it weren't for the symptoms I had before, I would agree with you. However, since it usually started without any problems in the past, and only occasionally refused to start at all, I don't think the timing is the issue.

Or is there a fallacy somewhere?

Best regards,
Erwin

I started disassembling the ESP (and thankfully, I started with the fuel pump cover), and that's when the cause of the problem became apparent.
Since I had already cleaned the MSW once and therefore resoldered the contacts, a cold solder joint came loose.
Soldering point renewed, MSW cleaned, and gaskets on the pump cover also replaced at the same time. -> It's running like new again.
best regards,
Erwin.