TDI Cold Start Problem Passat 3BG AVF

Hello!
I've decided to start the topic again from the beginning because I don't think the issue was explained properly.
Mein 3BG Passat (AVF TDI PD, Bj.2002) startet kalt seit dem Winter sporadisch nicht richtig. Man dreht den Zündschlüssel, der Anlasser dreht sich auch scheinbar durch, das Anlassgeräusch ist aber nicht (!) ein normales Anlasserorgeln, sondern begleitet von wahrscheinlich einem Zündgeräusch. Der Motor schüttelt dabei in reglemäßiger Frequenz und springt dann plötzlich und sofort (!) auf allen 4 Töpfen doch an und läuft sofort stabil und rund. Die Zeit dieses Orgelns/Motorschüttelns ist abhängig von der Außentemperatur. Das ist auch der Grund warum es jetzt gerade weniger oft auftritt und wenn es auftritt dann auch nur für einen Bruchteil einer Sekunde. Im Winter war das längste mal ca. 8sek. Im warmen und halbwarmen Betriebszustand tritt das Problem NIE (!) auf, und wenn der Motor läuft, dann läuft er perfekt (kein Ruckeln, volle Leistung, perfekter Verbrauch, lt. VAG-COM alles gesund)! Da das Problem jetzt schon lange besteht habe ich natürlich einige Erkenntnisse während der Fehlersuche gesammelt, leider aber das Problem (noch) nicht damit behoben. ..... und es scheint so daß es wirklich ein ganz spezielles Problem ist, welches noch keiner hatte. VoTherefore, I now require specialized knowledge from you experts.
If the error occurs for a sufficient amount of time, specifically for about 4 seconds, then the error will be logged.
"Die Sonne scheint hell, und die Vögel zwitschern fröhlich. Ein leichter Wind weht durch die Bäume, und die Blätter rascheln leise. Es ist ein wunderschöner Tag, um draußen zu sein."
VAG-COM Version: RKS 607.3-UD

Control unit part number: 038 906 019 GQ.
Component and/or version: 1.9L R4 EDC G000SG 1524.
Software Coding: 00002
Workshop code: WSC 05xxx
Additional information: WVWZZZ3BZ2Exxxxxx VWZ7Z0B4xxxxxx
"One error found:"
16705 - Motor speed sensor (G28): implausible signal.
P0321 - 35-00 - -

Self-test: 0 0 X X X
"Die Sonne scheint hell, und die Vögel zwitschern fröhlich. Ein leichter Wind weht durch die Bäume, und die Blätter rascheln leise. Es ist ein wunderschöner Tag, um draußen zu sein."

Now, let's move on to the findings and the parts that have already been replaced.
Spark plugs (were initially suspected, as the problem first occurred during freezing temperatures).
Battery OK.
Starter wiring OK.
Starting current draw is OK.
Quiescent current measurement: OK.
LiMa charge OK.
New intake manifold twin carburetor G71/G72.
Fuel temperature sensor G81, new.
New valve on the fuel filter (he unfortunately meant a different check valve. Does anyone know where that is located?).
Ignition switch, new.
G28 - New engine speed sensor.
It doesn't matter if the vehicle is full of fuel or if the tank is empty - it has no influence.

As I mentioned, the problem occurs more frequently, and especially for a LONGER duration (the engine cranks for a while before it finally starts), and this is particularly true at lower temperatures. It seems that the combination of temperature and the vibrations during starting affects the duration and eventually gets the engine running. When the temperature is higher (e.g., 13°C), the problem occurs rarely, and if it does, the cranking is only very brief. When the temperature is lower (e.g., 0°C), the problem occurs more frequently, and the duration of the cranking is also longer.

Here are some possible approaches:
a) Air in the system due to longer idle time. In principle, a good approach. However, what speaks against it is that it has sometimes happened that I started the car cold and the problem didn't occur. Then I only drove the car about 100 meters and parked it. After 5 minutes, I started it again, and the problem was there . Still, air in the system?
b) Determine whether there is a clicking sound during starting, or if it's a jerky start from the starter motor, which causes the engine to shake and creates a sound that mimics a clicking noise. A suggestion was to disconnect the camshaft sensor, start the engine when it's cold, and listen to see if the engine sounds the same as it does when the problem occurs.
c) Determine in which situations the G28 is stored in memory. It is definitely not the G28 itself that is being stored. Now, we need someone who truly understands all the conditions under which the G28 is stored.

The following sensors and parts have not yet been replaced:
KW-Temp. dual sensor G62
LMM G70
Camshaft sensor G40.
Engine control unit.
Starter.
Power supply relays (but which ones?)

I would be very grateful for any helpful information.

Therefore, the original post can also be deleted or closed.

Greetings.

In your case, I would look into things like compression, valve timing, or PD (possibly referring to pressure drop or similar). Otherwise, I'm also at a loss.

Hello!

Thank you for your tips. But regarding compression: Compression, even though it seems to be working properly and the idle speed control provides perfectly healthy values?
A suspicion remains that the problem lies in the fuel supply. Possibly due to a ball check valve that is not always 100% tight. This explains the occasional long cranking with ignition noise, because it seems unable to build up pressure quickly enough. However, I cannot find this mysterious check valve on MY AVF .

Greetings.

I read your post, but I can't help you. I'm just going to look into this tip myself. Does your car have a pre-lift pump that runs briefly before starting (I have no idea, I once had a 96kW model that made a short noise)? If so, then it shouldn't matter as long as there's no air in the line...

"Where I had compression problems (worn piston rings), it would sometimes start late and misfire. I don't put too much importance on the idle speed. It was actually better with the compression problems and the 6 worn injectors than it is now, even though everything *should* be okay."

Hi,

Yes, that "hissing" sound from the pre-lift pump is what he's hearing, but not always. I have no idea what it depends on. However, it's not related to the problem, because the problem still exists/existed even when it "hisses/hissed." I don't know if the variable-flow pump can immediately compensate for a leaky check valve. What would the check valve be good for then? The first AVF has one inside, and it certainly also has a variable-flow pump. }

Greetings.

Can you rule out the possibility of air being present? If you have a check valve, you probably need to allow for air to escape. Besides that, those are the only other things that come to mind.

If you know for sure that it's starting up correctly, just let it run for a short while. If you can provoke it to do so, you'll know where to look.

There was no mention of air in the . Only that the fuel flows back through a non-(always) tight check valve, and therefore the engine takes longer to start after a longer period of inactivity, because the fuel flowing back prevents the pressure from building up immediately . That's how I understood it. However, I can't find the check valve on MY AVF . The first AVF has it right next to the fuel filter.

A question about air leaks in the system. If there are no visible fuel leaks anywhere, can I assume that everything is sealed properly?

Greetings.

I don't know how it is with yours. Mine sucks directly from the tank. After the pre-pump, it might be leaking there in your case.

"However, mine can also draw air even in what appear to be tightly sealed return lines. Also, the valve on the filter often looks sealed, but can still draw air. Therefore, I suspect that the absence of visible leaks in the lines doesn't necessarily indicate a problem."

So, I can't say for sure, but maybe someone else will chime in who knows the answer...

Another question: Should I just replace the stupid diesel filter as a precaution? Is it a big deal, or not?

Greetings.

Other answer There are probably filters (like ATU, for example) that don't seal properly where the valve is located. Also, one of the seals might be damaged. I would try it, but with an original filter.

"Then there are probably sometimes problems with the hoses on the filter. I bought some special round hose clamps, because the standard ones tend to pull the hoses a bit awkwardly."

Did this problem suddenly appear, or was there a maintenance update or something similar happening shortly before?

Hello!

The problem suddenly appeared, and it wasn't due to any upcoming . The car was ALWAYS maintained at the }, and therefore original parts were always used.
I already replaced the valve at the top of the filter with a new one. The two seals are already installed on it. It's possible that the "tolerance" of the filter itself is too large, which might cause it not to seal completely at times. But what surprises me is that, if that were the case, wouldn't there be at least some "sweating" due to the pressure during operation?

Greetings.

In my case, there isn't really a high pressure in the return line... Also, what about air and diesel? Doesn't air flow more easily than diesel? I'll leave it for now, as I'm not getting any further with it.

".....but that should be fine. It's good to know about the diesel and air issue. I'll replace the filter; that's really not a problem anymore. I'd also like to replace the check valve, but I can't find it anywhere."
Unfortunately, I didn't have any time for anything this weekend. Therefore, I couldn't do any trading or try anything out.

Greetings.

See if it starts the same way after replacing the filter, as it does when the problem is present. If so, your vehicle may need to bleed itself occasionally, and I would investigate in that direction (if the problem isn't resolved by then).

I once heard something about diesel and air, but I'm not entirely sure about it.

Hey, great idea! Do you have any tips regarding filter changes to avoid major spills and leaks in the engine compartment?

Greetings.

Well, I have a VP44 injection pump, and with that, there's always a mess because I have to fill the filters and lines with diesel.

I think you can probably skip a step with yours (but you should definitely be sure!). If that's the case, I'll hold the new, empty filter next to the old one, and then disconnect each hose individually, and finally carefully remove the old, full filter. Then, I'll insert the new filter and that's it. Finally, I'll dispose of the old filter and its contents.

I'm not sure how the hoses are attached to your filter; I replaced the standard hose clamps with circular clamps.