Passat 3bg AVF geht sporadisch aus | Posts 16+

Hi,

Please check if you have installed the click frog correctly.
If you turn the valve around and insert the wire clip from the other side, then the valve will operate in the opposite direction.
The incoming flow to the fuel pump is immediately pumped back into the return line because it encounters less resistance than the path to the tandem pump.

If I'm not completely mistaken, the AVF (presumably a type of filter) is the one without the side fins on the "Knackfrosch" (likely a product name).

However, I don't see your trip to Spain being jeopardized, because the valve is actually only important in the winter to heat the diesel fuel from the engine and circulate it through the fuel filter.
So, it's not necessary in the summer. You can, if necessary, continue using a direct return system until next winter.
Either use a tightly sealed container (like a "Knackfrosch") or use a filter without the "Knackfrosch" (VW part number 191127401c).



Regards, Mpire.

Hello!
Thank you for your quick response.
I'm definitely sure that I installed the "Knackfrosch" correctly. It has a directional arrow indicating the flow, and the clip is very difficult to attach from the other side.

So, my vacation trip will be undertaken with the "blind" scooter, unless I find the problem by then.
However, I still felt uneasy. Because something must be defective or "weak," otherwise there wouldn't be a problem.
And if that failing component completely breaks down during the trip, I'll be stuck.

Best regards,
Oliver

Hello!

He died again today.
VDCS was immediately switched on and started! After about 2 minutes, it restarted. I couldn't find anything unusual in the measurement data blocks.
However, the diesel filter was empty again.
Now I have replaced the diesel supply and return lines with transparent hoses.
The pre-filter comes before the main filter and should be free of bubbles.
The return flow comes back with a large amount of fine bubbles. Whether with or without the "Knackfrosch," nothing will change.
But you can see from this that the diesel fuel is flowing nicely.

I don't think about any potential flaws in the wiring. As mentioned before, I have replaced all the rubber hoses in the fuel system and secured them with screw-on hose clamps.

Now, I'm not sure if it's normal for bubbles to form somewhere in the system and then be visible on the return flow.

r28oli wrote:

Now I don't know if it's normal for bubbles to form somewhere in the system and then be visible on the return flow?

No, that's not normal. The leak needs to be found and fixed. If it's not the hoses, then unfortunately, the pump is also a possibility.

Regarding the rubber hoses and their clamps: Did you really use round-profile clamps, or just "simple clamps from a hardware store"? Even when tightened properly, the latter are also susceptible to leaks.

Regards,
Guste.

Hello!

These are high-quality stainless steel clamps, which we also use in our company (I'm an installer).
I'm familiar with the problem of cheap cable ties from hardware stores.

Okay, I'll continue investigating the potential leak.

Thank you for your reply.

Sure, here's the translation:

"Best regards,"
Oliver.

Does the fuel level in the tank have any effect on the overall performance, i.e., does it matter if it's completely full or almost empty?

Hello!

No, he does it when the tank is full or almost empty.
The quality of the diesel fuel also has no influence.

Update:

I've now connected the supply and return lines from the tank using a short, transparent hose.
The diesel comes out cleanly without any bubbles and flows smoothly.
Diesel remains in the line when the pump is turned off.


Then I tested the inlet and outlet lines using the transparent hoses while the engine was cold.
The diesel fuel is flowing properly without any air bubbles, but the return line to the tank is blocked because the engine temperature hasn't reached 30 degrees Celsius yet.

Inlet and outlet at the filter on a warm engine:
The diesel fuel flows correctly to the engine, but the return line, after the fuel filter (often referred to as a "Knackfrosch" - a type of fuel filter), constantly has many air bubbles.
In the initial phase, diesel fuel leaks back when the engine is turned off.

As a final step, I made the "Knackfrosch" (a type of device) blind and connected the return line directly.
Good flow, bubbly return! When parking, the coolant leaks out.

Since all the hoses are new and definitely leak-proof, and the problem hasn't changed, I'm narrowing down the source of the error to the tandem pump and will replace it.

I'm encountering the following issue: The tandem pump is leaking and drawing in air.
As long as the diesel is cold, the "Knackfrosch" (a type of fuel additive) also goes into the tank. The bubbles that come back with the return flow fill the diesel filter with air, until it's almost empty, and then the pump sucks air from the filter, causing the engine to stall. At this point, it is also not possible to bleed the system because the return line to the tank is blocked.
That explains why a car with a directly connected return line, without a "Knackfrosch" (a type of valve), can run for days without any issues. Because the air bubbles can return to the tank.

Am I correct, or does anyone have any objections?

Best regards,
Oliver

Then the car would consistently perform poorly, and its performance would significantly degrade under prolonged periods of high demand.

Hello!

Hmm, he has power, and he also runs relatively smoothly otherwise.
I put in some extra effort today and replaced all the diesel hoses in the engine compartment with...
Replaced the transparent hoses and properly bled the system.

The diesel fuel flows from the tank through the filter to the tandem pump without any air bubbles, but exits the tandem pump with persistent air bubbles.

I'm completely lost now and I'm spending every day working on the car, tinkering, and testing things, and this has been going on for 6 weeks.
Today, I ordered a new tandem pump, gasket sets for the PD units, a screw set for the PD unit, a diesel temperature sensor with its bracket, and the original diesel hoses with the original hose clamps. I finally want to be done with this topic.
And above all, I want to embark on my trip to Spain with a clear conscience.

Edit: I also ordered a new DiFi without the "Knackfrosch" (a type of frog toy). Cars used to run without it, and I prefer it without this unreliable part.

Best regards,
Oliver

r28oli wrote:

I would then finally like to be finished with this topic. And above all, I want to embark on my trip to Spain with a clear conscience. Best regards, Oliver

"Go ahead and replace the wiring harness for the PD elements as well; it's not a big deal now. After all, you've already put a lot of miles on it, and the cables are exposed to oil and heat."
They can therefore become brittle quite easily. Since you'll be working on these components as part of your project, you might accidentally introduce a new problem, such as a broken cable.

Tip: When working on the AVF engine in a Passat, it's easier to access the wiring harness at the rear of the cylinder head if the tandem pump has already been removed. The space available for the splash guard is limited.

Regards, Mpire.

Thanks for the tip, but I actually replaced that about 4 weeks ago.
That was one of the first parts I replaced since the engine started acting up.

Perhaps the engine control unit (ECU) is malfunctioning.

Hello!

No, it wasn't MSG.
I replaced the tandem pump and the pre-charge pump, and I also re-sealed the PDE units all at once. Since then, it has been quiet.
I drove to Spain and back (4000 km) without any issues.

P.S.: I can't say for sure which part was ultimately the problem, as I replaced everything at once. But I no longer had time for experiments.

Best regards,
Oliver

Well, then he's passed the long-term test. I'm happy for you.
Thank you for the feedback!
hg
Herbert.

Since we're already talking about PDEs.

Was it easy to remove and reinstall, considering its age?