AVF TDI im Passat 3BG: Kaltstartproblem | Posts 80+

Simply put, the old hoses were not sealing properly. By replacing them with transparent hoses, the problem was resolved.

cksIT wrote:

It was quite simple, the old hoses were not sealed properly. By replacing them with the transparent hoses, the problem was then resolved...

I also suspect that.
According to Murphy's Law, it's also typical for stubborn and insidious errors: When you finally resort to THAT (last / only) suitable method to catch them, they'd rather disappear than face the shame of being discovered .

Stop, stop, stop, not so fast. I would have expected that, but I didn't get any NEW hoses; instead, the mechanic "added" hoses from the AFN to my AVF, essentially extending them. What that means is that the mechanic simply disconnected the two "old" hoses from the filter and installed two short connecting pieces from the AFN. The hose diameter is the same on the filter, and at the transition point between the AFN hose and the original AVF hose, regular screw-on hose clamps were used! The whole thing was routed upwards in an arc near the oil dipstick so that any air bubbles might collect at the top.

And now?

Could it be that the pressure control screw on the tandem pump was not sealing properly?

Greetings.

I would say the connection at the filter was not sealed properly. By disconnecting and reconnecting it with the clear hose, he fixed the problem.

It would be a possibility. But I also replaced the KS filter back then because of the error. And I had to disconnect the hoses and reconnect them to the new filter. Or the replacement of the "knack frog" on the filter: Again, lines had to be disconnected. These procedures didn't bring any success at the time. What I noticed is that the mechanic spilled something, and the hoses were a bit diesel-wet when connected. It's possible that it seals better now .

Greetings.

Quote:

The AVF hose was connected using standard screw-type hose clamps!

Could it possibly have been due to leaky clamps? In that case, 100 points to Mr. Antje, who suspected it almost a year and a half ago.

What else did we learn?

During a longer start-up attempt, the error 16705 - Implausible signal from engine speed sensor (G28): sporadic error is stored in the control unit, although the component is functioning correctly .

Unfortunately, I got my hopes up too soon! This morning, it happened again . It seems the problem is still there, but it's SIGNIFICANTLY better since the procedure (I had 3 weeks of peace, before that it was at least 2 times a day). However, what exactly it is remains unknown. Does anyone have any more tips?

Greetings.

I would like to replace the diesel filter and all the hoses (using clear ones). While doing that, make sure to tighten everything properly and check the hoses.
Maybe it wasn't properly secured again...

Mike3BG wrote:

Unfortunately, I got my hopes up too soon! This morning, it happened again. It seems the problem is still there, but it's SIGNIFICANTLY better since the procedure (there were 3 weeks of peace, before that it was at least 2 times a day).

If the error was so clearly influenced by the last action, it seems you at least worked on the right spot: probably the temporary parts have become leaky again in certain areas.

It's best to completely redo everything in that area. If possible, also incorporate a clear section into the return line: this will show bubbles if combustion gases are leaking past defective PDE seals into the diesel circuit, even if the supply line is 100% tight.
-> As long as there are no air bubbles in the fuel lines, a current fuel system problem can be ruled out.

Hello everyone!

The fuel filter was already replaced back then, without success. It's definitely not the filter itself. But one thing is certain, as Ulf wrote. The "area" where the problem lies must be the right one, because it's no coincidence that it improved significantly after the "action," including draining the lines. I've been looking inside almost every day to see if there are any bubbles. Without success, since it started up perfectly. But since it hasn't appeared for 3 weeks now, I haven't looked inside anymore, and what happens? The error occurs So, I can't say yet what the lines looked like.

@Ulf, you installed two transparent pieces. Also one in the return path. I can therefore observe both.

Greetings.

Hello!

Am Wochenende habe ich mal beobachtet. Im Rücklauf war eine Mini-Blase so groß wie ein Stecknadelkopf (max.2mm im Durchmesser) zu sehen. Angesprungen ist er aber dennoch ganz normal. Meine Frage lautet nun: ist das schon ein Indiz? Wenn ja, worauf? UNonlinear PDEs?

Ergo question: Is it permissible to have any air bubbles in the return line, or must the hoses be absolutely and completely free of air bubbles?

Greetings.

Mike3BG wrote:

Im Rücklauf war eine Mini-Blase so groß wie ein Stecknadelkopf (max.2mm im Durchmesser) zu sehen. Angesprungen ist er aber dennoch ganz normal. Meine Frage lautet nun: ist das schon ein Indiz? Wenn ja, worauf? UNonlinear PDEs? Ergo-question: Is it permissible for there to be any air bubbles in the return flow, or must the hoses be absolutely and 100% bubble-free?

It becomes interesting as soon as the diesel flows - that is, even when the ignition is ON (helper needed), when the pre-lift pump starts, and then when the engine has started and the tandem pump delivers the full flow rate.

A bubble that is immediately washed away is unlikely to cause any problems.
But if a larger and more sustained flow of gas is introduced (rather than a short burst or a constant flow), things become interesting: If the vacuum level (VL) and/or the reflux level (RL) are affected, the gas content changes with the speed, etc.

I would suspect a faulty PDE (Pre-Delivery Equipment) seal towards the combustion chamber if there is a constant flow of gas into the return line (RL) without any supply from the fuel supply line (VL).

Hello Ulf!

According to your theory, there should also be problems after the engine starts, right? But that's not the case. Once it starts, there are absolutely no problems, zero!

I still don't know if it's acceptable to have 100% 0.0 bubbles in the hoses. Because if that's the case, wouldn't even a tiny bubble indicate a leak?

Greetings.

Mike3BG wrote:

I still don't know if there can be 100% 0.0 bubbles in the hoses. Because if that's the case, wouldn't even a tiny bubble indicate a leak?

In principle, yes - unless gas is being "constantly" produced in the fuel system due to some reactions, like in a home heating system, where depending on the materials, you can sometimes release smelly "air" from the radiators at certain intervals.

Unfortunately, I don't know if something like that is possible inside an engine (and whether it could create individual bubbles).

"That tiny bubble is definitely not the problem. Many diesel engines have a small amount of air in the lines."

In my opinion, your problem lies with the hoses themselves (the altered starting behavior after modifying the lines).

I would completely replace everything, making sure it's all perfectly clean and new, and that should be it. It will only cost you a few euros, and you'll have peace of mind.

Okay, I know that the small bubbles aren't the problem, otherwise it wouldn't have started normally despite the small bubbles. My question was whether these small bubbles *could* be a sign of something, or whether such small bubbles are *always* normal.

You're right: I'll go ahead and replace the hoses around the filter.

Greetings.