AVF TDI im Passat 3BG: Kaltstartproblem | Posts 96+

"Normal" is relative again.
At least quite a few TDIs have small bubbles in the lines, and they still run normally.

If things stay the same, I wouldn't change anything. But in your case, it's reasonable to assume that it will get worse...

Hi everyone.

Unfortunately, I'm still not making any progress. It still malfunctions occasionally. And now, to make matters worse, the AGR valve is also "sweating." The silver part is slowly but surely turning completely black. So, I'm going to replace it. My question to you experts: Do you need to adapt a new AGR valve software-wise? Or can you just install the hardware and it will work fine?

Can a malfunctioning EGR valve also cause starting problems, for example, due to the valve sticking due to deposits?

Greetings.

Mike3BG wrote:

And now, to make matters worse, the AGR is also "sweating". The silver part is slowly but surely turning completely black.

Exactly where is the leak coming from?

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Does the new AGR system need to be adjusted software-wise? Or can you simply install the hardware as a plug-and-play component and that's it?

I don't see a need for adjustment as long as the same part number is used. However, adapting it to the maximum setting is always advantageous for the engine.

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Can a malfunctioning EGR valve also lead to starting problems? For example, can deposits cause the valve to stick?

If the shut-off valve is stuck closed, the engine will crank for about 3 times and then completely shut off.
This game can be repeated with 100% reproducibility until the battery runs out.
If you absolutely want to rule out any possibility of the valve being open, simply disconnect the vacuum hose and seal it.

Hi Ulf!

It appears that the black oil mist is coming out directly from underneath the membrane dosing unit. All other suspected areas are okay. Specifically, the seal at the 90-degree bend, the manifold, and the connection to the thin exhaust pipe are all okay and dry.

My suspicion is that it might be a stuck EGR valve. However, since I don't know exactly what happened to the EGR valve at the start, it's just a suspicion. It feels like something is "loosening" during the start. There's a rattling sound, "orgel-orgel-schüttel-schüttel," and then suddenly, it runs normally.

Perhaps you know more about this. Is the AGR valve open when only the ignition is turned on? Can the valve be activated without the engine running?
Since everything works when it's warm, I suspect that if it's related to the EGR system, a different state occurs during cooling, which the EGR system then occasionally causes to happen during cold starts. Could you list the possible scenarios, if you're familiar with it?
Fangen wir beim Leerlauf (Motor läuft Zustand=Warm) an. What position?
Then the motor is switched off. -> What position (resting position)?
Then it immediately restarts (idling) -> what happens to the EGR valve in that case?
What happens after a longer period of inactivity, and what happens when starting the engine? Could the EGR valve get stuck during this process and prevent the engine from starting? In principle, the engine shouldn't start if it doesn't, but sometimes the cranking time varies significantly.

Ulf, I'm not talking about the shut-off valve; it seems to be working normally. I'm talking about the EGR valve. It can't be the shut-off valve because if I stop the engine when it's warm and immediately restart it, it works. So, I assume that when I turn off the engine, the shut-off valve remains in the correct position, even if I don't start it again for 8 hours. It shouldn't change position during that time. And isn't it true that it's not activated during startup? It can't be!

Perhaps I don't understand the AGR system. Can only the shut-off valve completely interrupt the intake air flow to the manifold, or can the valve between the intake air and the exhaust also do this?
I just want to find out if it's possible for the AGR (Abgasrückführung) system to enter a state that makes it impossible to start the engine, or that causes problems.

We've already tried dealing with the shut-off valve. If an assistant manually presses and holds it closed, the engine then runs normally as if it's not getting any fuel. If you press the valve quickly in short bursts, it comes close to my starting problem. However, the shut-off valve can never be actuated so quickly in short bursts. The vacuum system and the diaphragm of the shut-off valve are very slow to respond.

But aside from that issue, a faulty EGR valve can't be good for the engine's performance, right? So, I definitely want to replace it. Whether it will actually make a difference remains to be seen. I'm almost certain it won't. I've already replaced too many things without success.

Was mich wundert ist, daß in der langen Zeit (seit über einem Jahr) das Problem nicht schlimmer wurde und nicht andere oder weitere Fehler in den Fehlerspeicher abgelegt hat. Auch im Fahrbetrieb hat sich nichts verändert. Ich dachte wenn ein Fehler auftritt wird er irgendwann sich deutlicher äußern oder auch im Fahrbetrieb Probleme machen. Nichts, rein gar nichts! Absolut keine weiteren Indizien. VeI'm sorry, I can't translate that. It appears to be a typo or an incomplete word. Could you please provide the full text you would like me to translate?
I only know that it tends to occur more frequently with decreasing temperatures. After they checked the tandem pump and installed the transparent hoses, I had three weeks of peace, but then it started again sporadically, although less frequently than before. However, there's absolutely nothing visible that would indicate a problem with the hoses. There's virtually no bubble formation either. It's probably just another coincidence, and the problem is likely located somewhere else.

Greetings.

Mike3BG wrote:

Maybe I don't understand the AGR system. Can only the shut-off valve completely interrupt the intake air flow to the manifold, or also the valve between intake air and exhaust?

There is no "valve between intake air and exhaust." The AGR valve only opens a bypass from the exhaust manifold to the intake side.

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Let's start with the idle (engine running, state=warm). What position?

Depending on the load, it is generally open at idle.

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Then the engine is switched off. -> what position (resting position)?

AFAIK, closed.

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Then it immediately restarts (idle) -> what happens to the EGR then?

Immediately after the start, the target air mass is high, which means the EGR must be closed in order to achieve the target value.

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What happens over a longer idle time, and what happens when starting? Can the EGR valve get stuck during this procedure and cause the engine to fail to start? In principle, the engine wouldn't start if it suddenly started running. So, the cranking time is sometimes very different.

As long as the engine is cranking, even if the EGR valve is open, only fresh air would enter the cylinders. Only the initial ignition cycles can reduce the amount of fresh air entering through the open EGR valve.

Hi Ulf!

Ich habe gestern die neue AGR gekauft und die alte nochmal angeschaut. Wow, die wird ja jetzt dramatisch schnell schwarz! Die eine Seite wo die Mebran für die Abstellklappe ist ist die AGR schon komplett schwarz (mattschwarz) und die andere Seite ist leicht schwarz. Woher genau es den Siff rausdrückt kann man jetzt nicht mehr sagen, aber kanns sein daß es bei dem kleinen Loch unter der Dose rauskommt? An den Anschlußstellen kommts def. NICHT out.

Now I understand how the AGR (Abgasrückführung) is constructed internally. The valve that looks like a throttle valve is the shut-off valve. And if I'm not mistaken... it's only closed when the engine is turned off, right? Otherwise, it's 100% open? There's no intermediate position? And the other valve, as you said, is just a bypass. That means it's impossible for that valve to be stuck or in a state that prevents the engine from starting properly, right? Because the shut-off valve is definitely open during startup, and the other valve shouldn't matter for starting, whether it's open, half-open, or closed!? Or am I mistaken?

Greetings.

Mike3BG wrote:

Yesterday, I bought the new AGR

. Then, don't throw the old one away: the membrane that controls the shut-off valve sometimes breaks and is not available separately. You would probably find someone who wants to buy it soon...

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It's no longer possible to say exactly where the leak is coming from, but could it be coming from the small hole under the can? It definitely isn't coming from the connection points.

Then, the leak (in the case of LD) would have to flow through the (worn?) valve stem guide into the lower half of the can and from there out into the open: that seems plausible to me.

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The valve that looks like a throttle valve is the shut-off valve. And if I'm not mistaken... it's only closed when you turn off the engine, right?

Correct.

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Is it otherwise 100% open? Is there no middle position?

Correct, unless something is stuck.

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Because the bypass valve is definitely open during startup, and the other valve shouldn't matter for the start, whether it's open, half-open, or closed!?

I already wrote that...

Hi Ulf!

Okay, I know about the membrane dosage. Mine is still perfectly fine.

Also die Abstellklappe ist bei meinem beim Starten 100%ig offen! That's for sure!
Okay, then the AGR probably has nothing to do with the starting problem. However, I also can't really imagine why some people experience jolting due to the AGR . Is there a logical explanation for that?

Perhaps I should also look into the problem of the "leaky/oily" tandem pump again. I mean... everyone talks about minor leaks in the lines that can cause this, but you can't actually see them, but on the other hand, is it normal for the tandem pump to visibly "sweat"? I'm still not entirely clear on that. What if the tandem pump "draws" air after a long period of inactivity? Why are you so sure that it can't be the case?
Ulf, just to be clear: when I start the engine and the problem occurs, it happens immediately. Therefore, if it's a fuel problem, I believe the lack of fuel must be directly at the common rail injectors and not somewhere earlier in the system. My theory is based on the fact that if the pre-pump is making a hissing sound, the problem also occurs. And the high-pressure pump (VV-Pumpe) at least delivers fuel up to the tandem pump, right? When starting, the tandem pump pumps fuel to the injectors at a certain speed. However, since the problem occurs immediately, I assume that there is no fuel at the injectors at all. Or at least not at all the injectors, because you can hear a combustion noise during cranking. What do you think of this assumption? Should I have the tandem pump replaced? And should I replace the injectors at the same time? That's definitely going to be expensive, I know that already.

Greetings.

Mike3BG wrote:

However, I also can't quite imagine why some people experience jerking or lurching due to the AGR Is there a logical explanation for that?

What about a lack of fresh air due to an excessively high AGR rate?

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What if the tandem pump "sucks in" air after a longer period of inactivity? Why are you so sure that this cannot happen?

Who is so sure about that?

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If the problem occurs immediately when I start the engine, then it's INSTANTANEOUS. Therefore, if it's truly a fuel problem, the lack of fuel must be happening directly at the PD elements, and not before.

ACK

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When starting, the tandem pump supplies fuel to the PDIs (pump-injector diesel) at a speed proportional to the engine's RPM. However, since the problem occurs IMMEDIATELY, I assume that there is NO fuel reaching the PDIs at first. Or at least, not all of the PDIs, because you can hear a combustion noise during the cranking. What do you think of this assumption?

ACK

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Should I have the tandem pump re-sealed? And should I replace the PDE's at the same time?

That will certainly be expensive, I know that already. } If there's "false" air in the area of the PDE's, you should be able to see it in the return line during the sputtering or immediately after starting (clear hose!?). As long as there are no bubbles passing through, I think we can rule out the air theory.

Hi Ulf!

Okay, so if that's the case, then one would have to look at the air in the return flow? Hmmm, it's difficult for me to start and observe at the same time .

Another question: Can PD engines "stall" briefly during startup? If so, would that be visible somewhere using VAG-COM?

You're welcome.

Greetings.

Mike3BG wrote:

Oh, so, meaning if then one would have to see the air in the return flow? Hmmm, that's something I find difficult to do at the same time, to start and to observe.

But it's solvable:
Disconnect the starter cable and attach a jumper cable that reaches the battery. Put the car in neutral, engage the parking brake, turn on the ignition, go to the engine, and hold the jumper cable on the B+ terminal .

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Another question: Can PD engines "stall" briefly during startup? If so, would you see that somewhere using VAG-COM?

Good question, next question.
If there's a problem, it's likely that the solenoid valve is stuck, which should ideally be detected by the continuous BIP (Basic Input/Output) evaluation, or at least lead to error messages.

Jetzt fällt mir nochwas ein. Ich habe es ja mal geschafft daß er den Fehler 3 mal hintereinander gemacht hat. Also Starten -> 5sek. orgeln -> Motor läuft. Dann Zündung aus und gleiches wieder. Das habe ich 3 mal hintereinender geschafft wobei die Orgelzeit immer kürzer wurde. Von 5 sek. runter auf 1sek., bis er dann immer anspringen würde, egal wie oft ich gestartet hätte, außer man hätte ihm wieder längere Zeit gegönnt. Könnte man dadurch nicht wiederum ausschließen daß es Luft ist? Denn das kurze Abdrehen und Wiederstarten kann doch den Kraftstoff nicht so schnell verschwinden lassen!? Die Orgelzeit wurde aber immer Kürzer. Nur was kann sich da in der kurzen Zeit so verändern daß die "Orgelzeit" sich verkürzt? Es muß auf jeden Fall was thermisches/mechanisches sein, welches nach ein paar Sekunden in einen Temperaturbereich kommt wo es dann bei allen weiteren Startversuchen immer klappt. Nur what?
"If only I had one more piece of information in the error memory, and not just the G28 code, which is really just a consequence of the problem because the initial speed is completely wrong."

Greetings.

Hello Ulf!

Sure, here's the translation:

"Quick, slightly off-topic note:"
The new AGR valve is installed, but the old one wasn't broken. It seems the problem was actually just the gasket between the AGR valve and the manifold. It was leaking on the underside, where it's not easily visible, and causing all the issues. Of course, I immediately installed the new AGR valve, and it fits perfectly. I'll clean the "old" one (it has a slight buildup) and keep it. What surprises me is that the buildup is relatively small. Especially at the manifold inlet, where you can see it clearly. It's a maximum of 1mm thick, no more. In the AGR valve itself, it's maybe 2mm. Does the buildup tend to decrease as you move closer to the combustion chamber, or does it tend to be more there?

"I've also noticed that when it's really cold, around -6°C, it has a bit more trouble starting. It sputters briefly, then runs perfectly again. I'm not sure if this is related to the problem, but I wanted to mention it. The glow plugs were replaced a year ago. I can't quite remember, but should a diesel engine start instantly at -6°C and run perfectly and smoothly right away, or is it normal for it to run roughly for a short time (like 2 seconds) before running normally? I don't really know because I almost always start it with the auxiliary heater. It always starts perfectly and quickly without any "coughing"."

Greetings.

What about the starting speed? Is the starter motor spinning fast enough? Could this be due to a weak (old) battery, a faulty starter motor, or poor grounding cables, etc.? That would explain why it starts better each time (after 3 attempts, as described above, for example)?

Hi,

Just to remind you: The starter is new, the battery is also new, and the wiring is absolutely fine.

Short question: What happens if the VTG (Virtual Test Generator) "gets stuck" during startup? Does it then start up with the same hesitation, or should it start normally as if nothing were wrong?

Greetings.

The VTG (vehicle traction generator) has no effect when starting.

Edit: Did you replace the PD cable harness yet, or not? If you haven't replaced it yet, try unplugging and replugging the connector a few times to rule out a loose connection.