Kniffelaufgabe: Ursachen für unrunden PD-BUK-Kaltstart? | Posts 112+

Were there different versions or data states for the product with the code 038906019NT?

Hello everyone,
Hi Ulf!

Thank you for the long-awaited log .
Here are some questions: Were there any noticeable misfires? What was the engine temperature? The log doesn't seem nearly as extreme as some of the other data you've uploaded before.

@Steffen_W: Thanks again for the offer, but I can't even manage to install KlausMausi's bra extension on my T5 right now *sigh*.

Our T5 is always preheated using a standing heater, so the "cold start" doesn't hurt quite as much .

Best regards from Munich!

dieselschrauber wrote:

Hi Bertil, "Then those definitely aren't VAG TDIs for me, because as far as I know, all of their standard diesel particulate filters (DPFs) are closed systems. In fact, I don't know of any engine that comes with an open filter from the factory. The only way to get around it is if someone has drilled a hole in the DPF or otherwise damaged it." ...

It doesn't matter if it's a VAG engine, another brand, a closed system, or an open system. A filter simply can't be perfectly sealed. And when you look at the exhaust in the light, you can clearly see it emitting black smoke. This is even true for direct injection gasoline engines, which produce significantly more smoke.

Quote:

Samrt was the only one, to my knowledge, with an open DPF.

No, that's not true!
At least the diesel version of the first-generation Subaru Outback also had an open DPF system. When this engine was later installed in other vehicles, it received a closed DPF system.
I am even aware of other vehicles that were produced with an open system as standard equipment.

Quote:

A filter can only be so dense; it won't always be perfectly effective. And when you look at the exhaust in the light, you can clearly see that it's emitting black smoke. This is even true for the direct injection gasoline engines. They produce significantly more smoke.

You are absolutely right. The best example is the FSI engine.

RedR32 wrote:

You are absolutely right about that. The best example is the FSI engine.

With TFSI, it's even more evident.

DieselBär30x wrote:

Hi Ulf! Thank you for the long-awaited log . Questions regarding this: Were misfires audible?

What was the engine temperature?

Quote:

The engine had an estimated temperature of 5°C after approximately 12 hours of standstill (according to the log, it briefly showed MWB7 -> 8°C). Right after starting, it hesitated for about 1-2 seconds. But I think this is because the "ready" mode (which keeps a low voltage on for those who want to avoid re-heating the glow plugs after they've turned off) was already switched off before I finally got VCDS to start logging. Then the glow plugs only receive power again when the engine starts firing, and they weren't fully heated up at the moment of starting.

The log doesn't look nearly as extreme as some of the other ones you have uploaded before.} Perhaps it's a result of the modified software that I mentioned earlier.
"I can't say for sure yet, as the stuttering with the OEM software didn't show any clearly identifiable temperature dependencies."

Enjoy your meal! / Have a good lunch!

It would be interesting to see a 13-18-23 log again, under the same conditions, using the original software.
If the values here were to consistently deviate from the expected range, then your theory would be confirmed .

Another question: Have you ever measured the pressure of your tandem pump, as Steffen_W mentioned?

Best regards from Munich!

DieselBär30x wrote:

Another question: Have you ever measured the pressure of your tandem pump, as Steffen_W wrote?

No.
It still hasn't quite become clear to me why this inappropriate pressure should only manifest itself during cold starts at temperatures below approximately 10°C .

The software actually provides more insightful explanations if you look closely.

No.
It still hasn't quite become clear to me why this inappropriate pressure should only manifest itself during cold starts at temperatures below approximately 10°C .


Sure, because the ventilation system on your BUK is guaranteed to work even without a tandem pump.

'Sorry, Ulf, but that would be the first thing I would check.'

It seems you're completely unaware that the fuel system can draw in air at temperatures below a certain point. However, it's a fact that metal contracts when cold. Therefore, it's entirely possible that at a specific temperature, a seal may no longer provide a perfect seal and allow air to be drawn into the system.


The software provides more insightful explanations if you look closely.

Sure, just because something won't stick in your head, the software is always to blame...

goettmann wrote:

It is certainly completely unclear to you that the fuel system can draw in air at temperatures below a certain point. But it's true that metal contracts when cold. Therefore, it is quite possible that above a certain temperature, a seal may no longer seal 100% and the system draws in air.

OK, even I understand that .

If air were to enter the system, it should be visible as bubbles if you temporarily replace the two hoses between the differential and the engine with transparent sections (which you would naturally have to observe during engine start-up).
Because it would be less trouble for me than finding/borrowing a suitable pressure gauge and installing it on the pump.

Quote:

Sure, just because something doesn't make sense to you, the software is always to blame...

So far, it appears that the BUK software is at least involved in the problem.

If air were to enter the system, then it should be visible as bubbles if you temporarily replace the two hoses between the differential and the engine with transparent parts (which you would naturally have to observe during engine start)?

Not if it is 'disposed of' by the pressure differential equations (PDEs). Otherwise, yes. However, if the pressure of the tandem pump is incorrect, then the intended venting mechanism will not function correctly either.

The BUK SW, as far as I know it, is fine. The kind of 'mistakes' that you suspect are not present in it. If that were the case, all BUK systems and related types would have this characteristic. However, that is not the case.

goettmann wrote:

If the pressure of the tandem pump is not correct, then the intended venting principle will also not work correctly.

What does the correct venting procedure look like (if you are allowed to say)?

Quote:

As far as I know, the BUK SW is fine. Things like the "issues" you suspect are not present in it. If that were the case, all BUK and related types would have this characteristic. However, that is not the case.

Relatives of the BUK, in the sense of my assumptions, do not exist, AFAIK, mainly because it is the only 1.9-liter engine with the "550 EU4 PDE" configuration.
For example, in the software of BPW (using the same PDEs), I cannot detect any similar patterns.

The issues with my assumption, in my opinion, only become noticeable when the LRR (Low Rolling Resistance) tire requirements (due to tolerance or wear) exceed certain thresholds. And this seems to occur primarily in my case after cold starts below approximately 10°C engine temperature.

The LLR is initially inactive during the cold start process (at least in the area you're looking at). That's not the right approach.

And there are plenty of relatives (similar models). Your STG (Steuergerät - control unit) is a derivative of a mass-produced product. There are some others out there with similar specifications. The fact that yours has features that others don't has nothing to do with the basic functions of the ECU (Engine Control Unit). These are largely the same as those of its 'colleagues.'

If it were a software error, then every BUK should have had it. However, since I know of at least three that don't exhibit this behavior, it probably isn't the case.
Your BUK simply has unfavorable coinciding tolerances. Or it may have a defect.

Hello Goettmann!

goettmann wrote:

... If it were a software error, then every BUK would have it. ...

From conversations with Steffen_W, I know that another AXC (a cheap exotic component in the T5 system) also experiences significant runout issues after starting at temperatures below 0°C. Again, the LLRR values are incorrect, the PDE statuses were "128" before startup (which seems to be the normal starting state for the AXC only??), and finally, the PDE switching times here were also firmly in the negative range.
Therefore, for me, Ulf's software bug doesn't seem entirely implausible (at least it's still better than "Climate HOAX," etc. ). Furthermore, it seems he has managed to address the problem by modifying the rule parameters (?).

Before I start speculating further about our T5, the tandem pump pressure must be measured first. Steffen_W is absolutely knowledgeable about PD-TDI systems, and he will know why he's telling me this... Only after that has been verified to be 100% correct will we investigate other potential causes.

By the way, I'm convinced that it's faster to replace the pressure plug of the tandem pump with a gauge than to swap out any hoses for transparent versions (and, as experience shows, you'll likely end up introducing even more air leak issues in the process) .

Best regards from Munich!

DieselBär30x wrote:

Before I start speculating further about our T5, the tandem pump pressure must be measured first. Steffen_W is absolutely knowledgeable about PD-TDI and he will know why he's telling me this... Only after that has been checked to be 100% correct will other potential causes be investigated. By the way, I'm convinced that it's faster to replace the pressure plug of the tandem pump with a gauge than to swap out any hoses for transparent versions (and, as experience shows, you'll likely end up introducing more air leak issues in the process)

. Hmm... how do you see a potential temperature dependence of the pump pressure? Wouldn't it be possible that you measure "everything OK" with a (semi-)warm engine, but the pressure is still incorrect when the engine is cold?
Or is something like that not constructively possible?

@Deus:
I had overlooked your question until just now... so, I only know of one data point regarding the BUK system.

goettmann wrote:

During the initial start-up, the LLR is initially inactive (at least in the area where you are searching). That's the wrong approach.

I mean the LRR = Idle Speed Control -> MWB13.
If it weren't active, why are the diagnostic data sometimes immediately reaching their maximum values of +/- 3 mg after starting up (see previous log images)?


Here's another question about explaining drafts:
Can air in the area of the fuel tank filler neck be discharged towards the tank by running the fuel pump for a sufficient amount of time, i.e., by repeatedly turning the ignition on and off?
Or does the full flow rate of the tandem pump need to be used to flush air out of all areas of the fuel system?

Of course, it will then be measured with the engine cooled down and (for now) not in a fully warmed-up and error-free condition .

Best regards from Munich!