Kniffelaufgabe: Ursachen für unrunden PD-BUK-Kaltstart? | Posts 176+

Just because a battery is six months old doesn't mean it's good.
The quality of the batteries has deteriorated significantly due to the scarcity of raw materials and their location in Asia.

I suspect that your problem is caused by a faulty battery.

Skoda Doc wrote:

Have you ever tried connecting a new battery? Bitte gib mir den deutschen Text, den du übersetzt haben möchtest. The quality of the batteries has decreased significantly because the raw material is scarce and located in Asia. I tend to think that your problem is caused by a bad battery.

Hmm... Currently, I don't have much motivation to buy another functional battery as a spare, but if that changes soon:
What reliable advice can you give me regarding a good battery?

Quote:

Even with good batteries, it can happen in extreme cold. The voltage drops briefly at the engine control unit, which then causes a delayed or jerky start.

A symptom in this direction would be, in my opinion, a disruption of the data stream during cold starts, but that hasn't happened to me yet.
I know: While continuous log data are not a guarantee of consistently sufficient voltage, I believe they are at least an indicator.

Currently, it appears that batteries sold as genuine parts have a better quality than freely available batteries from the same manufacturer.

Banner is complete garbage as an aftermarket product, but it's acceptable when purchased as a genuine Volkswagen part.

Here are two more logs for the Bärli vans from the T5 transporter series. One is a 62 kW BRR and the other is a 75 kW AXB. Another AXC is currently unavailable. Both drivers reported that these transporters start smoothly and without any issues in the morning, after they have cooled down overnight.

It's also possible that the control unit "forgets" the NW (negative torque) and kW (kilowatt) settings due to undervoltage, which could be the cause of the problem. Not everything can always be explained with diagnostic tools and logs.

Today, another AXC arrived, and I was able to extract a PD start log from it. Unfortunately, it was only running warm, and on top of that, it had a box attached.
This T5 also suffers from rough engine running immediately after starting.
With a warm start, it fires up cleanly.

I can only advise you, Ulf, to finally get out of your chair and measure the pressure of the tandem pump. I don't understand why you're putting so much thought into it and developing theories and scenarios when a simple measurement might solve the problem?

I also experienced this behavior during cold starts, with an unstable idle and incomplete combustion. The measured values were generally okay, but since the tandem pump was already leaking, it was replaced with an original part, and I haven't had any problems since.

Zak1976 wrote:

I can only advise you, Ulf, to finally get out of your chair and measure the pressure of the tandem pump; I don't understand why you worry so much and create theories and scenarios when a simple measurement might solve the problem?

If you're going to do it, then you want to measure the pressure precisely while it stutters during a cold start. And for that, you need cold nights on the weekend. So far, I haven't found any source that will lend me a suitable pressure gauge over a winter weekend.

In addition, the car did not cough nearly as badly throughout the entire winter of 2010/2011 as it does in the video.
I don't know if it's due to the modified software and/or the transparent tubing used for air bubble control, which may finally be sealing properly against air leaks. In fact, one end of the original equipment manufacturer (OEM) hoses was so loose on the pipe connection that it could be rotated back and forth without much effort.

Quote:

I also experienced this behavior in my car during cold starts, with an uneven idle and incomplete combustion. The measured values were generally okay, but since the tandem pump was already leaking, it was replaced with an original part, and since then I haven't had any problems.

Did you happen to measure/compare the pressure output of the new and old pumps?

You rocket scientists! Change the pump and see what happens, you guys...
You probably also weigh the yogurt containers when you're shopping, subtract the tare weight of the container and lid according to a list you keep, and put them back on the shelf if the industry has cheated you out of 0.8g .

Deus Violentia wrote:

You rocket scientists! Change the pump and see what happens, you know...

Unfortunately, my parts bin doesn't have a (working) tandem pump available, and buying one based on a hunch is too expensive considering the current problems I'm still dealing with.

But maybe I can still find an old gasoline engine compression tester somewhere that I can modify to fit the pump's measurement port...

I still have another pump; it was leaking a bit, which is why I replaced it with a new one, but I didn't experience any motor problems with it.

Hi Ulf, I can't seem to find my post about the pressure gauges from my brother-in-law's old Touran anymore when I search. I had posted a bunch of them before, and I still have two left. They are professionally crimped with an adapter and a 60mm gauge up to 10 bar. Let me know if you need one.

Hi Ulf,

Okay, here's a question about your video: At 0:32, you can hear a strange noise. What is that? Do you have a moose in your garage?

Regards,
Sure, here is the translation of the text from German to English:

"Micha"

Mephisto wrote:

Hey, I have a question about your video: At 0:32, you can hear a strange noise. What is that? Do you have a moose in the garage?

No, that's the sound of my significant other protesting the exhaust fumes while she was making the video.

Zak1976 wrote:

I can only advise you, Ulf, to finally get out of your chair and measure the pressure of the tandem pump.

Thanks to the offer from teileklaus, I now have the pressure readings, measured with a cold engine:
- Idle speed at 850 rpm: 4.5 bar.
- As the engine speed increases, the pressure rises; approximately 7 bar is reached at around 1500 rpm. The pressure no longer increases further, apparently because the pressure relief valve opens.

What I noticed: The pressure gauge needle is fluctuating wildly, often with amplitudes of 2 bar.
Okay, I'm not aware of any damping devices for the pressure waves generated by the pump in this type of system, but I was still quite startled when I saw the needle jumping around so violently.

Why don't you change the tandem pump...?