Lupo 3L AYZ engine failure - Troubleshooting ideas and solutions

Hello,

After finally getting the Garagenlupo (a car likely restored or modified in a garage) registered last Friday, the first drive wasn't great. After about 200 meters, the car stopped running and wouldn't start again.

Background:
I bought it at the end of last year. Since then, it's been sitting in the garage and has only been run occasionally at idle or driven around the yard, but not on the road. In July of this year, I replaced everything that was part of the major service, including the air and diesel filters. After that, I started it up again after a long time and noticed an uneven idle. It was accompanied by voltage drops occurring every few seconds, which were resolved by replacing the lighting machine.

Current issue:
Last Friday, he drove 200 meters in the first attempt and then stalled – and he didn't restart afterwards.

On Saturday, I first filled the tank with 5 liters of fresh diesel – before that, the fuel level was almost at the point where the warning light would come on. It started again, but it took a short moment. I let it idle for about 10 minutes in the yard, occasionally adjusting the gas pedal – everything seemed normal. Then I drove it out onto the street again, and this time it ran for about 1 kilometer before it stopped, and wouldn't start again afterwards. This time, I left the cabin and walked back – pushing it in 35°C was too exhausting.

Approximately 3 hours later, I went to the vehicle. It started up again. On the way back, he felt like giving up again, but luckily, the road started to go downhill at that moment, and he was able to keep going – he then managed to recover.

Back at the farm, I let it idle for a bit longer and gave it plenty of gas several times. It went smoothly, without any issues.

Now I'm wondering where the problem lies, and my initial thought is that it's related to the fuel supply. As a hobbyist who is encountering this problem for the first time, I would appreciate a brief collection of ideas and suggestions on how to proceed with troubleshooting.

I found out myself that the valve on the diesel filter sometimes causes problems with the Lupo 3L – I've already ordered it. Another tip I found online suggests replacing the O-rings in the fuel tank strainer, but I couldn't find anything about that in the parts catalog, and I haven't gotten any further information on the relevant forum yet. I also got some transparent tubing, hoping to discover some air bubbles.

"In what order would a professional proceed now, and in what other areas besides fuel supply should one consider potential issues?"

Bye.
peace

Are there any error codes stored in the error memory?
That would be the simplest way to do it, at least for now.

Hello,

Sorry, I forgot to include that information. Error memory is empty.

Bye.
peace

If air can be excluded, check if there is still voltage present at the control unit (STG) in case of a fault.

Hello,

What's the best way to check the air pressure using a transparent hose connected to the tandem pump?

Does the troubleshooting process for voltage loss at the control unit point towards relay 109?

Bye.
peace

Fuel vent okay?

Hello,

Quote:

Tank vent

. Is that the valve on top of the filler neck? I thought that was important for refueling. Does this also provide ventilation for the tank to prevent a vacuum from forming, or how might there be a connection between this and the problem?

Bye.
peace

Hi,

First, install a transparent hose between the filter and the tandem pump. There should be no air bubbles visible.


Regards,
A.

Autoservice wrote:

Check if there is still voltage at the control unit in case of a fault.

I just remembered that, while it wasn't starting, I was able to access the control unit with VCDS - so its power supply should have been guaranteed.

Bye.
peace

pax wrote:

I just remembered that, while it wasn't starting, I could access the control unit with VCDS - so, its power supply must have been working.

Right. Unless, of course, the rotating bar in the top left corner of the VCDS screen happened to stop moving while the engine was idling.

Hello,

Sure, here's the translation:

"Just a quick clarification:"
white Lupo -> problem child
blauer Lupo -> reference vehicle (same model year, same engine, etc.), runs reliably.

Here's a quick summary of today's news:

White Lupo: Replaced the valve on the diesel filter - this valve redirects fuel either back to the filter if the fuel temperature is below 60°C, or back to the tank if the fuel temperature is above 60°C.

2. White Lupo: A short, transparent piece of fuel line was installed between the connection of the fuel line and the tandem pump -> a relatively large number of large bubbles appeared there, and the number of bubbles increased at higher engine speeds.

3. White Lupo: A transparent piece of tubing was installed completely between the diesel filter and the tandem pump -> the air bubbles are coming from the filter itself.

4. White Lupo: The fuel flow to and from the filter is directed through transparent hoses into a small, separate container filled with diesel -> still many large bubbles, very little fuel is being drawn in, and very little is reaching the container, the height of the container is varied (below, above, at the same level as the tank) - no change.

5. White Lupo: Keep the engine running, even at higher RPMs - eventually, the valve on the diesel filter switched - now, a good amount of diesel was being sucked out of the reservoir and pumped back in through the return line - the bubbles in the supply line became fewer and smaller, but there were still many small bubbles overall, even when accelerating.

6. White Lupo: Removed the recently replaced new diesel filter, reinstalled the old diesel filter - same situation, then reinstalled the new diesel filter.

7. White Lupo: During the run, the valve was removed - after a short time, large bubbles reappeared in the pre-filter; the valve was reattached to the filter - the bubbles then became smaller and less numerous.

8. BLUE Lupo: A short, transparent section of fuel line was installed between the fuel supply hose and the tandem pump -> small bubbles were also present there, and the number of bubbles increased with higher engine speeds - the bubble situation is comparable to the white Lupo after opening the new valve.

9. White Lupo: Original hose clamps reinstalled, all previously separated sections are now secured with the original clamp and an additional hose clamp. Feeling confident, I took it out on the road and drove approximately 12km with a 10-minute break - everything worked perfectly (for the first time).

10. White Lupo: After several hours - it had grown dark and cooler - while attempting to start the journey home, the engine cut out after 150 meters and could not be restarted.

11. Conclusion: No idea!
- With the old valve, it once shut down after about 200 meters, although it hadn't been idling for a long time before that, as far as I remember - so the valve should still have been in the filter preheating mode.
- The second time (with the old valve), it stopped working after about 1 km, preceded by a longer period of struggling - the valve was likely in the tank inlet mode.
- With the new valve, it heats up (return flow to the tank) and operates normally.
- With the new valve and the "pre-filter warm-up" mode, it stopped working after a short time.

Basically, I can only speculate that I might be wrong, and that the second failure last week actually occurred during the filter preheating mode – after all, there were estimated to be 6-7 liters in the tank. Possibly, they weren't fully warmed up yet.

Then I would have an air problem in the return line, which would affect the supply line through the filter. Is something like that even conceivable?

And what does "blasenfrei" actually mean? Does this refer to larger bubbles, approximately 1cm or more in size, or does it also include smaller bubbles/foaminess?

What would be the next logical step?

Bye.
peace

"Bubble-free" should have very few visible bubbles, or only tiny bubbles are acceptable; in "retirement" mode, up to about 1 cm of bubbles is allowed. Check the sway bar bushings on the tank! Everything seems fine when the car is stationary, but the problem might appear while driving, especially when taking turns. Is that the case for you?
If you're too lazy to check - does a full tank solve the problem?

Quote:

white Lupo: The valve on the diesel filter has been replaced - it redirects fuel back into the filter when the fuel temperature is up to 60°C, and to the tank when the temperature is above 60°C

"Does it only switch at 60°? Is that normal for a PD valve? Mine switches at a much lower temperature." It is normal for the diesel supply to not be vented as long as the thermostat valve is in the preheating position, as the air is circulated within the system.
hg
Herbert.

Hello,

Regarding air bubbles: Small bubbles at idle, with a tendency towards foaming (many small bubbles), and then appearing as larger bubbles when accelerating (in the range of approximately 2000-3000 RPM) indicated, based on yesterday's tests, that the system was in an "IO" state.
Larger bubbles are present both at idle and at higher engine speeds (where they are less visible due to the flow velocity), and these bubbles, which extend across the entire hose diameter, are considered unacceptable if they are longer than approximately 1 cm.

Does "1cm of bubbles in the resting state" mean the engine is stopped?

If the "bubble interpretation" is acceptable, then I don't necessarily consider the "Schlingertopf" to be the culprit, because otherwise the 12km he easily rode yesterday, including uphill and downhill sections, curves, and right-of-way situations, wouldn't have worked. It happened to me twice, both times immediately after exiting the gate and driving straight for about 150-200 meters.

Thermostat valve: According to SSP223, page 38, this is indeed the case for the Lupo 3L, while other valves switch at 30°C.
This could lead to the idea of temporarily installing a bypass this summer and short-circuiting the valve. Is there anything wrong with that?

Bye.
peace

Will the "Knackfrosch" thermostat (designed for 1.4L TDI engines) fit? It opens at 30°C. If it fits, the engine would warm up faster, which could speed up the bleeding process. First, make sure the cooling system is properly bled. When idling, the little thing hardly gets warm, so you have to wait forever.
hg
Herbert.

Hello,

The diesel filter seems to be the same, so the other valve should fit as well. Normally, it shouldn't be necessary to bleed the system - at least, that's not what the service manuals say.

Yesterday, the valve switching worked because I had siphoned diesel from a small external container and also redirected it back into that container. The approximately 1.5 liters of liquid quickly warmed up.

If I have time today, I'll try the bypass method again. However, I'd rather not use the aquarium hose I used yesterday to search for leaks, and I need to see if I can find a proper fuel line somewhere.

Bye.
peace