Lupo 3L AYZ Not Starting? Troubleshooting Tips

Hello,

After finally getting the garage-built car registered last Friday, the first drive didn't go so well. After about 200 meters, the car stalled 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 only running occasionally at idle or being 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. This was accompanied by a voltage drop occurring every few seconds, which was resolved by replacing the alternator.

Current issue:
Last Friday, he drove 200 meters in the first attempt and then stalled – after that, he wouldn't start again.

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 fuel 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, but it only ran for about 1 km before it stopped, and it wouldn't start again afterwards. This time, I left the car there and walked back – it was too tiring to push it in 35°C.

Approximately 3 hours later, I went to the vehicle. It started up again. On the way back, it almost stalled again, but luckily, it was going downhill at that moment and it was in gear - it then recovered.

Back at the farm, I let it idle for a bit longer and gave it plenty of gas several times. Everything seemed to be working fine.

Now I'm wondering where the problem lies, and I'm initially leaning towards a fuel supply issue. As an amateur mechanic, I'm encountering this type of problem for the first time, so I would appreciate a brief collection of ideas and suggestions on how to proceed with troubleshooting.

I was able to find 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 is to replace the O-rings in the fuel tank strainer - however, I couldn't find anything about this in the parts catalog, and I haven't received any further information on the relevant forum yet. I also bought some clear tubing, hoping to spot any air bubbles.

How would a professional approach this problem now, and what other areas, besides fuel supply, should be considered?

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 mention that the error memory is clear.

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

Is the fuel tank vent working properly?

Hello,

Quote:

Fuel tank vent

. Is that the valve on top of the filler neck? I thought that was important for refueling. Does it also provide ventilation for the tank to prevent a vacuum, or what might be the connection to the problem?

Bye.
peace

Hi,

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


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 was likely guaranteed.

Right. Unless, of course, that rotating line in the top left corner of the VCDS screen happened to stop spinning while it was cranking.

Hello,

Sure, here's the translation:

"Just to clarify again:"
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 directs fuel back to the filter when the fuel temperature is below 60°C, and to the tank when the fuel temperature is above 60°C.

2. White Lupo: A short, transparent piece of fuel line was installed between the outlet hose 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: Fuel is fed to and from the filter via 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 container 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 outlet 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 12 km 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 car stopped working after just 150 meters, and it could not be restarted.

11. Conclusion: No idea!
- With the old valve, it once shut down completely after about 200 meters, and before that, it didn't seem to be idling for a very long time, 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-heating filter" 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 while the system was in the preheating mode – after all, there were estimated to be 6-7 liters in the tank. Perhaps 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 conceivable?

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

What would be the next logical step?

Bye.
peace

"Bubble-free" should have very few visible bubbles, or mini-bubbles are normal. When the car is stationary, there should be no more than about 1 cm of air. Please check the expansion tank. Everything may be fine when the car is stationary, but the problem may appear when driving, especially when taking turns. Is this the case for you?"
If you're too lazy to check - does filling up the 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 below 60°C, and redirects it to the tank when the temperature is above 60°C

.
"Only at 60°? Is that normal for a PD (Pumpe-Düse) system? In my car, the thermostat valve switches at a much lower temperature. It's normal that the diesel supply isn't vented as long as the thermostat valve is in the preheating position; the air is simply circulated within the system."
hg
Herbert.

Hello,

Regarding air bubbles: Small bubbles at idle, with a tendency towards foaming (many small bubbles) when accelerating (in the range of approximately 2000-3000 RPM), indicated to me, based on yesterday's tests, that the system was in the IO (Initial Operating) 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 correct, then I don't necessarily consider the "Schlingertopf" (presumably a vehicle model) to be the culprit, because otherwise the 12km it easily drove yesterday, including uphill and downhill sections, curves, right-of-way situations, etc., wouldn't have worked. Also, twice it stopped directly after exiting the gate, after about 150-200 meters of straight road.

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

Bye.
peace

Will the "Knackfrosch" (a type of coolant temperature sensor) from a 1.4L TDI fit? It switches at 30°. That would make bleeding the system faster. First, make sure it's properly bled. When idling, it barely gets warm, so you'll be waiting forever.
hg
Herbert.

Hello,

The diesel filter seems to be the same, so the other valve should fit as well. Normally, there's no need to bleed the system - at least, that's not mentioned in the service manuals.

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

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

Bye.
peace