Parking Heater Runs Briefly, Then Full Power, Off

Golf V - 2.0 TDI (BMM) Model Year 2008, manufactured in 2007, DSG (KCU) transmission, Variant (estate/wagon), VIN: 1KZ8M320904.

The standard-installed auxiliary heater starts up, but when it tries to switch to full power, it produces smoke next to the car and then shuts off.

Actuator diagnostics for the circulation pump, dosing pump, glow plug, etc., show plausible values. The circulation pump and dosing pump can also be heard.
The circulation pump was replaced about a year ago.

Now, of course, nothing is possible anymore because, after xx failed attempts, a safety mechanism prevents further attempts.

The error log also contains something like this:
"Fuel level indicator 002 - lower limit exceeded."
...and I don't remember ever turning on the auxiliary heater.
programmed it, only to find that the tank was almost empty, with only reserve fuel remaining.
Based on that, I'm assuming that no diesel fuel is actually being delivered.
The error first appeared two days ago, when the temperature was around 0 degrees Celsius.

It's obviously very inconvenient with the weather being like this, especially when you need it.

The issue with the fuel level indicator occurs as soon as the low fuel warning light on the fuel gauge illuminates.
Since the auxiliary heater is smoking, there's also fuel involved. I would start by investigating the area around the glow plug.

Best regards,
Michael.

Xoctron wrote:

The problem with the fuel level indicator occurs as soon as the reserve light on the fuel gauge comes on. Since the auxiliary heater is smoking, there's also fuel involved. I would start by investigating the area around the glow plug. Best regards, Michael

I think you didn't read everything I wrote above.

hkss wrote:

Xoctron wrote: The problem with the fuel level indicator occurs as soon as the reserve light on the fuel gauge comes on. Since the auxiliary heater is smoking, there's also fuel involved. I would start by investigating the area around the glow plug. Best regards, Michael I think you didn't read everything I wrote above.

I've already read everything you've written.
However, the problem with the fuel level indicator also occurs even when the heating system doesn't turn on, as soon as the low fuel warning light comes on.

If the heating system didn't receive any fuel, it wouldn't produce any smoke.
Possibly, the flame arrestor could be clogged. This would prevent the fuel being pumped from igniting, and the heating system would go into fault mode.

Xoctron wrote:

hkss wrote: Xoctron wrote: The problem with the fuel level indicator occurs as soon as the reserve light on the fuel gauge comes on. Since the auxiliary heater is smoking, there's also fuel involved. I would start by investigating the area around the glow plug. Best regards, Michael I think you didn't read everything I wrote above. I've already read everything you've written. However, the problem with the fuel level indicator also occurs even when the heating system doesn't turn on, as soon as the low fuel warning light comes on. If the heating system didn't receive any fuel, it wouldn't produce any smoke. Possibly, the flame scanner could be damaged. Consequently, the injected fuel no longer ignites, and the heating system goes into fault mode.

Thank you, the idea with the flame arrester is a good one. My auxiliary heater is having some issues.
high usage / used very frequently.

This car has a Webasto heating system installed, not an Eberspächer. This eliminates the issue with the filter.
The error indicating a low fuel level, even when the heating is turned off, occurs because this heating system also functions as a temperature-controlled auxiliary heater. As such, it experiences the same problem with insufficient fuel.
Okay, so: Unlock the heating device, clear the error code, and continue to monitor it. If the error messages related to flame interruption reappear, or if the heating system continues to malfunction, the burner assembly is the first thing to check.
However, a flawless fuel supply is a prerequisite.

Okay, I had to read it a few times, but I now understand that even if I drive conservatively/start the car in a cold state – even if I haven't used the heater – the error message is still being recorded and stored.
Das hätte man dann aber besser und logischer programmieren können z.B. wenn Motorstart und läuft, dann Zuheizer und kein Fehlereintrag wg. Reserve.
Well, if that's the case, then that's the case.

"However, what has been extremely noticeable to me over the past 3 days is that my engine is not reaching its operating temperature. The temperature gauge stays at 75 or a maximum of 80 degrees, even after driving 40 km (at a low RPM range)."
The thermostat display isn't showing 90 degrees like it usually does. At 65-70 degrees, the needle even stays in place for a long time, and it doesn't increase much until after about 30 kilometers.

I'll log the temperatures tomorrow using VCDS and take a look at the data.

"I realize that I no longer have a supplementary heater, but I'm surprised by how drastic the effect is. Now that the supplementary heater is broken, is there perhaps a safety circuit that's overriding the thermostat for some reason?"

...thermostat malfunction... there is no circuit breaker for it.

hkss wrote:

Ah, I had to read it a few times, but I now understand that even if I drive conservatively/cold-start - even if I didn't use the heater, the error message is still being recorded and saved.

The heating unit has a dual function when installed at the factory. Firstly, it functions as a standing heater, which operates whenever it is switched on. Secondly, it also functions as an auxiliary heater.
If the engine is running and the outside temperature is below value x, and the coolant temperature is below value y, then the heater will automatically start and heat the cooling circuit until the coolant temperature sensor reports value z (or exceeds it). Then, the auxiliary heater will turn off again. This is roughly how a simple program works. Some models also operate with two heating levels. This happens without any intervention from the driver. If the auxiliary heater doesn't receive any fuel because the tank is empty, it will react in the same way as a stationary heater. It will turn off and generate an error.
If this situation occurs, there might also be air in the system. In that case, you need to bleed the system again. You can do this by repeatedly turning the heating on until it circulates properly. If it gets blocked, you need to unlock it again. If nothing else is broken, you might have solved your problem with this.
In the Golf V with the original auxiliary heater, you can prevent the auxiliary heater from starting by setting the Climatronic to "Econ" mode. In this mode, it will not activate. This is particularly useful for vehicles that are driven short distances, as it helps prevent the heater from becoming clogged. It is also helpful when there is little fuel in the tank. Older models did not have this option, and there were frequent problems with the auxiliary heaters.
Vehicles equipped with factory-installed fuel-powered auxiliary heaters do not have an electric pre-heating function. If the auxiliary heater fails, this is very noticeable on the coolant temperature gauge. There is no protective circuit or similar device that interferes with the thermostat's operation in this model. The thermostat is a classic design, not a temperature-controlled type, and is located at the front of the engine block, next to the oil filter.

So, my tank was never completely empty, and I can rule out air in the line.
After 4 attempts to start, all of which proceeded in the same way (see the original post), I'm going to...
Let's just assume there's a defect, so I don't need to try starting it again.
The past winters have all gone very well, to our complete satisfaction.
My car is currently experiencing its very first cold starts in its lifetime.
My car has never been started in cold weather before.
Above +4 degrees Celsius, the water is initially preheated until it reaches 80 degrees Celsius. This has always been the procedure, and there has never been an exception in my case.
That's why I said that my auxiliary heater has already accumulated a very large number of operating hours.

But regarding the thermostat:
Today, I logged the water temperature during a 28km ride.
Cold start after being parked overnight.
I didn't like that the engine started to slow down at a coolant temperature of 60 degrees Celsius.
The temperature in the radiator is rising. Otherwise, well, after about 25 kilometers, it started to slow down.
It once reached 70 degrees Celsius for the engine coolant, which I really didn't like.

I'm going to rephrase the story about the empty tank again :
The heater draws its fuel through a separate line directly from the fuel supply unit in the tank. However, this fuel intake point is located higher than the fuel line leading to the engine. Therefore, the heater stops receiving fuel before the engine can run normally on reserve. This is intentional to prevent the heater from emptying the tank. If this happens, it is possible that air will be drawn into the line, which can cause the heater to malfunction. This occurred for you at 92,000 km. See your own post in the original thread.
"Did the heating unit function properly between kilometer readings 92,000 and 102,000 (entries indicate flame interruptions)?" If the answer is "yes," then the burner assembly is a prime suspect. Unfortunately, with a Webasto TT, you can't get away with using small parts like this Eberspächer flame filter. And for many Webasto heating systems originally installed in vehicles, you can't buy the individual parts from retailers; you have to purchase them from a car dealership.
If I were to repair it, I would still compare the actual flow rate of the dosing pump to the target value before ordering any parts, just to be sure.
You should probably focus your efforts on getting the auxiliary heater working and leave the thermostat alone for now. The auxiliary heater is an important component of the temperature control system, and in your case, it's known to be malfunctioning. Get it fixed first and then see how the temperature behaves. You can easily replace the thermostat later.

Thank you very much, Steffen, for the detailed explanation - now it's clear to me - thank you.

Since I still have the premium warranty, I "have to" get it checked and repaired first.
I'll get back to you and let you know.

P.S. Update: I just spoke with the repair shop.
Get the burner and thermostat replaced, and the bill will be settled.
regarding the warranty service - I only have to pay my share.
Well, I think that's actually good news. I can't just...
To have bad luck

Here's the log from the exact same route and under the same conditions as before, but this time with a new thermostat.
Changing the thermostat was a good idea of mine.
The new thermostat seals pretty well up to 50 degrees Celsius, and it only starts to open gradually at 60 degrees Celsius. The old thermostat was already letting coolant in at 30 degrees Celsius. That probably meant the warm-up phases were much longer before.

Wenn ich nur wüsste wie man von der 1. und vom 2. Log file only.
It would be helpful to combine the two data points from the engine/radiator coolant into a single file, so that they can be observed together using VC-Scope in the same flow diagram.
Unfortunately, it's not possible to call the VC-Scope function twice.