0 litres VCDS fuel level
 

I always trusted the fuel level in litres measuring block in VCDS.
So yesterday I was driving back home when the fuel level warning came on. I knew, or thought I knew that from that point I had 12 litres left. The trip computer said I had 100km range left and as it happens I had 102km to home. We all know the trip computer is a bit cautious and that there can be several miles range left when range drops to zero.
So the range did finally reach zero after 99km (pretty accurate?), and Indrove on for another 3km to home. Got home fine no funny noises from engine or fuel pump, so I assumed must have a few litres left in there.

Out of curiosity today I checked the fuel level in VCDS and was disturbed to see 0.0 litres left!

I am sure there was at least a litre left in there as there were no signs of fuel starvation.
Just to be on the safe side I take a5l fuel can down to local pertol station only 6mins walk away, fill up 4.5l.
After I pored it in the tank I check VCDS fuel level, now the reading keeps toggling between 3l and 4l, so split the difference he fuel gauge thinks there are only 3.5l in the tank when in reality its at least 4.5l more likely over 5l.

Conclusion, VCDS fuel level reading is not accurate either.

Should be easy to check.
Go to petrol station, read what VCDS says is in the tank then fill up at the pump. Add the VCDS and fill up figures, then compare with your tanks capacity.

I would compare the VCDS figure for the full tank against how much fuel you put in there.
The VCDS output and the trip computer probably use the same reading and there would have to be a safety factor in the reading to ensure the car doesn't actually run out of fuel.
Going the 103/99 km range may have put the tank a half litre below the safety level, which explains your 3-4 litre reading maybe?

Fuel tank is comparably low and wide and irregular shape and the cars is not always level, there is no way you will get very accurate reading and it has to be on the cautions side to avoid running out of fuel. For a floating sensor to register anything there need to be certain level of fuel to lift it. If you have dry tank you can probably put couple of litres of fuel in it and the sensor float will still be resting on the bottom of tank and showing no fuel. I think you would do better with load cells checking weight of tank than 2 floating sensors.

Isn't there also a way of doing this with voltage drop along a wire indicating volume?
Can probably still fluctuate like a floating sensor though...

I didn't realise they use a float, that's reassuring that even when it reads zero there are a around a couple of litres in there. Not that I plan to use them :)
I will try the full tank trick, to try and calculate how much was in there. That's assuming the VCDS reading at full tank is accurate also.
The petrol station is only 0.4 miles away so I won't lose much fuel getting there.

I do wish they used a more accurate sensor, and a litre display on the dash, it is more reassuring to see how much fuel is left rather than some range calculation. They could have displayed both anyway!

Yes, the straight tube that goes in through the fuel pump housing contains a float for fuel level. This is a float within the tube.
On the left hand side of the tank is a second float which is more like a toilet cistern float valve, this is only for the low fuel warning light.

They are both connected in series adding up resistance

I didn't know that!
My observation of the left hand float is that it doesn't go high enough to register a full tank.

Ainar you know everything about these cars. +++
You must have taken one apart and put it back together again. :)
Its good to know these things.

Agreed :D +++

So I filled up to full tank today. I only managed to put 77.3l in and now I am a bit confused. Stopped straight after to VCDS the fuel level it shows 89l.
x + 4.5 + 77.3 = 89

If the fuel level sensors are to be trusted that means there were roughly 7 litres still in there when the range read zero and VCDS showed 0 litres.

Perhaps it is possible? Kind of reassuring, but also annoying, I assume I could have used at least another 3 of those litres without endangering the fuel pump?
Thats roughly another 15 miles!

I reckon, that on both my S8's there was usually about 2 gallons (9L) left in the tank when range was showing as zero.

That sounds about right. I find it so strange that so much fuel is left when range and fuel sensor reads zero. Unless its needed to save the fuel pump, but why so much?

I only ever ran out once in my D3, I had done about 50 miles some of which were towing a trailer since the tank showed zero, and then it only ran out as I was going up hill!!!!! Started up when I put a gallon in that I had in the boot, probably the only time I got more than 90 litres in that fill!

There is a big margin for sure, and I suspect the tank and filler is marginally more than 90L also +++

I reckon if you brim the tank and have good tank and sender seals in a D2, it has an extra 1 to 1.5 litres in the filling neck too. Probably similar in a D3. I doubt these are calculated in tank capacity....

Good info, thanks guys +++

Next time it goes to zero I won't worry so much.
I wonder how mpg calculations change when we account for all this unused fuel, does that mean our cars actually do better mpg than we think?

I think on most petrol cars of this vintage mpg is calculated solely using data from MAF sensor.

I didn't know that, thanks +++
Damn that probably means its fairly accurate.

......Picking up on this debate, the S8 stuttered and spluttered with 40 miles range left yesterday.

Last time this happened was 11pm Christmas night about 3 miles from home and it stopped completely.

At that time I went through the whole fuel pump not seated issue, assumed that at some stage it had been monkeyed with...... but not to be.
Turned out the top had popped off the driver side fuel sender tube and the float seemed to stick/jam near the bottom of the tube.

Anyway duly fixed and reassembled. Ran around for a couple of weeks with 5lt in a can in the boot to see if it was sorted. Ran the car right to 0 miles range, needle firmly in the R zone without a hiccup.

...fast forward to yesterday, thankfully this time I was about 500yds from a petrol station and nursed it to a pump.

Filled to more or less the brim - 88.35lt
Spec sheet states 'Tank capacity 90lt'
Rough calculation 40 miles range @ 20mpg = 2 gallons/9.09lt left
I doubt I'm getting 40 miles on 1.65ish litres of fuel!

Looks like I'm going fuel sender purchasing/hunting :-(