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-   -   0 litres VCDS fuel level (https://forum.a8parts.co.uk/showthread.php?t=12934)

Goran 24th April 2017 08:54 PM

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.

tonupkid 24th April 2017 09:48 PM

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.

27litres 25th April 2017 04:28 AM

Quote:

Originally Posted by tonupkid (Post 127102)
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?

ainarssems 25th April 2017 08:16 AM

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.

27litres 25th April 2017 08:28 AM

Quote:

Originally Posted by ainarssems (Post 127118)
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...

Goran 25th April 2017 11:18 AM

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!

27litres 25th April 2017 12:43 PM

Quote:

Originally Posted by Goran (Post 127126)
I didn't realise they use a float

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.

ainarssems 25th April 2017 02:14 PM

They are both connected in series adding up resistance

27litres 25th April 2017 03:10 PM

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.

Goran 25th April 2017 09:44 PM

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.


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