Miles in tank display - adaptation?

[CliveH]

Following on from Dippy's excellent thread explaining how to calibrate the MPG reading using Vagcom, I wonder if there is a similar adaption procedure for this?

Driving back from the Northern Meet last night, my DIS was showing 10 miles left in the tank as I came off the motorway, so I pulled into the garage on the way home and put £5 of 95 in to get me to work the next day.

The display then promptly read 45 miles to go, which may have been a bit optimistic, but lets say at least 30.

I drive home, 1 mile away, park on the drive, let the engine idle for a few minutes, the display still shows 45 miles (and no low fuel warning sign).

The next morning, I switch on and it shows 0 miles. Our drive is on quite a slope, so I think it must be the angle of the car and position of the fuel sensor. But no, I drive the 15 miles to work (in the knowledge that I've put over a gallon of fuel in it) and it shows 0 miles all the way...

I've had these sort of discrepancies before.

Is there an adaptation procedure, or is this display just pants...?

[SimonS4MTM]

Mine always shows less in the morning, it then creeps back up as I drive to work. I drive 20 miles to work.

I put it down to running rich on cold starts.

[davidhodgkinson]

no clive, it's the car telling you that you should'nt go to work as you've got no petrol

It just left enough for your trip home last night so you could sleep sound...

[tartan_rob]

Is this not more likely to be due to a fuel sender unit issue? I'm sure I read this before where the sender unit gets stuck for a short time and reads slightly more (or less) fuel than in reality. This does not stuff up the range calculation as it is simply working out figures on the info it is receiving. I'm not sure but this might be to blame. Clive - Does your fuel gauge on a decent run move normally and then move really slowly or quickly? That is a sure sign....

[CliveH]

Thanks Rob,

I would say that the range display typically shows a rather optimistic figure when the tank is full (anywhere between 280 and 320), which becomes progressively more realistic as the tank empties - I get between 220 and 250 (I dont get many long runs in).

The most quirky point is around 50 miles range, when it can suddenly go haywire - but this only normally happens after switching off, as above - turn on the next morning and, hey presto, it's all gone - down to 0 miles...

[KayGee]

Clive mine does first bit, shows 320-360, ends up around 260. Doesn't do 2nd bit. Does sound like sticking fuel sender unit.

[CliveH]

Thanks Kevin.

It's something I can live with - just thought there might be some calibration type thingy that might sort it out. Where's Dippy...?

[Dippy]

No you can't change it.

Think about it. The ECU is making a forecast of how many miles you can travel on what fuel remains. It can only do this based on historical data, and with the algorithm it has been programmed with. I tried to find out what this was, and what data it is based on, but couldn't find anything.

My guess is that it's quite simple, and has been geared to encourage you to fill up sooner than you need so that Audi don't get customer complaints of "I ran out of fuel when the DIS said I have 10 miles left". I think it uses a rolling average. So if you give it a good blast, the figure will come down, but will recover a bit afterwards. However it must retain an overall average in the algorithm because it never fully recovers.

I think it also changes when the fuel is low. When I have a full tank or a half-full tank, I can persuade the range to go quite high by driving off boost. However when it is 30 miles or less, it NEVER goes up even if I totally baby it. Also, turning off the engine when the reading is very low can totally screw it up, as Clive has seen.

[CliveH]

Thanks for that Dippy. Top post, as usual

That's my guess of how it works, just don't understand why it freaks out when you turn off on a low tank - as you say, maybe it's just the programming (Audi) being cautious.

I had an even weirder situation with my 2.6 quattro - when it got very low, the next time you looked at the DIS the reading would show 1,350 miles range! (yes, really) This would slowly (well over about 10 miles) return to 0 - at which point you knew you really were f***ed...!

[shineydave]

on a similar vein (and i'm not sure whether i'm imagining it) my fuel gauge actually seems to go UP if i drive steady. ie. if the DIS range remaining reading goes up the fuel gauge seems to go up in tandem. i don't seem to get anomolous readings on the range remaining though other than i can never get much above 50 litres in the tank when the DIS shows the range as zero.

[Dippy]

Well the ECU drives all the gauges on the dash, so it COULD have some programming in it that will adjust the gauge position according to other parameters. Mine does change position by surprise sometimes but I'd been putting it down to a possibly dodgy fuel sender.

Food for thought.

[CliveH]

on a similar vein, my fuel gauge actually seems to go UP if i drive steady. ie. if the DIS range remaining reading goes up the fuel gauge seems to go up in tandem

I get the same - thought I was imagining it!

[ChipHazzard]

When I fill up my tank range normally shows 230 miles, is this a potential problem in the offing ????

[CliveH]

Does it always show that?

What is your normal driving pattern (eg town, motorway, mixed, nail it everywhere, etc)?

[Scotty]

My understanding was it's largely based on the last 30 miles. If you are therefore working in a city and then once out going up the motorway, by the time you get near home, the prediction will increase as the last part of the journey is more efficient.

It seems to make sense on the readings I've got, not only in the S4 but also in the TT before.