VRS fuel gauge strange goings-on!

[boylers11]

I've done 140k miles in my ownership and normally get between 55-60 mpg which gives me 550-575 miles per tank (going by the gauges). The needle and digital readings are normally fairly consistent for the whole tank, even when it goes into the red....

 

....for the last few months, for the first 450 miles, the gauges still act as normal, however, when I get to 80-90 miles left in the tank, I still get 55 odd mpg, but within a few miles, the 'miles left' drops very quickly to zero and the needle drops from being just in the red to absolute zero. 

 

When I brim the tank again (e.g tonight 37.6 litres), it suggests that there is actually more fuel in the tank than the gauges indicate. 

 

Whats going on please?

Is it likely to be a float issue or something more complex?

[YS53]

I must admit that I do not usually let the gauge get below 1/4 before I refuel.

However - on our 09 Fabia1.9tdi - when I did let the tank get down to the red sector it always did drop off alarmingly quickly (only did it a couple of times as not worth running out in a diesel LOL),I always expect car fuel gauging to be non linear and that the lower half tankful will not last as long as the top half,which is why I usually keep it over the 1/4 mark.

[KenONeill]

@boylers11 - When you're low on fuel, do you get a warning light? Have you checked the "available range" on the computer? (both if applicable)

[boylers11]

Hi Ken, yes, the warning light comes on at 80-90 left on the 'available range' but then drops to zero within 15 miles or so even though I'm averaging 55-60 mpg

[boylers11]

For 135k miles, both gauges have worked perfectly, it's just in recent months that the 'red zone' has started acting up

[Breezy_Pete]

12 hours ago, boylers11 said:

Is it likely to be a float issue or something more complex?

Sounds a bit like the wiper of the potentiometer that the float moves is lifting off the track at the bottom end of its travel.  You might be able to spot this if you take the pump/gauge sender out of the tank and operate it by hand.

[boylers11]

2 hours ago, Wino said:

Sounds a bit like the wiper of the potentiometer that the float moves is lifting off the track at the bottom end of its travel.  You might be able to spot this if you take the pump/gauge sender out of the tank and operate it by hand.

Thanks Wino, I'll have a look at the next weekend and report back!

[KenONeill]

@boylers11 - As Wino says, or else the bottom of the track is dirty. Either way his suggestion covers how to proceed for diagnosis.

[JWvrs25]

mine does the same. need to check the fuel sender 

[YS53]

I guess I have a deep mistrust of fuel gauges LOL - both in Aircraft and Cars,I know how inaccurate they can be especially at the low end of the scale.

The Fabia almost caught me out last month - we were planning to take our  02 Polo from Perth down to Gatwick to catch a holiday flight.

The night before we left I started the polo engine and it was making a worrying noise so decided not to thrash it for the 9 Hrs/480 miles to go dahn sarf.

We did a last minute swap to the Fabia 1.9tdi which was not full of fuel but the range readout was high enough to lead us into thinking we could drive to Beare Green,W Sussex - fill up there and then take the back road into Gatwick via Charlwood.Even on the M25 the range readout was ample for Beare Green but just before we left the M25 at Chertsey the Fuel 'Bingo' Light came on with 80 miles range showing - I was not worried because we only had 30ish miles to go.However the range kept tumbling rapidly (not helped by some roadworks and some slow traffic) and we actually pulled into the Petrol Station with 15 miles range showing (should have been 50ish).

It would not normally have happened because normally we would fill the car to the gunwales the night before or fill up on the way,anyway lesson learned - not to trust the range readout !  

Edited 18 October, 2018 by YS53

[Breezy_Pete]

2 minutes ago, YS53 said:

both in Aircraft

Reminds me of the saying I learnt during my paragliding days

"There are a lot of old pilots, and a lot of bold pilots

But not many old, bold pilots"

 

[YS53]

Wino - One of the nice things about flying Gliders was no fuel gauge to worry about 

[KenONeill]

@YS53 - Lesson learnt; the "available range" is actively updated as you drive.

[YS53]

Absolutely Ken - and I do not usually rely on any electronic 'Mickey Mouse' readouts,in my particular case (similar to the other 2 cases on this thread) - it is the rate of change of the achievable range that is the problem - bearing in mind that it is with a 'hot' engine and without an intervening engine stop/start.

I always zero the trip meter when I refuel and I then am aware of my 'safe' range,the trip meter however on this occasion had been re zeroed whilst somebody tried to reset the service indicator and I did not have a primary reference for 'safe' range.