Low fuel light goes out again?

[Supervinnie40]

I think it has been asked before, but I couldn't find a topic that really answered my question.

Today it happened to me again. When I got in the car the Low Fuel light started glowing. I knew this day was coming. I have been dreading it for weeks. Some day it just had to happen, time to fuel her up.
However, thanks to trial and error I know how far I can drive when the light burns and I knew I could easily drive to work and go to the petrol pump on my way back.

Then something strange happened, the light went off half way to my work. And it stayed off. During my lunch break I went to the shop (5 minute drive) and the light didn't go on.

At the end of the day I left to go home, and the light went on as soon as I got in. Before I got to the petrol pump, the light was out again.

This makes me wonder, why does that happen? Is the light programmed to go out after a certain amount of time? Which wouldn't make much sense. Is the first indication based on a guess, and it adjusts itself later while driving?
I can see how the light can go on or off while driving (for example when you drive up or down a steep hill, or when taking a very sharp corner), but this would fix itself as soon as you are flat again.
Also, I live in the Netherlands, our biggest hills are the sleeping policemen in our streets.
I can also understand why the ligt can go on and off when it sits at the edge of the sensor. But I drove about 40 kilometer since it switched on for the first time. This means the fuel-level dropped about 3 to 4 liters since then, yet it still went out after about 20 minutes.

And its nothing new, it has done this since as long as I remember (just never thought about asking this question).

So what is happening? Can anybody explain this?

[Mickyboy]

From personal experience of lots of 75’s and ZT’s I tend to find that the car calculates how much fuel you have based on various bits of info, like speed/engine temp/throttle demand/temp/boost pressure etc, so when you first drive off it calculates a low figure to be on the safe side.
Mick

[clf]

fuel gauge uses amongst other things fuel economy. When starting off the engine is cold, and at its most uneconomical, so the light is on. After driving more economically or warming up, the needle can sometimes be seen to rise. I know certainly, mine can! Some days I can come home and mot realise I need to get fuel on the way home (I only buy it from one place). Then in the morning, the light comes on, perhaps showing on the IPK 50 miles remaining. I know it will be fine for my 10 mile commute, so it becomes a challenge to see how far above 100 miles remaining by the time I get to work. My best has been 48 at home, and 112 by the time I reached work :d:

But yes it will go out, when the light first comes on, take a mental note of where the needle is, then drive economically for 10 miles or so, and then check out where the needle goes . It will come back on again eventually though, it doesnt make fuel lol.

[lady zed]

I find the 75 quite generous when the fuel light comes on. I can usually get at least 60 miles.......when the fuel light comes on in the scenic you literally have to stop at the next petrol station.......

[Supervinnie40]

I always used the average of 10% left in the tank when the light goes on. In my case that means 6.5 liters, which should be good for 84.5 kilometers. Enough to drive for almost 2 days, having to fuel up on my way back.

My previous car had a sensor in the tank, it would say the tank was half full if I parked it on a slope :P. The 75 seems a bit smarter.


On a totally offtopic note: love your signature Lady zed!

[Supervinnie40]

Originally Posted by clf

I know it will be fine for my 10 mile commute, so it becomes a challenge to see how far above 100 miles remaining by the time I get to work. My best has been 48 at home, and 112 by the time I reached work :d:

But yes it will go out, when the light first comes on, take a mental note of where the needle is, then drive economically for 10 miles or so, and then check out where the needle goes . It will come back on again eventually though, it doesnt make fuel lol.

Do you mean you drive further then the IPK says you can? :P

[clf]

Originally Posted by Supervinnie40

Do you mean you drive further then the IPK says you can? :P

No, well I dont know. I have never let it go long enough to find that out lol. But what happens is this

I start the car, light comes on. I curse myself for not noticing the night before coming home driving past the fuel station lol .

I check the IPK, which will display for an example, 55 miles remaining.

Drive from my home 1 mile to the main road and it will then display 50 miles remaining! The needle on the gauge has not noticeably moved.

It is more or less a straight run for the remaining 9 miles to my work, with 4 miles of motorway. So as I am able to maintain a relatively comfortable speed, fuel economy rises. After another few miles I can check it again, and the display will read perhaps 70 miles remaining, and the light will have gone off.

Going onto the motorway, and maintaining a steady speed, it will rise even further. In that 10 miles commute, the remaining distance normally doubles.

The fuel gauge needle will also rise, as an example from 1/8th of a tank to 1/4 tank showing.

However, if I drove hard, or drove around town in an uneconomical way, it will rarely rise above that 55 miles remaining, and normally will drop quite significantly, sometimes 3 miles for every 1 traveled.

Ultimately the gauge and IPK is based on what the in tank float says is left, and calculated based on current fuel economy.

My ex's father was fascinated by it. We went out one night with half a tank of fuel, and drove for around 15 miles. He was offering to pay for fuel, which I declined, but when we reached our destination, I showed him how the gauge had risen noticeably above the half way line lol. (I was always super observant of speeds and economy with him in the car lol). He asked how it did it, I told him it made fuel as we drove along!

[HarryM1BYT]

Originally Posted by clf

No, well I dont know. I have never let it go long enough to find that out lol. But what happens is this

The fuel warning light, the gauge and the IPK miles remaining, all seem to be based on a calculation. The calculation seems to be one involving the tank sensed levels from the two floats, the distance being travelled, plus the instantaneous quantity of fuel being injected. On initial starting from cold, all three will be quite pessimistic, but once out on the road - the light can go off, the fuel gauge reading can rise and the IPK indicated range can rise quite drastically.

[Supervinnie40]

I started this morning with 774 km remaining. I ended up at work with 830 km remaining. I consider that a win

[clf]

Originally Posted by Supervinnie40

I started this morning with 774 km remaining. I ended up at work with 830 km remaining. I consider that a win

Told you, they make fuel :p

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