Xantia – fuel evaporating

[lancia58]

Hi

I am not 100% sure but I looks while the car is parking some fuel is evaporating. I look for fuel leaks and there arn’t. Can it be that while the car is parking fuel is evaporating ?

Thanks
Zohar

[Mandrake]

What leads you to believe fuel is evaporating ?

[lancia58]

The fuel gauge shows this

Zohar

[Mandrake]

Can you be more specific ? Do you mean the reading is less when you return to the car than when you parked it ?

Fuel gauges are not very accurate, unless fuel was pouring out on the ground you're not going to see a difference in reading when parked that isn't due to the inherent inaccuracy of the gauge.

The minimum change that a fuel gauge could show would be approximately 5 litres and I doubt you're losing that much due to evaporation!

[CitroJim]

Modern cars don't evaporate their fuel. The system is pretty much sealed and vapours in the tank are captured in the charcoal canister.

There's strict regulations on fuel evaporation as that is one of the major ways hydrocarbons pollute the atmosphere...

If you are evaporating a huge amount then you would smell it.

I agree on fuel gauges in Xantias. They never read the same twice

I note the gauge reads differently on all of mine when restarting from rest.

[Chlorate]

Xantia fuel gauges are funny creatures. I have a feeling there's a bit of coding built into them somewhere so they "remember" what the fuel level was when it first set off.
For example I'm usually parked on a little hill at home, so when the fuel level is low the light will come on and stay on until I've parked on something flat and started up again.

[CitroJim]

Xantia fuel gauges are funny creatures. I have a feeling there's a bit of coding built into them somewhere so they "remember" what the fuel level was when it first set off.

I don't think they're that clever It's more a quirk of cheap and poor design rather than anything deliberate!

I do find S1 gauges a bit better than S2 gauges but not by much...

[Mandrake]

Xantia fuel gauges are funny creatures. I have a feeling there's a bit of coding built into them somewhere so they "remember" what the fuel level was when it first set off.

I don't think they're that clever It's more a quirk of cheap and poor design rather than anything deliberate!

They have more smarts than you give credit for Jim.

As I'm sure you know, a fuel sender is positioned such that it is heavily influenced by the fuel tilting in the tank from the car leaning to one side or the other, for example parking on tilted ground, or cornering during driving.

There is averaging with a very long time constant (maybe 10 minutes or more) applied between the reading the sender sends and what the gauge displays, if there was not the gauge would be swinging wildly up and down every time you cornered while driving and would be wrong if you were temporarily on tilted ground.

I don't know what they use on a Xantia or nowadays with digital gauges, (probably just a couple of lines of code in the BSI!) and I've never traced the circuit for one but I'll bet in the old days of analogue gauges it was nothing more fancy than an RC filter with a long time constant and a buffer transistor to drive the gauge.

There is also some sort of "reset" circuit (possibly as simple as a transistor shorting the R in the RC when the key is first turned on) which brings the gauge quickly to the real reading when the key is turned on the purpose of which is to allow the gauge to come to the correct reading instantly after filling the tank - under the assumption that the tank is filled when the key is off!

The low fuel warning light on the Xantia also latches - I used to park on a slope at my previous work and it was common when the tank was getting a bit low that when I'd arrive there would still be a significant fuel reading (averaged reading while driving to work) but when I'd start up to go home not only would the low fuel warning light come on and stay on for the entire journey home, the fuel gauge would read exactly zero a well.

When I got home on more level ground if I turned off for a few seconds and turned on again the warning light would go out, and the fuel gauge would jump back up to normal and I could then drive many miles without the warning light coming again...so the warning light latches until the key is turned off.

I've noticed when the warning light comes on during driving that the gauge instantly drops from a 10% or so reading to zero, so there is some connection between the two such that the gauge is artificially set to zero, perhaps to reinforce to the driver that they really need to get fuel even though 10 litres are left...

Most likely all that is happening to Zohar is that he parks on a sloped section of ground (leaning downhill towards the right hand side of the car) and before the key is turned off it is holding the correct averaged fuel reading during driving, after the next start he is getting a lower than normal reading due to the tilt of the car, which will take many minutes of driving to gradually correct. (Unless it was low enough to trigger the warning light in which case it will not correct until turned off and on on level ground)

Zohar - if you do indeed park on sloped ground, try parking 180 degrees in the opposite direction, if this is the reason for your fuel gauge difference it will read too high instead of too low next time you start!

But seriously, I would not worry about it. You're not going to be evaporating fuel without a very pungent smell...

[Ben82]

There is also some sort of "reset" circuit (possibly as simple as a transistor shorting the R in the RC when the key is first turned on) which brings the gauge quickly to the real reading when the key is turned on the purpose of which is to allow the gauge to come to the correct reading instantly after filling the tank - under the assumption that the tank is filled when the key is off!

My c5 gives me enough time to think "oh crap nothing has gone in" before going "ooh extra fuel" lol still catches me out on occasion when I forget it takes a moment for it to readjust to being filled up.

[CitroJim]

Gosh, thanks Simon I never, ever realised there was so much to a fuel gauge...

I guess it's because I never let any of mine drop below an (indicated) half ever and have only seen the low fuel light on once...

I am OCD about keeping tanks full of petrol... When the needle gets near half-full I start to get nervous...

[Mandrake]

Sadly Jim, I'm all too familiar with the operation of the bottom end of the fuel gauge, and the low fuel warning light and I are on a first name basis. Things have improved since the new job, previously I could rarely afford to fill the tank.

I'm sure the Xantia gauge must have electronic averaging or peak hold averaging of some sort but now I think back I remember the fuel gauge in the GS had some sort of mechanical averaging, perhaps a heavy mass loaded but balanced spindle or some sort of vicious damping as the gauge was quite slow to rise when the key was turned on and slow to fall when turned off, in the order of 10 seconds or so... Despite that I remember it used to change reading during any prolonged cornering through long sweeping bends. The Xantia gauge has a lot longer time constant but is quick to rise and fall with the key so I assume its an entirely electronic low pass filter of some sort.

[Northern_Mike]

Sadly Jim, I'm all too familiar with the operation of the bottom end of the fuel gauge, and the low fuel warning light and I are on a first name basis. Things have improved since the new job, previously I could rarely afford to fill the tank

Same here. In times not too distant I could never afford to put more than about £25 in the car. This is a habit I still have, even though I could fill it. There are two reasons..

1. It makes me drive more economically having less fuel in.
2. Up until recently when we were poor I drove tat that was likely to break down or simply die. I didn't want a dead car with a tank ful of fuel to retrieve before I got rid!

[CitroJim]

That last point makes good sense Mike...

I recall a V6 we broke a few years back. It was nearly full of fuel.... That was a bonus for us...

[dnsey]

The averaging response of old style guages was achieved very simply. The meter movement was a hot wire type, with significant thermal inertia, so relatively rapid fluctuations were not registered. The system was sensitive to voltage variation though, so a crude voltage stabiliser was usually incorporated.

[Mandrake]

Ah of course, I'd completely forgotten hot wire meter movements existed. That will be exactly what the GS used... not so a Xantia though, so there must be some sort of electrickery involved.