An observation

[Jocko]

I wonder if any other Mk 1 owners (and possibly 2 and 3 owners) have noticed the following.
You return from a trip, and the fuel gauge says X; the following morning, the fuel gauge says X - a lot.
Yesterday I returned from my usual trip to visit mother, looked at the gauge and thought, there is enough for another two trips. This morning I got in, and the gauge is considerably lower, and it looks like one more trip over will be it. If the car wasn't locked and in a locked garage, I would think someone was syphoning some off.

This reminds me of a story (well, the thread will wander off, so may as well wander from the start!).
Back in the days before locking fuel caps, a TV personality (I cannot remember who) told the story of a neighbour who bought a brand new car and boasted about how many mpg he would get (could be me). So the storyteller and another neighbour hatched a plan, and every couple of nights, they would add petrol to the car's tank. This continued for the entire winter and only stopped when the light nights returned. The poor owner suddenly noticed this drastic drop in mpg and had the car back to the dealer. Of course, they could find nothing wrong. He then went to another dealer in a nearby town with the same result. The poor chap was getting distraught before they owned up and told him about their ploy. Great to have neighbours like that.

[springswood]

I've not had that with my fuel gauge. Sounds like the float in the sender may be leaking.

I do like the fuel prank. Makes me wonder who's been putting fuel in your tank all these years 

[Derkie54]

Not doing many miles at the moment due to lockdown but a previous VW I had used to do this.
Filled up to a certain level and without driving a few hours later it was lower on the gauge.

[Jocko]

Sounds like the float in the sender may be leaking.

I think if that were the case, it would just go down and stay down. And when I fill up, it always returns to the top.

[UKjim]

Not noticed it on our Mk1.

As regards the fuel prank, I'd rather have a neighbour that puts petrol in rather than taking it out!


Sent from my iPhone using Tapatalk

[embee]

Are you parking on a slope? Gauges usually have pretty heavy damping so don't respond very quickly, if you are driving on level roads then park on a slope it will still show the "level" amount, and slowly alter to reflect the slope after you've left it.
Don't know about the Jazz, but my Yaris shows more fuel when going uphill, it will often gain an extra bar on the gauge...... and promptly lose it again when it levels out.

Just a thought.

[culzean]

There is a thing on analogue gauges called 'hysteresis' - what it means is that due to friction or magnetic lag when a needle is falling it reaches a different position to when it is rising.  So as the fuel level in tank drops the needle is falling and it takes up a position,  but when power goes off instruments and is re-applied when you turn ignition back on the needle is rising and reaches a different position on the scale.  The same can happen with a float,  that due to friction it can reach a different stable position when rising to the one it will reach falling,  due to the movement of the vehicle and fuel in the tank sloshing a bit, and if you are going uphill or downhill means the position the float reaches can be rising and falling regularly,  but damping ( slowing down the response ) in the mechanics means that the needle on the gauge does not move noticeably while you are driving.

[Jocko]

Are you parking on a slope?.

No. The car is parked in a garage with a level floor. There is no gauge movement when driving (although when the low fuel light comes on, it is usually when cornering).

[Jocko]

when power goes off instruments and is re-applied when you turn ignition back on the needle is rising and reaches a different position on the scale.  The same can happen with a float,  that due to friction it can reach a different stable position when rising to the one it will reach falling

The thing is, it doesn't happen when you turn the ignition off, then, shortly afterwards, turn it back on. It is only evident after a couple of hours. And the float doesn't move because the car is sitting with a stable amount of fuel.
It is as if the float is in a separate part of the tank from the outlet, and there is only a tiny hole to level the fuel (I know this is not the case as the filter and the float are mounted on the same assembly).
Just strange but not a great issue.
Another observation is that I have to cover about 100 miles before the gauge moves of the top stop after filling up, but that could be because the fuel in the filler pipe has to be used up before the float would start to move.

[sparky Paul]

There's a circuit in the instrument panel using a couple of capacitors to heavily damp the reading, ironing out any bounces, bumps, and hills, etc.. After the car has been stood for a few hours, the capacitors have dissipated, and the reading matches the sender voltage.

No magic going on 

[Jocko]

I understand that, but why does the gauge not return to the original position when the capacitors charge back up?

[sparky Paul]

I understand that, but why does the gauge not return to the original position when the capacitors charge back up?

Some part of it may be because the gauge was over-reading for the position of the sender where you have parked up. In other words, the tank sender is reading a lower level where you parked than while driving, and immediately after parking up, you are still seeing the driving reading.

There is another issue in that the way the simple circuit used charges the capacitors. There will also be a resistance involved to limit the current to and from the capacitor to give the required RC damping - and a voltage will be dropped (or lost) across that resistance. When the sender is moving up and down, the error will be both positve and negative and will average out to give a truer reading, but when the sender is static, there will always be an under-reading error, from the discharged state.

Hope that makes some sense. After reading it again, I'm not so sure about it myself!

[embee]

I don't think modern digital based systems use capacitor charging principles for meter damping any more, it's all done electronically, time averaging etc., it's your algorithms ain't it.

[sparky Paul]

I don't think modern digital based systems use capacitor charging principles for meter damping any more, it's all done electronically, time averaging etc., it's your algorithms ain't it.

I think there were still quite a few analogue circuits in the mk1 dials, there's a schematic floating about somewhere.

The effect is essentially dead-band damping, which will always give you a negative error on a step change from zero. Perhaps the algorithms employ a similarly simplistic control method.

[culzean]

This phenomenon has happened on every car I have ever driven. The needle is higher when you switch off than when you switch back on.