New to me car, and after filling up the needle doesn't move at all for what feels like ages, then it starts dropping normally, then the last quarter disappears alarmingly fast. First car I've had that does this so obviously. Is the sender failing, or is this a normal thing that I've just never noticed before?
Best thing you can do is stop reading the gauge and start tracking the trip meter. Fill up, zero it, and after a few tanks you'll know that your car does around 550 km comfortably and 620 if you're being brave. That number is more accurate than any gauge and it doesn't lie on hills.
Also note the odometer at each fill and do the arithmetic once. Consumption creeping up over months is one of the earliest signs something is wrong, and the gauge will never tell you that.
Normal, and it's mostly the shape of the tank.
The sender is a float on an arm that sweeps through an arc, and the tank is not a neat cylinder. It's a moulded shape that fits around the exhaust, the suspension and the floor pan, so it's wide in some places and narrow in others. Where the tank is wide, burning a few litres barely lowers the level and the float hardly moves. Where it's narrow, the level drops quickly for the same amount of fuel.
On top of that the float can only travel so far, so a genuinely brimmed tank holds more fuel than the sender can distinguish from full. That's your first 80 km.
Manufacturers also deliberately damp the signal and bias the top of the gauge, partly so it doesn't swing around every time you go up a hill and slosh the fuel to one end.
The tank shape explanation makes the fast last quarter make sense too, which I hadn't connected.
Right. And there's usually a deliberate reserve below the empty mark, often 5 to 8 litres, because running a fuel pump dry damages it. They'd rather you thought the car was pessimistic than have you calling for a tow.
Also, brimming the tank right to the neck after the pump clicks off is worth avoiding on modern cars. It can push liquid fuel into the vapour recovery system, which is an expensive part to replace for the sake of an extra two litres.
@warranty_wes · last wk.
What would worry me instead: a needle that jumps around while you drive, drops to empty and comes back, or a gauge that reads full with a tank you know is half empty. Those are sender or wiring faults. A slow top and a fast bottom is just geometry.
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