Car fuel gauge oddity

[Clive Arthur]

I put some petrol in the car today , not much about 10 litres.

It didn't seem to register on the gauge, that's a bargraph with limited resolution but even so that was strange. Stranger still was that the 'miles to go' figure didn't change either, it stayed on about 90 miles.

I've never seen that before but I usually put more in, albeit infrequently, it's not used much. The only difference today was that I parked at the pump on the 'wrong' side because the garage was busy, and wasn't close enough to get the nozzle in fully and it was upside down. The petrol did go in the car and not on my feet.

I had assumed that the fuel gauge was a float on an arm jobbie, but maybe it's more complicated. Any ideas?

It's an 04 Citroen Xsara Picasso.

[ARW]

I am interested.

The hire van that I used to move a piano never altered the fuel gauge or the "miles to go" range on the dash despite shoving 4 gallons of fuel into it from fuel cans.

[Bob Eager]

I got a similar effect when I filled up my AdBlue tank. It righted itself a mile or two leter, though.

[Jethro_uk]

No answer, but last year, when our car was in for repair, we were loaned a (really shitty) Toyota, with the usual stipulations about returning with same level of fuel.

I drove 6 miles in it, but mysteriously the gauge dropped a bar, and the estimated range dropped by 20 miles. I put the minimum 2L in before it was collected and the gauge and range didn't shift. I put another 2L in and still nothing happened.

I kept the receipt ready for a bunfight with the hire company and called them before it was collected to avoid any problems, but even when they collected it 2 hours later, the gauge hadn't changed.

It was a *really* crap car.

[mm0fmf]

fuel used is calculated by the ECU by how many injections and how big they were. So as you drive it knows how far and how much fuel used. It can then use that for mpg. Remaining range is updated as it knows how much it had and how much is being used.

The initial range comes from how much in the tank and how much is used. i.e. tank reads full, car sets max possible range from known last mpg. As you drive, fuel use is calculated and mpg/range updated. Next time level changes, i.e. sender indicated next level, 9/10ths full, range is re-evaluated based on current mpg and known remaining fuel.

If you add fuel but not enough to cause the sender to move to the next level, the range calculations will not know you have added more fuel so the range wont increase but will keep decreasing as you drive. The range will re-calculate at the next time the sender level changes, probably upwards.

[Brian Gaff (Sofa 2)]

What you are really saying then is that its a bodge in the first place to avoid a finer grained measurement of the fuel level in the tank. Bring back analogue. Brian

[Steve Walker]

I've had the same thing with a Zafira on more than one occasion. It's righted itself either later in the journey or the next time I have used it.

SteveW

[The Natural Philosopher]

seen this before. what you see on te dash is heaavily damped to prevent the numbers going up and down and frightening punters but there is an override that will instantly register large changes.

It should sort itself out.

[The Other Mike]

The fuel gauge on the dash will in all probability have no direct electrical connection to the sender, instead it could be driven from an ECU output. The dash computer may even just sniff a CANBus signal representing that fuel level, interpret that and then drive a bargraph, a stepper motor or a moving coil device with a pointer on the end.

The actual level from the fuel tank sender might be digitised to 8 bits or 256 levels, or even more, so the possibility of putting say a litre of fuel in a tank holding 60 litres and the actual level measured by the ECU not changing is pretty small.

But the gauge itself might only be driven to one of only say sixteen discrete levels. In the past they were heavily damped, now that is effectively done in software.

The initial range after refuel is calculated from the actual measured fuel level and the rate of usage from immediately before the refuel, and updated with the actual usage when the engine starts. If you coast down hill for many miles before refuelling your range can be drastically overstated.

Similarly if you dispense small amounts (maybe 3 litres) of fuel from a can into the tank, the range instantly updates on an ignition on event without the vehicle starting and turning a wheel, even after it has been sat unused for several months.

But as ever your specific vehicle range measurement techniques can vary.

--

[fred]

Fuel tanks on cars look more like some internal body organ out of ones gut. The very irregular shapes make them hard to measure the contents.

Generally I find they are forgiving and will go a little further than anticipated except for one humpty dumpty Hillman Hunter I had which would stop dead once they needle hit zero

[Andrew]

That model commonly suffers from failure of the sender device.

Neigbour has been asked to fix a couple, and when he discovered the price of the replacement sender unit, the owners decided to just use the trip meter to warn them when to refuel.

[Commander Kinsey]

That sounds like a very shoddy design. Is it a Renault? My fuel guage moves more quickly for one half of the guage than the other (unfortunately I can never remember which), as for the miles to running out, it's complete and utter nonsense. When I fill the tank it says something like 450 miles. I can only do 300 on a full tank.

The only useful thing that French piece of shit ECU does is show me a very accurate average speed, which means when driving along a motorway with SPECS cameras, I can get stuck behind something, then drastically exceed the speed limit for the rest of that section, while keeping my average speed legal.

[ARW]

I would have expected 4 gallons to have moved the sender unit.

However it was a hire van and I only used it once.

My car will pick up the addition of 1 gallon of fuel from a can.

As to the range:-) Both the van and the car do weird and wonderful things with that. I still have not decided if they recalculate after time or mileage.

Both can knock 20 miles off the range after either a short steep up hill run or just a bit of speed but then adds 20 miles back on as you either go down the other side of the hill or take your foot off the accelerator when you hit the 50MPH roadworks.

The car ALWAYS knocks 5 miles off the range when you start it up. So in theory 120 starts of the engine should use a full tank of fuel.

[Clive Arthur]

The next day the 'miles to go' was still 90 miles and I commenced my 100 mile round trip journey.

Up till about 30 miles, mostly motorway, the miles to go crept up to 130 miles, then started to decrease. Of course, this figure does often increase a little depending on the changing MPG, but this amount was quite unusual, as if I was being topped up by an invisible tanker as I drove, which I suppose can't be ruled out.

Anyway, I got home with 80 miles left which is about right for that journey.

I do like that old car. One of the passenger doors stopped locking for about a year, didn't matter much, then it fixed itself at the same time as the various dashboard messages decided to change to German.