How do cars know how much fuel is in the tank?

snowdog said:
Most cars simply have a floater though, in some cars like a 106 it's actually hilarious, one moment you're bombing through a corner and the petrol light goes on as it's nearly ''empty'', the next moment you have half a tank when driving calmly :D.
Click to expand...

Never had that problem in my Saxo, the gauge has damping on it so quick changes in readings shouldn't cause you an issue.
 
Firestar_3x said:
Never had that problem in my Saxo, the gauge has damping on it so quick changes in readings shouldn't cause you an issue.
Click to expand...

It's a 1994 106 Rallye of a mate that does this a lot, rag its nuts of and the petrol light goes on, drive calmly and you have half a tank a couple mins later...
 
My colts were like that, give it the beans and it would quickly drop about 1/4 tank, and slowly creep back up after you stop being a goon.

The BMW is a bit different, hoon it and the gauge will drop a bit, it won't ever come back up though. However, it will stay in that position for ages without falling until it sort of catches up.
 
DampDog said:
It's not the standing that bothers me. It's watching the blinkin numbers spin round to a bigger number every time I fill up. It seems daily there's a change in price. 1979 when I started driving it was 0.98p a gallon.
Click to expand...

I find it best not to look. :D
 
Clarkey said:
Yes I have, both scabby old jap scrappers with steel tanks. Both times, the bottom of the tank was seen to be absolutely gleaming clean.
Click to expand...

I concur.

Both the tanks I've removed from old cars (1998 Scorpio Cosworth - rusted around the filler neck a 1987 Sierra 2.0i Ghia - broke for parts) have been absolutely spotless inside with no sludge or carbon build up or anything.

Also, both tanks had a stainless mesh filter on the outlet and the fuel filter would protect the injectors/engine anyway.
 
I have a Passat which decided on Sunday that I had a little over 1/4 tank left. Since then it has worked its way back up to somewhere over 1/2 tank as I have done another 20 or so miles.

I know it has a float in the tank as I have had it out in the past but there is obviously something else at work here.
 
Ian_Eb said:
I have a Passat which decided on Sunday that I had a little over 1/4 tank left. Since then it has worked its way back up to somewhere over 1/2 tank as I have done another 20 or so miles.

I know it has a float in the tank as I have had it out in the past but there is obviously something else at work here.
Click to expand...

Fuel gauge could be based on the "range" worked out by the ECU rather than directly off the float.


57 Plate Mini Cooper S that I had before the A3 seemed to do something similar - the fuel gauge was made up of 10 LED segments - if you got down to say the last 2, but then drove efficiently the range would increase and upon restarting the car would on occasion go back up to 3 bars until the range reduced again.
 
DampDog said:
And get ready for a bill. Run the tank to empty and all the dirt and crap from the bottom of the tank will get dragged up into the fuel filter or worse. If you're not bright enough to put enough fuel in for as journey you shouldn't be behind the wheel.

Why do you need to know "exactly" how many mile you can do on a tank. There are so many variables. Driver, traffic, weather, even temp.
Click to expand...

the risk of crap in the bottom of the tank is fine in theory, but irrelevant in reality. This isn't the 1960's.

The miles I achieve per tank have been consistently within ~50 miles of each other over the last 25,000 miles. To me it is very useful information to know.

Dogbreath said:
It's not good for fuel pumps if the tank is run dry either. The fuel runs through the inside of the motor to cool it.
Click to expand...

Again the logic is sound, but in reality pump failure is unlikely enough to be discounted. Does your screenwash pump die everytime you run out? ;)
 
Tom_ed1987 said:
the risk of crap in the bottom of the tank is fine in theory, but irrelevant in reality. This isn't the 1960's.
Click to expand...

Cheers, marked it on my calendar, 1976, it is 1976 isn't it??;):)

I'm sure I've just seen the Sweeny on the tele?

You're right though with modern vehicles the chances of picking up some grime are quite slim, but as is the way with new motors the potential bill can be big. What's the old addage "an ounce of prevention is worth a pound of cure" I still wouldn't run mine down to empty intentionally.
 
Last edited:
Armageus said:
Fuel gauge could be based on the "range" worked out by the ECU rather than directly off the float.
Click to expand...

I know what you mean but it is supposed to be a level gauge and as such should reflect the actual level in the tank and nor some calculated level.
 
Ah ok, they must have changed the fuel gauge function in the B6, like you say you'de expect it to be linked to actual tank level, it is in B5.5.
 
Indeed. It has moved up a little more again which is odd considering I have driven around as well.

Computer has always been pretty good though. I generally run until it says 0 miles and it consistently takes around 65 - 66 litres. Only once did it actually let me put 71 litres into the 70 litre tank :)
 
You must log in or register to reply here.
Back
Top Bottom