BBC Focus Magazine

[Croatguy]

When my fuel gauge is near empty, I have noticed sometimes that the fuel gauge will sometimes fluctuate ever so slightly back towards F and then return back to the original spot. I have noticed this in pretty much every car I have drove. Anyone know what causes the fuel gauge to move back up? Is it do with the fuel tank or are the gauges just overly sensitive? Any mechanics out there?

[M Paul Lloyd]

Fuel tank gauges on cars are generally not very sophisticated and when the tank is nearly empty the remaining fuel is more prone to sloshing around (sloshing is a technical term by the way) which causes false readings to appear. I always try to drive with the tank at least 30% full as running it low runs the risk of compromising the pressurised fuel sysyem that a lot of modern cars now have.

I'm no car mechanic but I do know about engines and fuel and stuff.

[MikeG]

I always try to drive with the tank at least 30% full as running it low runs the risk of compromising the pressurised fuel sysyem that a lot of modern cars now have.

This "compromise" won't have a permanent effect will it? No air bubbles permanently clogging the system?

I have noticed the same effect in cars, but only when the car isn't level. If the car is going uphill, it may appear as though the car has more fuel. Driving downhill makes it appear more empty. I only trust the reading when its level.

[M Paul Lloyd]

This "compromise" won't have a permanent effect will it? No air bubbles permanently clogging the system?

I do not have first hand experience of this but have been advised by someone who did allow therir tank to run dry that it can lead to air entering the system which requires expensive work to bleed the system.
So I'm very reluctant to risk it.

I only trust the reading when its level.

Very wise and one reason why car fuel gauges only give a rough guide as to what is in the tank, even cornering can alter the apparent fuel level. You wouldn't put up with that in an aircraft where you want it down to the very last kilo/litre regardless of your position but then again they have far more sensitive measuring systems and baffle plates to help stop the fuel sloshing around.

[BrIDo]

The fuel gauge in the car is a strip of electrical contacts, with a runner. A metal contact slides up and down on the runner, which is attached to a float. As the level in the tank rises and falls the float remains on the surface and the contacts provide an electrical signal indicating the height of the fuel level. When going up or down a hill the fuel 'sloshing' about can make the float rise higher or lower depending on where it's sitting. This is why on some cars going down hill can increase the reading on the gauge (sensor at front of tank), while on others it will decrease it (sensor at the rear).

One of the issues with regards to running the tank dry was rust. The top of the tank above the fuel line, as some people rarely fill their tanks up, would rust and the rust would fall into the fuel and sit at the bottom of the tank. When the tank ran dry rust would be dragged in along with the last drops of fuel and cause blockages.

Ofcourse most tanks now are plastic so this problem is largely eradicated.

[Geekit]

If you still run your car on cheaper less refined versions of fuel you can get a lot of sediment or MBG (micro biological growth) in your tank once this MBG has set in it is extremely hard to remove from a system and can cause injectors to become blocked! Reducing performance and essentially causing miss fires in your engine however thankfully this MBG floats.

[Nails]

Me being a thicko is wondering how some digital fuel guages are not effected at all by hills...

[Geekit]

The electrical probes in the fuel tank at different levels use conductivity to figure out a level within the tank so a quick splash onto the probe is factored out... and also more modern cars have simple pressure plates in the tank and weigh the amount of fuel remaining and convert it into a read out

[M Paul Lloyd]

Ah you see, I'm showing my age and ignorance now, thing is the last time I looked inside a cars fuel tank the sensor was a small float on a hinged arm ..................... the type they evidently have fitted now sound more like the things I recall from aircraft......