Low fuel question

[MarcoRodriguez]

Hi Everyone,

Quite new here. I have been reading about the hazards of driving with low fuel, such as letting garbage & water get sucked into the fuel pump. I was wondering whether that is the general consensus on this forum too, and what do people mean when they say "low fuel"? Is this defined as the point that the petrol gauge reads empty (not sure if that changes between car models), or is it something like the remaining 1/3 of a tank or less? I ask this because I have seen some people recommend not letting the tank fall below 3/4, which seems over the top to me.

[Hiro]

Sucking up dirt and water etc when fuel level is low is an urban legend. Fuel pumps are always situated at the bottom of the tank, and petrol floats on water so if there was water in the tank the pump would suck it up regardless of the fuel level. In addition, even if dirt has settled to the bottom of the tank (cars make quite good agitators when driving so this would only be after a prolonged wait) then there is a sock on the fuel pump pickup AND at least one other fuel filter in the line before the fuel would reach the engine, thus preventing any kind of damage.

The issue with running modern cars out of fuel is mainly three-fold:

1) Modern high-pressure in-tank fuel pumps are lubricated by the fuel itself. Run it dry and you risk overheating and damaging the pump

2) There isn't a "reserve" of fuel like a carby bowl, so the instant the fuel pump starts to starve, the engine will struggle and cutout, which leads to

3) Running the car almost out of fuel can create very lean combustion, which can damage the engine and the catalytic converter.

There is nothing wrong with running a car down to the fuel light comes on, but pushing it any further is opening you up to the above risks. Once you drive your car alot you will learn just how much fuel is left once the light comes on and how far you can push it, but being so pedantic that you fill up the tank when it drops to 3/4 is just ridiculous, and technically will cost you money in the long run (perpetually full tank means more weight to carry around, which means poorer fuel consumption - not a lot, but it could add up over the life of the car)

[MarcoRodriguez]

Ok. Thanks heaps for answering that, I've been wondering for a while and just wanted to clarify. What were you going to say in point 2); and does any water enter the fuel line if it is sitting at the bottom? If it does then why is it an issue for water vapour to enter the line and cause rusting?

[Hiro]

2) leads to 3)

As for fuel in the lines, the last thing you'd worry about is rusting. Water is not combustible and can cause damage if it makes it to the engine.