Running your car out of gas isn't just the embarrassing mishap that it used to be; it can end up costing you a pretty penny in repairs. Even running your car low on gas all the time can net similar results, which is one more reason to make sure you don't run out -- as if you needed one apart from the chiropractor bill.
Fuel isn't just your fuel pump's reason for being; it's also serves as a coolant to carry heat away from the pump motor and its associated bearings and vanes. Fuel entering through the pump pickup goes through the pump vanes. Those vanes are usually made of some sort of plastic and ride inside of a plastic case. As long as fuel flows through the pump, the vanes can't get any hotter than the fuel. Once that goes away, well, you can imagine what happens.
While not particularly common today, engine-driven pumps were once the norm for most engines. An engine-driven or "mechanical" fuel pump uses a diaphragm actuated by a lever on the camshaft to suck fuel from the tank and push it to the carburetor. While they can be a bit difficult to prime after running out of gas, the pump diaphragm doesn't rely on cooling fuel flow the way an electric pump does. And even if it did, it's not going to do anything when the crankshaft isn't turning.
On most older cars, turning the ignition key to the on position would automatically engage the fuel pump and turn it on. Under normal conditions, leaving the key in the on position with the engine off would simply cause the fuel pump to recycle fuel through the system and stay as cool as it normally would. But this obviously isn't true if there's no fuel in the tank and you leave the key on. So the moral is: If you're out of gas, shut the car down ASAP and turn the ignition key to the "Off" position.
Many modern fuel pump controllers use a program that will automatically shut the fuel pump off if the line fails to pressurize. This is as much of a safety mechanism as anything else, since a pressure failure could just as easily indicate a severed fuel line as an empty gas tank. Less common are fuel pump timer systems, which will shut the pump off after a few seconds if you don't engage the starter.
Constantly running your car low on gas can damage the pump as surely as running it out of gas. There's a lot of junk floating around in your gas tank -- rust scale, dirt, metal particles, dead bugs, you name it. The fuel pump utilizes a "sock" pre-filter to catch this debris before it goes into the pump, but sock filters can eventually weaken and get cut by debris in the tank. When that happens, the debris goes into your pump, and you're into the local garage for $400. Keeping the tank at least a quarter full at all times will keep that debris dispersed in the fuel rather than allowing it to concentrate and collect around the filter.
