There’s no denying that fuel prices are painful at the moment. If they get any higher, your local petrol station could graduate summa cum laude by the end of the year. But as tempting as it might be to splash just a liter or two in the tank every time the needle hits the bottom of the gauge, or waiting until the engine starts sputtering on parking ramps, you’re really not saving yourself anything by doing so. Let’s look at the ways that running your car with a nearly empty tank can actually cost you more in the long run.
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1) The fumes
If your car runs on petrol, gas vapors can and will build up inside the tank when you park under the sun. While modern cars have an EVAP system to vents these vapors when the engine is running, pressure built up when parked can cause a pretty bad smell when you open the cap, especially if there’s a lot of empty space inside the tank for the fuel to evaporate into.
2) The sediment
Over time, sediments and impurities can settle at the bottom of your tank. While this isn’t as big an issue with newer cars with plastic tanks and filler systems, having the fuel slosh around in the bottom of the tank increases the risk that settled contaminants will get sucked up into the fuel pump, clogging the filters prematurely.
3) The water
Water contamination is a very real issue with cars. You can get it from contaminated fuel, but also from the air around you. While modern tanks are sealed, opening the tank more often to fuel up lets more moisture-laden air in, and that moisture can condense and build up, especially if you leave a lot of empty space inside the tank and don’t use your car for long periods of time. Sucking that water into the engine can cause poor running, whether you have a petrol or diesel vehicle. Worse, on modern common-rail diesel vehicles, water ingestion can damage sensitive - and expensive - high pressure fuel pumps and injectors.
4) The heat
Fuel, just like the other liquids that go through your engine, helps cool it down. Evaporating gasoline carries away with it a lot of heat. But even when it doesn’t evaporate, liquid gasoline can hold a lot of heat. Many vehicles have fuel return lines that dump hot fuel back in the tank, removing excess heat from the injector rail. When running low on fuel, the amount of heat circulating between the tank and the fuel rail is, obviously, much higher than if the tank is at least half full. This also increases the chance of gas vapor build up in the fuel system, which can lead to poor running. With fuel return systems and electronic engine control, vapor lock isn’t the same big boogeyman it was in the days of carbureted engines, but hot fuel is still no fun.
5) The heat, part two
Aside from the engine, the fuel pump needs cooling as well. That tiny electric motor runs non-stop during operation and can get very hot. Normally, the pump is fully submerged in fuel, which keeps it cool. But at very low fuel levels, parts of the pump will no longer be submerged, leading to overheating and exposure of rubber bits and gaskets to corrosive ethanol fumes. Habitually running at very low fuel levels can lead to premature pump failure. Exposure to corrosive fumes doesn’t help, either.
6) Do it right
To keep your fuel pump—and your engine—happy, you should aim to at least have a quarter to a half tank of fuel in hand at all times. Running out of fuel once or twice won’t destroy your car. But it’s not something you should make a habit of. You can definitely go the other way, too: consistently filling your tank to the point where you can see the fuel sloshing just under the cap can clog the charcoal canister and damage the EVAP system. In buying fuel, as in life, moderation is the key. Eat until you’re full, and no more than that. Consistent full tank fill ups cycle new detergents and cleaners in the system, flushing out built-up sludge and water contamination. It is also more economical than buying a few liters everyday. And less stressful as well.