Sometimes it’s tempting to try to eke out as many miles from your tank of gas as possible. But you shouldn’t do it. This video showing the innards of a fuel pump will help you understand why.
Your car’s fuel pump sends gasoline from the tank to the engine. It’s a crucial component in getting your car running, but it has a tendency to fail and leave you stuck on the side of the highway—especially if you like to run your car low on gas.
You can see why running a car low on gas might cause problems by looking at YouTuber speedcar99's tear-down of a fuel pump module:
The key bits of a fuel pump that tend to cause problems when you run your car low on fuel are the strainer, fuel filter and electric motor.
As you can see in the video above, the electric motor is actually cooled by the fuel. Gasoline from the tank enters in through the strainer (or “filter sock” as he calls it in the video), gets squeezed via a round impeller-style pump (shown below), and is pushed through the electric pump motor to cool the copper windings that make the motor work.
Running a vehicle out of fuel means air, not liquid gasoline, has to do the job of cooling the windings. Since flowing gases tend to have lower convective heat transfer coefficients than liquids, they aren’t able to remove heat from the windings as effectively, meaning the fuel pump electric motor can overheat, melt its windings and ultimately croak.
On top of that, fuel acts as a lubricant for the pump, so running air through the system could cause premature wear of rotating parts.
In addition to overheating and prematurely wearing out the pump, running a car low on fuel tends to pull up the sediment that normally sits at the bottom of the tank. (This is a bigger issue with older metal gas tanks that tend to corrode, especially when empty and thus exposed to moisture in air.)
UPDATE: It’s worth noting that the sediment collecting at the bottom is really more of an issue on older vehicles that draw fuel through the bottom (my old Jeep clogged its fuel filters and ruined its pump because of this very issue).
You don’t want this getting into the pump.
Sediment can clog the strainer or gunk up your fuel filter element positioned after the pump. Once the fuel filter is clogged, sufficient fuel will no longer get to the engine, and your car won’t run. As the fuel flow rate stagnates, eventually cooling for the electric pump can again become an issue.
Replacing a fuel pump thoroughly sucks. It often requires you to drop the fuel tank and spend lots of cash on a new replacement part. This is easier with older cars at least, as they tend to have access panels in the body or at the front of the tank. If you own anything vaguely modern, though, everything gets costly fast. If you have to replace a fuel pump, you’ve got some loins to gird.
So don’t run your car low on fuel. Keep an eye on that gauge (which gets its reading from that float-based voltage divider setup shown in the video), and try to keep the “running on fumes” to a minimum.
This post has been updated with additional information.