It's a car-culture legend, passed down from generation to generation, that an angry person with a bag of sugar can leave your car dead on its axles. Add sugar to gas tank, turn its fuel into a sugary petroleum mess, and wait for the owner to start the car and blow the engine.
It's also a myth.
Sugar doesn't dissolve in gasoline. If you add it to gasoline, it stays in granular form.
"We have not seen an engine damaged or destroyed by sugar in a gas tank, nor heard of any truly plausible or established cases of this happening," says Mohammad Fatouraie, manager of engineering at Bosch, one of the auto industry's main suppliers of fuel system components.
The Thing about Filters . . .
A sugar crystal is about 200 microns, a measure of size for small particles. Filters in a car's fuel system capture particles much smaller than that, so suspended sugar granules in the gasoline would be caught by any one of several filters before they ever made it into the engine. There's a fabric, sock-like filter surrounding the fuel pump pickup in the gas tank, an in-line fuel filter at the tank pump inlet, a filter on the high-pressure fuel pump in the engine bay, and filters at the inlet of each fuel injector.
Even in a carbureted engine, which doesn't have fuel injectors or their individual filters, there's a low chance that sugar would ever make it that far into the engine after all the other filters in the system.
Sugar is roughly twice as dense as gasoline, says Fatouraie, so some granules wouldn't even make it all the way to the filters. Particles denser than fuel settle in pockets and corners of low-velocity flow, and there are many low-velocity pockets between the gas tank and the engine. If someone dumped sugar in your gas tank and you removed the tank to clean it out, you'd see a lot of the sugar granules collected on the bottom. It could clog the in-tank filters and prevent fuel from flowing properly, and while it's possible that prolonged running of a car with clogged filters could burn out the fuel pump, Chris Louis, director of engineering at Bosch, says it's unlikely to reach that point.
If you knew someone dumped a lot of sugar in your gas tank, you'd just have to drop the tank to clean it out and replace the sock filter. You may as well test the fuel pump, to be safe, and if its flow rate doesn't match the factory specifications, you'd replace it.
Your engine would be fine.
So What Can Destroy an Engine?
So if you shouldn't be worried about sugar, what should you worry about being added to your tank? People posit that dumping water into a gas tank would cause the kind of damage that sugar can't, because engines need their fuel to combust and water prevents that. They're right, but it'd take much larger quantities of water to do serious damage than an angry car vandal with pitcher could carry.
After all, ever since E10 was mandated in North America, there's been water in every gallon of fuel you put in your tank, says Fatouraie. You've seen E10 on gas pumps, and it means that 10 percent of every gallon you pump is ethanol, a corn-based alcohol fuel that appeared on the U.S. market in 1990. There are other popular gasoline-ethanol mixes at pumps, too, such as E15 and E85 that are, respectively, 15 percent and 85 percent ethanol.
Alcohol is very hygroscopic, meaning it absorbs water easily. Moisture-laden air inside the gas tank will pass water into the fuel's ethanol, and so any car you see on the road today already has some water coursing through its fuel lines. It's just not enough to cause damage. Even adding water outright to a gas tank—cue our angry pitcher-toting vandal—would cause no harm, says Louis, as long as it doesn't dilute it so much that there isn't enough fuel left to combust and power the engine.
It'd just displace some of the liquid fuel in the air/fuel mixture in the engine's combustion chambers, but oxygen sensors and on-board computers would automatically compensate for the leaner mixture and the engine would run fine without injuring itself.
If the piston can't complete its stroke in the chamber because there's so much non-combustible water, the engine becomes hydro-locked. It'd cause considerable damage, Louis says, but under normal cases the engine would stop operating before failure is catastrophic. Like the sugar myth that inspired it, this myth is based more on urban-legend.
People often say every myth starts with a grain of truth, but there's nothing concrete to this one. Early mentions of sugaring someone's gas tank to get even with them date from the 1950s. Physics haven't changed. All it adds up to is a big waste of sugar.