How does putting sugar in a car's gas tank also destroy the car's engine?

Just a rumor.

As it turns out, sugar doesn't dissolve in gasoline. Pouring sand into the gas tank would have about the same effect as pouring in sugar. The sand or sugar might clog up the filter, and that could disable the car, but it's not a sure thing.

http://auto.howstuffworks.com/fuel-efficiency/fuel-consumption/sugar-in-gas-tank.htm

Putting Sugar Into a Gas Tank
•Just about everyone has heard of the notion that putting sugar in a gas tank will destroy a car engine. But is it really true? While putting sugar in a car's gas tank won't always destroy an engine, it can cause some major damage and, in most instances, cause the victimized car to stall and require major repairs.

What Happens to the Sugar
•Sugar is a scourge to car engines because it doesn't dissolve in gasoline. This means sugar poured into a gas tank will get sucked into the fuel lines and begin clogging up a vital part or parts of your engine's fuel system. It can disable the fuel filter, the fuel pump and the fuel injectors, depending on how much sugar is loaded into a gas tank and where it accumulates. Wherever it ends up, the sticky ball of sugar will block the fuel flow and cause your engine to stall and not start again. And once a mechanic takes the engine apart, repairs can be expensive, especially if the sugar reaches your carburetor or fuel injectors. You'll also need your fuel tank emptied and cleaned and your fuel lines flushed out or replaced. So while sugar won't exactly destroy an engine, it can destroy parts of your engine that are expensive. Not surprisingly, the sugar-in-the-gas-tank tactic was developed in the early 20th century as a guerrilla war tactic to disable enemy vehicles.

Some Exceptions to the Rule
•There are cases where sugar can completely ruin an engine. If enough sugar gets by the filter, pumps and fuel injectors, it can get into the engine itself, and build up in the valves and piston rings. Once there, it will eventually harden into a mess that will require a complete engine overhaul or a new engine. But the chances of that happening to new car models is slim because most cars today have sophisticated fuel filter systems designed to catch contaminants and prevent them from reaching the engine. The bottom line is that putting sugar in a gas tank really can cause some damage. The only variable is how much damage it causes.

https://www.popularmechanics.com/cars/car-technology/a33237683/sugar-in-gas-tank/

Snopes says that it is an urban legend.

http://www.snopes.com/autos/grace/sugar.asp


Who I am to believe?

I've been a mechanic for well over 30 years and I have never seen sugar used as the basis for revenge. I once poured a 1/2 cup of sugar in the gas tank of a single cylinder Briggs and Stratton engine and in ran fine for a couple of hours till it ran low on fuel and then the sugar plugged up the fuel pickup strainer. Sugar may cause some damage to the fuel delivery system in more expensive engines but it will never be the catalyst of a catastrophic failure.

If you read down in snopes, it does say the sugar can clog the filters, etc. Ordinary granulated sugar would be stopped by any modern fuel filter, but there are many 'classic' vehicles that don't have any fuel filtration at all. If you know the size of the hole in a carburettor's jets, it is possible for a small granule to pass through the largest of these holes, but the smaller ones would just be blocked so the engine would be running very unhappily anyway. Granules that did pass through wouldn't clog valves, as they would be passing through borne by a gust of wind, but would carbonise in the heat of the cylinder and deposit - as carbon used to in the old days of having to decoke your engine after only about 40 or 50,000 miles. Icing sugar would get through a carb much easier. It would be stopped by the modern micropore filter - and if enough was present so would the engine as no fuel would be getting through. Basically, both are correct. (Baloo knows about fuel filtration, having had a van with two filters that he fitted to the fuel line. One morning he noticed that someone had syphoned two gallons from the tank. He just laughed. That was the end of a minor spate of syphoning in the area. What they got was mostly petrol, but also a whole load of horrible brown gunge. Whatever they put it in would have given trouble for ages if it ran at all... He also knows about decoking and when it's best to leave the carbon well alone.)