Q: Lately in my neighborhood some pranksters are pouring sugar in a few of my neighbors’ cars at night! What damage does that do to a fuel system? How much sugar in the tank is deemed harmful? Does the gasoline absorb it? I’m asking because some of my neighbors have different results after the sugar episodes. Some cars run fine and some have had expensive repairs done. What exactly happens to a fuel system when foreign contaminates are poured in and how much does it take to be deemed harmful?

A: This is an intriguing and certainly unfortunate situation. Any contaminate added to a fuel system will cause problems. Stories of sugar destroying engines or caramelizing carburetors have been told for decades, but there seems to be little evidence sugar is any worse than other granular or liquid contaminates. In 1994 a researcher at the University of California, Berkeley, proved once and for all that sugar does not dissolve in gasoline, so with adequate filtering, it should never reach the engine.

Automotive fuel systems use a nylon sock at the fuel pickup tube or fuel pump, in the fuel tank, which prevents ingestion of moderate to large foreign material (150 microns and larger), and an inline filter to block particles as small as 10-30 microns. A quarter cup or more of sand, dirt, sugar or any contaminate would likely begin to clog the sock and/or filter, resulting in a high speed/load power loss, possibly a check engine light, and ultimately poor or no performance at all speeds.

Renewing a clogged fuel filter isn’t hugely difficult, but repeated renewal would certainly become costly. A clogged sock can result in fuel pump failure (perhaps a $300-$1,000 job) due to cavitation (the strong suction of the pump excessively reduces inlet pressure, causing fuel to vaporize), and fuel tank removal and cleaning can add more to the repair bill. It’s possible also for tiny contaminates to sneak through the filter and gradually foul the small screen/filters within the fuel injectors.

If you have a non-latching fuel filler door, I’d consider a trip to the auto parts store to purchase a locking gas cap. These were invented in 1932 by the Stant Corp. to prevent fuel theft and work well. Gas caps vary considerably, but locking versions are available for most applications for about $15 to $30. Extra keys can be easily made for all the drivers in the house, and a labeled spare slipped into the glove box. With the high cost of repairs and potential consequences of vehicle downtime or stranding, a locking gas cap makes a lot of sense for everyone.