As car parts go, the gas cap keeps a pretty low profile. In the
old days, it was possible to drive away from a
gas station without it, necessitating a trip to the auto parts store or
junkyard for a replacement, but automakers started tethering them to the necks of their fuel tanks, so even that rare
cause for concern faded away. It's hard to think of a simpler, more reliable functional component of an automobile.
Of course it wouldn't be Real Life without complications.
New Cars
In modern cars, the gas cap is part of the emissions control system. Its function in that regard is to prevent raw gasoline fumes from escaping into the atmosphere. It's a worthy task, because not only are the fumes toxic in themselves, they contribute to the formation of ground-level ozone, which is a significant health hazard in most urban areas. Jurisdictions that require emissions testing generally include a gas cap pressure test as part of the regimen. If your car fails the test, you may be able to buy a new gas cap and go on about your way, but some places will just fail the vehicle and force you to be retested later, at full price. This could cost 30-40 bucks or more in many areas.
The automakers have become aware of this issue, and many new cars have sensors that measure the pressure-holding integrity of the fuel tank. A bad gas cap can now cause your check engine light to come on. Those cars will record the event in their on-board computer systems, and your smiling contemporary service agent will not have to waste time trying to troubleshoot the problem.
Old Cars
Before we started worrying about all this "environment" stuff, fuel tank design was much more straightforward. As the level of gas in the tank drops, you have to admit air to take its place, so gas caps were vented - that is, they had holes in them. Fat old DeSoto and Studebaker mechanics like to amaze one another with stories of plugged gas cap vents that created enough negative pressure in the tank to starve the fuel system and cause cars to stall out.* What an adventure that must've been!
Other Tidbits
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In the seventies, many people bought locking gas caps, because gasoline became expensive enough to steal.
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Stant Corporation is the largest maker of gas caps in the US, and hence, the world.
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Gas caps tend to be on one side of the car or another. If you are driving an unfamiliar car, it is easy to drive up on the wrong side of the pumps at the gas station.
- Huge old cars often had the gas filler nozzle in the center, behind the license plate or something. People were much less intelligent in those days, and it was quite hazardous to jockey those land yachts around at a filling station. Savvy modern drivers and their more agile vehicles have obviated the need for this strategy.**
*Conversely, plugged vents can cause pressure to build up when the temperature in the gas tank rises. On a hot day, you may get enough pressure to force gasoline through the fuel lines in some of those old cars, flooding the carburetor with the car parked and locked. It's alays something, isn't it?
**In the interest of veracity, I should mention that the real reason that gas caps are never in the middle anymore is to prevent your car from exploding when somebody rear-ends you. See Ford Pinto.