Can gasoline leak from the fuel tank due to a loose gas tank cap?

Short answer: yes... sort of.

What comes out of a loose or faulty gas cap is gas vapor. Vapors of gas rise above the puddle of gasoline in the tank and hang in the air. When the pressure in the tank is too high, vapors enter the fuel vapor canister through a small hole in the gas tank filler neck. In the past, vapors were simply released through the filler cap, but that was before anyone knew about the effects of gas vapors on air quality.

In addition to reduced air quality, the loss of fuel vapors adds up to significant fuel losses over several years. The fuel vapor trap allows vapors released in the fuel system to return to the fuel tank.

How to prevent gas vapor from escaping through the gas cap

The gas cap on every vehicle must have signs either on or next to it explaining how it should be used to close the fuel tank properly. The most common way to check for leaks is to listen for the clicks the cap makes when tightened. The average is three clicks, but some manufacturers use caps that click once or twice.

A loose gas cap can also cause the "Check Engine" light to come on, so if the light comes on randomly (or right after refueling), re-tighten the gas cap before doing any further diagnostics.