I had to replace the (in tank) fuel pump on my car. I removed the
entire sending unit and had to order the part. In the meantime I put
a 16oz plastic disposible cup in the opening to prevent dirt, insects,
and anyhing else from getting in the tank, as well as vapors from
escaping. The tank still contained about 2 gallons of gas. The cup
made a nice tight fit in the hole and while I waited for my part to
arrive at the parts store, I just left the cup in there as well as
putting a rubber car mat over the opening just in case it rained.
The next day I removed the car mat and found that this cup was full of
holes, soft, and misshaped, and ready to fall into the tank. I was
able to grabe it before falling in.
I realize that gasoline will melt plastics when it contacts them, but
no liquid gas ever contacted the plastic cup, just the vapors. How
could gas vapor cause the plastic to melt?