I just checked, and it turned just over 1.5 times before clicking.

 

Also make sure the seal is good where the cap meets the filler neck.

 

The guy I bought the car from said that there was a problem with the gas cap, and that he was going to get it fixed before we picked up the car (by his local mechanic, who specialized in European cars). So I just checked out the cap again, and it's actually brand new and made by an American company called "Stant". After rummaging around through the car, I found the original Volvo gas cap with the spare wheel, and put it on instead of the aftermarket one. It tightened about the same amount (1.5 turns) before it clicked.

The seals on both looked fine, although the seal on the original cap was faded. Which cap should I use?

While I was doing this, I noticed that there was a hole in the plastic below the filler neck. Is its purpose to drain spilled gas while you're refueling, or is it something else?

EDIT: I didn't notice any pressure or gas escaping when I took the cap off.

 

Oh hell.

I know I said earlier that the gas tank wasn't leaking, but it certainly is. I just looked under the car, and the bottom half of the tank was wet (presumably with gas, but I didn't check), and every 15 seconds a decent sized drop would fall from the center of the tank.

Could this have caused the p0455 code? And how much should I be prepared to hand over to a Volvo dealer?

 

Yes that will cause the code.
Usually a hose goes bad causing the leak.
Sometimes the tank cracks as well.

If it is the hose then it really isn't all that bad maybe like $100.00 to have it installed.

 

...and make sure to have it fixed asap, as the tank has now become a fire hazard.

Stant cap should be fine, as they are also sold at FCP Groton. Or, you could perhaps buy a Stant cap at a local auto parts store and use it as a "donor" for the O-ring[8D].

Speaking of O-ring, one O-ring for a B747's IDG (Integrated Drive Generator; equivalent of alternator) costs approx $100. The generator itself ranges between $100,000 to $300,000.....

Gentlemen, stay away from airplanes as much as you can!!!

Just kidding (but the prices are for real[8D]).


JPN