When temp gets hot here in NM, I hear boiling gas from the tank and when I remove the gas cap, it hisses and lets out fumes.

What going on here?

 

The gas is not boiling it is the vapors expanding with the heat. I would have the venting system checked because these fumes should be dissipated in the system so that the pressure doesn't build up in the tank.

 

If your gasoline inside the tank was boiling, I would be amazed that you're still alive...

On the flipside, your venting system may in fact be working properly, and the hissing you're hearing may not be gas escaping, but air being sucked into the vacuum created. However, If your car was in a lot of stop-and-go traffic on a hot day, and your fuel level was low, that creates a lot of vapors in the fuel tank. The EVAP system is designed to keep these vapors in check by drawing it back into the engine and consuming it as fuel. That said, you should heed Jagtoes advice and check your venting system just to make sure everything is fine.