All models of the NSX have a semi sealed Evap system. None of the Evap control systems are 100% sealed because they need to admit air into the tank to allow for fuel being used up. They are sealed (sort of) to block air and vapours from exiting the tank into the atmosphere by the Evap systems two way valve.
If you store your NSX, when the temperature drops the fuel and air in the tank will contract and it will suck outside air in to the tank to make up for that. However, once it has sucked outside air in it will stabilize and the two way valve in the evap system will close and prevent that air from exiting the tank when the temperature warms up unless the air pressure exceeds the setting of the two way valve in which case it will vent into the charcoal cannister and release pressure. So unlike in the old days the tank can breathe in once; but, its ability to breathe out is restricted as the temperatures go up (that is why you get that pfft when you open the gas cap on a hot day). The reduced breathing in and out as the temperature goes up and down every day while in storage reduces the moisture getting into the tank.
I like to store the car with as much gas in the tank as possible to minimize that air space. That minimizes the amount of initial breathing in as that air contracts as the temperature drops. It also minimizes the pressure increase on those warm days which may cause the two way valve to open to vent into the cannister which could then result in air being sucked back in when the temperature drops again. Chances are that with a full tank the 2 way valve may never burp into the cannister all winter long so fresh air never gets sucked back in.
Liquid fuel contracts as the temperature drops; but, it contracts less than air. If I put my NSX into storage at 10 C and the temperature drops to -40C during the winter the gasoline will contract by about 5% . However, the air will contract by about 21%. If the 70 liter NSX tank is full of gasoline it will want to suck in 3.5 l or air as the temperature drops from 10C to -40C. If the NSX tank is completely empty it will want to suck in 'approximately' 14.7 l of air as the temperature drops from 10C to -40C. You cannot 100% eliminate air getting sucked into the fuel tank as the temperature drops; but, you can minimize it which can minimize or eliminate the breathing problem.
If you have a 1995 or later OBDII car the OBDII system will let you know whether that all critical 2 way valve is working and the fuel cap is sealing by generating a CEL if it isn't. The earlier cars are designed as sealed systems; but, will not tell you if the 2 way valve has failed or the cap is not sealing.
