Gas tank pressure buildup

[Zick]

I already put on a new gas cap but the gas tank is building up what I would call excessive tank pressure.
It almost blows off the cap when removing it.
What else would be causing this pressure buildup?

[jakegday]

its supposed to build up pressure, thats how the fuel system works. if you take the gas cap off and leave it off your check engine light will eventually come on because it cant build pressure

[Zick]

No it's not suppose to build up this kind of pressure. Yes it does pressurize the fuel lines but it's not suppose to pressurize the fuel tank.

[ponyracer]

Is this excessive build up you're talking about only happening when you take off your cap when you're about ready to fill your tank? Your tank could be just extremely low. Our 94 Trooper and our VW do the same thing when we let the fuel level get way too low.

[Blue Bowtie]

The tank should be in a slight vacuum after the engine has been running and the control system is in closed loop mode. However, after sitting, the tank may build pressure. This is normal. The tank pressure control valve (part of the EVAP system) should hold pressure to 1-1.25 PSI before allowing vapors to escape through the EVAP canister and filter out the hydrocarbons. If you are really concerned, you could connect a pressure gauge to the tank and see what kind of pressure is being developed, or connect a scanner and see what the tank pressure/vacuum sensor is indicating.