The only time I can think of that there would actually be pressure in the gas tanks is when the car has been sitting for a while, especially in the heat, and the gasoline has begun to evaporate. Normally when driving the car, the fuel pump would try to create a vacuum in the gas tanks as the fuel was being used to run the engine. In the old days, vented gas caps were designed to allow air INTO the gas tank as that vacuum was being created inside.
I don't believe modern cars use vented gas caps any longer. But there has to be some mechanism to compensate for the vacuum being created in the gas tanks while driving. As I mentioned before, there is a canister that is used to recapture fuel vapors from the fuel system that has been known to emit a gasoline odor.
Anyway, hopefully the new gas cap will fix the problem.
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