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I own a secondhand Mitsubishi Mirage Asti V 1998. When refueling it I noticed negative pressure built up in the fuel tank. A bit force is required to loosen the fuel cap and air sucks in when uncapped.

Is this normal or problematic? Could the EVAP system have some sort of clogging?

Hi there,
This scenario is quite common, as the fuel being sucked from the tank by the fuel pump will create a vacuum. This is because the tank and breathers are not totally open to the atmosphere and vent under certain conditions.