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![phoenixhou](themes/aaAle/images/interface/profile-pic-placeholder.47cf740a.png)
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?
![Anon](/assets/aa/aa-avatar.png?m=1466990309)
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.