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Negative pressure in tanks and gas caps?

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Do modern cars (2006+ Miata) still have a negative pressure valve in the gas caps? The intention would be to avoid excessive vacuum in the tank (that could destroy the tank or make the pump struggle).


Over pressure is handled by the EVAP I believe.


Am I asking for trouble If I replace my 100% stock fuel system with an after market filler tube and cap ? (Still has venting for fueling and overpressure venting via EVAP)



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Yes, most modern caps do have over/under pressure reliefs in them.  Ours open at 3.7psi of overpressure, and -0.5psi underpressure.  However, this is used strictly for emergency situations only.  All air in/out of the tank goes through the canister, to capture the HCs on the way out, and release them back into the system on the way back in.  So, if the cap relieves any pressure or vacuum, it's because the vent is not working properly.  Make sure the vent is good and you can replace the cap without any issues.

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