The gauge will have a bleeder/cap.
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Pierce it so it's open to atmosphere.
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A liquid gauge when warm will give the wrong reading if it's sealed! The pressure in the gauge will effect the reading as the liquid warms.
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I've pierced mine, I get the same readings hot/cold.
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Vac hose disconnected I would run between 36-40psi.
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I'm still testing things out
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(but 36psi is fine, just be sure you can check your AFRs!).
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