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Q:

ilden bad on 1999 Mazda 626

Rookie cbe0621eac06868b3efe0d8d1d3611e23c60d3114864ea2ec19a68cfbd3eebab
ok so were on the intake manifold and does that causes it to hiss?
(1) Answer
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The hissing sound is most likely a vacuum leak, was all the old gasket removed prior to refitting the new gasket? Again try pouring a little engine oi where you hear the hissing sound with the engine running, if the oil gets sucked into the engine and the engine briefly runs smooth you have a leak from the gasket..
With the engine running you may hear a slight "hiss" which is fuel running in the fuel rail.