I have a hissing noise when the engine is running ie driving or idling ,it has stalled out in idle ,the rpm hold at 650 hot ,replaced all sensors except for o2 , did the intake manifold gaskets and the pletum gasket as well and I'm lost for words , I also have this open vac port on the drivers side behind the intake and idk what it does or where it goes ,but if it block it off the engine stalls faster or if I link it into to intake the engine stalls again ,so I'm clueless ,please help me I'm going nuts ,