I own a 3.5L V6 G35 and have noticed a very strong gasoline smell when I take off the upper plenum. It is as if you put your nose in the gas tank fill hole. I know this is not normal and could be dangerous. The engine has very high NVH and when I used an OBD tool, random misfires were detected on 2 or more cylinders (error code P0300). This occurred immediately after I changed the valve head gaskets, which were leaking previously. Which hose/gasket would be the most likely culprit? What would be the first thing I should check?

 

I'm assuming you disconnected the injectors when changing the gaskets? Make sure they were reconnected correctly. That's my first guess...

 

Strong fuel smell in the intake is probably a leaking injector. Tear it down to the lower intake collector, prime the fuel pump (key in ON position) then use a flashlight and paper towel to see if fuel is drizzling out of an injector.