Poor gas mileage and stinky exhaust

[Nifty50]

Hello Pathies! I'm new here so please excuse any stupid jokes or general ignorance. I recently picked up an 02 Pathfinder LE with 150k on it. It's in great shape cosmetically and I'm trying to get the engine tuned up to its running tip top. The first thing I want to adress is the issue I mention in the subject. Per my calculation it's getting around 9 MPG and the exhaust is smelling pretty gassy. It does not seem to be lacking in power and I do not see any gas leaks on the floor of my garage. My question for the community is where would you start on this? I am thinking that I will do fresh plugs and when I do that I may also do the valve cover gasket (not that I think that would affect my mileage, but one id leaking quite a bit and I believe I have to do some disassembly to get at the plugs.) But I am thinking that if the plugs were the sole issue it would be running poorly. It seems to be running very smooth. Maybe its just sending too much gas to the engine? More than can burn? So O2 sensors? Where are the important ones on this engine (vq35)?

Thanks for any insights. Really stoked about this car and the community here.

[macgiver]

Does " gassy " mean gasoline , would = running rich . OR "gassy" is the Fart smell , I believe is from out of spec NOx levels going out your exhaust ?
OR someone eating toooooooo much Mexican food? .....( my 'stupid' and 'ignorant' )

[VStar650CL]

Pull the spark plugs and see if any or all of them are rich (black). If only certain ones are black then you probably have issues with individual injectors (try a can of SeaFoam on a near-empty tank before throwing parts at it). If they're all black then look at things like the IAT sensor or fuel pressure regulator that can make the whole system run rich.

[Mike W.]

If you're getting 9MPG I would think you would have to have an OBDII code, that is the first place I'd check and that should point you in a direction. At 150K it's overdue for O2 sensors which are nominally a 100K part, although not many here seem to have changed them. I have and while the drivers side was a pain, the pass side was much worse. Little change in MPG or performance and it would be the fronts you would change, the easy to get to rears are for OBDII monitoring only and have no role in mixture or running. But check for codes first, otherwise you're just throwing darts blindfolded.