I have a 2005 35th 350Z in Ultra Yellow with 70k miles and notice that when I take it out for a drive, when I return there is a layer of what appears to be exhaust soot on the rear bumper. My initial reflex is thinking my car is running rich or not fully burning the fuel. Is this normal or does my car need to be tuned, maybe just new spark plugs? There do not appear to be any modifications made to the car, so I have a hard time thinking it needs tuned. On the other hand, I have only owned it for about 3 months, so maybe I am just a noob and should suck it up since I am lucky enough to have such a cool car.

 

The rear of the 350Z collects lots of dirt. Probably due to aerodynamics. If you are concerned have an air-fuel test done. That way you know for sure.

 

I have the same problem with my uy. The rear gets dirty before anything else. Many z's I have seen have the same problem. It's just more noticeable on the yellow orange and white for sure.