I recently had my spark plugs changed by a mechanic. I wanted to see if he in fact did change them. Well, it appears that he did. However, my #3 spark plug was rather dirty (sooty). All of the other plugs looked fine. I only put about 1,000 miles on the car after the new plugs were installed. My question is, what would cause the fouling of that spark plug? A bad valve or a bad fuel injector? Also, would this explain why my car (97 E320 w/ 98K miles) idles rough?

 

yes, this causes rough idle.
sooty plugs means rich or lean mixture depending on how you look at it.
only the fuel injector can cause it or air leak.

 

i think you got the bad value seal......
if your car dont have the big smoke when you driving