as far as I know, it's normal. I changed mine last year for the second time. The first time I did it, they were all covered in carbon. When I did it again a year later, after two went bad due to a bad batch of glow plugs according to my local tech, they were covered with carbon again. Always buy Bosch when buying glow plugs, I'd prefer paying a little more and doing it once. By the way, the brand of plugs that went bad were "Monark Diesel." Has anyone ever had experience with these? They were only a few dollars less than the Bosch's when I bought them from a trusted shop. Not the quality I expected though.