I just received a new set of h9dc0's for my '90 300se (184k miles) and found that the plug in cylinder #1 was heavily fouled, while all of the others looked relatively normal. I decided to install the new set and drive around a little. The idle was so smooth that I thought the car had stalled while waiting for the green light at the intersection! I thought that it was incredible, as the passenger side engine mount is collapsed, and the driver's side is torn (I've already ordered them).

Anyhow, around 15 miles later (hey, that's pretty far here in Hawaii ), I pulled the plug in cylinder #1 and found that it had already started to gain a shiny black coating on the insulator. The plug itself was a little wet, and there was a clear liquid on the threads. I tried sniffing it but couldn't decide if it was fuel or motor oil.

A few days later, I pulled the same plug again and found that there were now a number of black "clumps" stuck to the insulator. The slightly rough idle that I experienced before changing all the plugs returned, so I think that the engine is misfiring in that cylinder.

I've done a lot of searching on this forum and it looks like I might have to get the head done, but what I'd like to make sure of first is that it is in fact oil fouling rather than gas fouling. Is there a sure-fire way to determine this? Maybe I could try and light it while it's wet?