My Fiesta IV -99 (Zetec 1.25) has been running a little choppy for a while. Not very much, but I thought it was time to do something about it. So I replaced the fuel filter and ran a bottle of fuel cleaner through the system. I also put in new spark plugs. I noticed that one of the old plugs was in much worse condition than the other three. Ok, time to start it up and see how it runs... Much worse than before! The engine fires up just fine, but it runs really choppy now. I drove it for a day, I could barely get it up to 110 km/h, and the fuel consumption is almost doubled. The next day I pulled out the new plugs, and one of them (same "problem" cylinder as before) was blacker than midnight. After only one day! Black and dry. I replaced it, and also put on new ignition cables. The car seemed ok for the first 0,5 km, but then the problem was back. Yesterday I hooked up an OBD2 scanner, but it didn't show any errors. Any suggestions?