I had rough idles on cold start, so i decided to replace all my spark plugs this past weekend.

i replaced by my plugs with NGK Iridium Ix Spark Plug (BKR6EIX), pic below:



I'm getting rough idles now constantly. Did i use the wrong plugs. I didn't gap them, i assumed they were pre gapped to fit my car, 03 X5 4.4i. While searching online, some ppl suggested to clean the coils with dielectric grease. Help!!!!