OK update:

I had emissions test performed again today, and passed with flying results. Both HC (ppm) readings at idle and 2500rpms were below 20, the limit being 200! Considering I failed the emissions prior to switching to the proper spark plugs. But in all fairness (according to my experience) I must say that the NGK plugs lasted atleast 15K before showing noticable signs of failure. But one questions still remains, I was hoping that the proper spark plugs would return my high idle to normal again, but so far no luck.

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190e 2.6 1990. 300,000 KM
C230 K 2000, 130,000 KM