To THIS question, I can speak with experience. In my car -- exact symptoms you have -- I changed it with NO difference. I also unplugged the idle valve for a test and had no difference. My plugs were changed 5,000k ago, and no difference.

brookspw

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1994 C280, dark green metallic
105k miles

1991 190E 2.6, Black
191,500 miles
(sold to another forum member)

2003 Chevy Tahoe LT, Redfire Metallic
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1989 Mustang GT Cobra Convertible
43k miles