If the front end is good (ball joints, tie rods, drag link...) it is probably bad tires, a bent rim or tires out of balance. I changed almost ll the body rubber including lower a-arm bushings and I didn't have any vibration before I changed them. I also changed the rear sub-frame bushings and differential busing and it helped wander, but no vibration. I have a car that had a bent rim and it showed up after chasing vibration. You can balance a square but if it's not concentric it will set op a vibration. MHO...

Regarding idle, I changed almost the whole ignition system but the big change to idle was changing all the fuel injectors. It was my last resort. A vacuum leak will cause problems there too.

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Steve
1985 Mercedes 500SL Euro (Gray market)
1995 BMW 520i Euro (Gray market)
1992 BMW 525it Wagon
1994 Honda Del Sol Si