I have been told "they all do that". THe 450 & 500 series engines with the electronic idle control are the ones he was referring to. At idle, mine will run perfect and silent at certain tempratures and at others it will slightly cycle up and down but so little I can't see it on the tach. I figure if your tach is moving then the RPM must be going up and down. I think you car has a manual idle control. If the tach chcles but the motor rpm doesn't change it is most likely the tach. Best luck...

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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