it has 75,000 original miles, but it has a rough idle. when i take it out of gear the idle smooths out
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Another cause of rough idle can be the vacuum lines...these lines are now 21 years old and they tend to get brittle and leak around the connections causing a roughness in the idle. Other causes for the rough idle could be in the ignition department....plugs, wires, etc. The car idles better when out of gear because when in gear the extra load brings out the worst of any problems.

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1986 560 SEL (159K miles)