'95 M3: high RPM at Idle

["Michael Ting" <lupin@xxxxxxxxxx>]

I changed the spark plugs three days ago, with standard Bosch
Supers, which is
the OEM for my car. I tightened the plugs to 20 foot pounds.
Afterwards, everytime I want to turn off the engine after a
long drive, and
put the transmission in 'N' or 'P' (it's an auto), the RPM goes up to about
1000-1500 rpm.
Normally it's around 700-800 rpm.

Is this normal after a spark plug change??

BTW, this only happens AFTER driving. 
When I first turn on the engine, idle is normal in the 700 range. 

Thanks,

Michael Ting
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