Quick input sincerely appreciated here...I have spent literally hours with the Search function and reading threads en masse:

88 420 SEL w/138K mi. I'm 3rd (new) owner.

Starts fine, idles rough 'till warm. In Neutral, idle pushes 1100. In Drive, idle @ ~550.

I've replaced rotor & cap, have wires on the dining room table tonight and will gap new plugs tomorrow.

Any further suggested action if this doesn't help?

Sorry to bring up the "mundane tune up question" but no Haynes guide available on this model, and my $25 aftermarket copy of a supposed "shop manuel" wasn't worth it

PS Happy New Year