rough idling

[The Gunslinger]

howdy all!

So the only thing mechanically wrong with my 1990 560SEL, is when it first starts up cold after sitting for a good 5 or 6 hours, it starts ROUGH, like so rough the car feels like its rocking for a few seconds, then it smoothens right out to a very minute rough idle, but its still there. When im driving, its smooth as silk and doesnt do it at all. Its only when im in park or in nuetral that this idling problem seems to occur. Ive put new plugs, plugwires, and distributor cap in it, and they seemed to help a little but it didnt completley go away. After the cars been driven for a while, parked, and turned off say 10 to 15 mins later, it starts up fine without the rough idle.

Needless to say this is confusing. Suggestions?

 

[Myros]

It sounds like you're over-rich

not for having a 560, although that does require a wallet of some proportion.
I think your plugs are getting wet at start-up and not firing fully. As the mixture leans off, your plugs spark better and the engine smooths out, as you describe.
Maybe worth having a look at the cold start injector, and maybe the idle control valve/airslide.
The acid test is of course start her up, then kill the engine while it's misbehaving, pull a couple of plugs either side and check if they are wet with fuel.