Is there supposed to be a ball-bearing jammed in my EGR hose?
 

I have been fighting with my 450SL for about six weeks now trying to get it to idle better in D. It shakes the car in Drive but ok in Park. SO far replaced:plugs, wires, Dist. cap and rotor, additional air valve, fuel filter, Injector seals and holders, checked all vacuum hoses, set timing to TDC, checked fuel Dist. screens. It still idle's pretty rough and hesitates on acceleration. Ay ideas about why the previous owner may have put the bearing in the EGR. IS it supposed to be there I took it out and reconnected the hose and still stumbles on Accel and shakes.

thanks for any help.

Prevent porting of vacuum to the EGR to prevent it from ingesting exhaust gases at partial throttle - a common "fix" for diesels.

While this is likely to help some diesels, it probably does little for a gas engine car ... as you can tell, it is obviously not the problem with your SL.

The shakes
 

Check the vacuum advance/retard on the distributor. If the diaphragm is leaking, it can be a big air leak and cause hesitation and rough idle. It sounds like you either have a vacuum leak or a burned /leaking valve on the engine. Have you checked the compression in each cylinder to verify the engine mechanicals are OK?

If the compression readings are all ok, it can still be a worn camshaft lobe, but those do not happen suddenly and is way down the list of things to check.

The EGR and all the other fuel and vacuum system have to be in perfect order for a smooth idle in D. If even a small amount of EGR leaks past the control valve at idle, it will not idle smooth.

I do not know specifically how the EGR gets back into the intake system on your engine, but many MBZ's had it flowing through passages in the cylinder head to each intake valve pocket. If some of those get plugged up, then all the EGR flow goes into the others and really causes bad hesitation and a rough running engine just off idle.