Sinceyou do have an idle problem,
I'm quite certain the culprit is the O2 sensor and the fuel pressure regulator.
Good advice from carman850, problem is on the KE Jetronic injection of the 190E you don't have a fuel rail to attach a measuring device to. You'll have to check if the regulator is producing a vacuum on the little black rubber hose that comes out in the front.
I had the same problem on the 88 300E, which has about the same injection setup. Turned out to be injectors, fuel pressure regulator and the O2 sensor. As long as you don't have a hard start issue, the regulator might not be involved though.
As far as I can remember the fuel pressure regulator is not too cheap, runs about $160 or 180 the last time I bought one. Expect the O2 sensor the cost about $130. These prices are for OEM quality german made products from either fastlane or megaparts.com. I wouldnt put anything less on my cars.
For now you can try to adjust the fuel/air mixture with a 3mm T-wrench. There's a hole in the center of the airfilter in which to insert it. Press the wrench down slightly until it catches and then turn slowly and release the pressure to see what the effect is. Right is richer, left is leaner.
You might have to take the air filter off and drill a little metal ball out of the neck that houses the adjustment screw to get to it. ONce the ball is out there's a little metal plate in there. You can just take that out. Its not attached to the surrounding metal.
Dont turn the screw too much though or the computer wont register the cange or screw up the setting even worse. Just a tiny bit at a time. A hair-turn can make a big difference.
You can also get a smell of your exhaust. Rotten eggs is too rich.
A car on the rich side runs a tad more powerfull, but will screw up emission control equipment like cat and O2 sensor. Expensive 5min of more fun.
To cheaply test the O2 sensor my ex-boss (now independent MBZ mechanic in HIlo, HI, MB tech since the 60's, trained in Germany, worked for MBZ of Hollywood) tells me to disconnect the O2 sensor cable and see if the idle changes. No changes, ergo O2 sensor bad. I'm not so sure about that, but you can try it.
Best of luck!
If you need more specific directions, I'll mail you a picture.
81 300 SD grey/palomino 168K miles
84 190E 2.3 black/grey 64K miles (wrecked)
85 190E 2.3 maroon/palomino 92K miles
88 300E desert red/palomino 204k miles
(made to look like a '94-95)
92 400E desert taupe/creme beige 120K miles
(converted to 94-95 E420 looks)