190e 1993 model has been to the doc for rough idle so bad the passenger side hood was visibly shaking. changed oil from regular 10w40 to quarter state synthetic 20w50, replaced motor mounts, added lucas gas treatment and replaced spark plugs. Improved for a couple of weeks and is now back bad as ever.
Any advice as to what to do next?

How about plug wires, distributor cap and rotor? Check for vacuum leaks around intake by using carb cleaner spray and listening for smoothing or pickup in idle?

Tim

I had a rough idle a few years ago and it turned out to be a leak in the vacuum line by the intake like Tim said.
good luck

Mercedes dealers have been "superceding" spark plugs on some cars to, as far as I can tell, manage their inventory a bit better (maybe fewer plugs for more cars, I don't know). The problem is that when you go to the dealership for, say for example a W9DC, you may walk-out with a W9DC0. The 0 in this case means it is a different plug (larger center electrode) and for example on my car, it caused a hellish idle problem. The engine miss was so bad that the hood bounced up-and-down as you described. After putting back-in W9DC's the problem went away instantly.

The moral to the story is to take a magnifying glass (figuratively, not literally) to the exact part number on your spark plug. If it is not precisely the original Bosch part number that is in the owners manual and originally sold with the car, then change it.

After checking-out the other fundamental issues in the ignition system (per other posts)-- distributor, rotor, wires, etc, you might try some fuel injector cleaner and running the engine for a nice hard ride ASSUMING that your acceleration and driving is miss-free, ping-free, and knock-free (the engine is running well when normal driving). I like Chevron's Techron fuel injector cleaner-- I recently found a vacuum leak and corrected that (another possible cause of your problem but you should exhaust all ignition possibilities FIRST) and also put some Techron in there and drove the car hard. The combination of the two (or just one or the other, I'll never be sure) increased gas mileage incredibly and allowed me to lean the car back-out at idle to maintain a smooth idle.

The order of troubleshooting for a problem like this I think best is 1) Ignition 2) Fuel System Cleaning (e.g. fuel injector cleaning) 3) Fuel system operation and Vacuum. Some mechanics like to sell you new fuel distributors, injectors, and manifold gaskets so be careful in step 3.

All suggestions sound excellent and as best I remember none of them have been done. I am especially interested in the difference in spark plug problem that was mentioned. But I am concerned because the original spark plugs that were in the car have been discarded - so it will be impossible to know exactly what came out. Any suggestions to be sure I get the ones I need if I can't compare to what was there before?
Once again, thanks to all of you experts! Maybe if I sound like I know something my mechanic will do a better job!

If you still have the owners manual for the car, you should check what it says. They typically specific precisely the BOSCH spark plug required. If you don't have an owners manual, you can ask other folks on the board and also cross-check with the Chilton's manual which is USUALLY but not always correct. Don't put the champion equivalent in there, etc. Also, only use the original copper spark plug, do not put platinums in there-- those have been known to cause issues. And finally, the correct plug wires need to be in the car-- use only the Mercedes/Bosch plug wires, they are built much better and have the correct resistance for the plugs that are in the car. On my 380 SL, there is resistance built into the spark plug wires, that's why the plugs have no resistance in them.

Yes, I do have the owner's manual and I shall check that tip out about the proper spark plugs pronto.
Thank you, thank you for all the pointers. I'm so glad to be learning about what my car needs rather than just being dependent upon the advice of my mechanic.
This is a great site!