Cleaning out the injectors
 

Any idea on how to clean injectors on a 1991 560 SEL. I notice also that the car is idling a little rough, small shaking here an there.

Chevron Techron is the easiest solution. Just pour a bottle with your next fill up.

Other than replacing the injectors I have also heard of good results with ultra-sonic cleaning.

I've successfully cleaned injectors at home. I have access to an ultrasonic cleaner and a variable voltage supply. The injectors open at around 3v. DO NOT USE 12V!

A google search will reveal many shops where you can mail your injectors and they'll clean and flow balance the set of injectors for about $25 per injector.

Check for vacuum leaks as well. That could cause a shudder at idle

You wouldn't want to spend too much money having CIS injectors professionally cleaned. It isn't very expensive just to buy new ones.

I hear this all the time. Am I the only one here who thinks 58 bucks an injector is not cheap?

No, I guess that's not cheap, but they can be found for considerably less than that.

CIS injectors are not electronic; just spring-loaded poppet valves.
They pop open at about 50-60 PSI or so, if memory serves.
I made a simple cleaning kit from a piece of clear tubing and some carb cleaner. Just attach the tube to the injector, fill it with cleaner, and stick a nozzle from your air compressor on the other end. A zap of 50-60 PSI air will squirt the cleaner through the injector.
That being said, It didn't help my warm start issue, and neither did the new injectors I later bought. So, I can't vouch for it's effectiveness.

DG

I guess it's relative.
It's not that it's "cheap". But it's very close in price to having them professionally cleaned.

$58 seems expensive, but I haven't shopped them in a while.

Last time I checked, it was $30 to have them cleaned and $40 to buy new ones. At those prices, and injectors with 200K miles on them, I'd opt for new(also, mine's only a 4 banger). That being said, I cleaned mine in a similar fashion to what S-Class Guru did, and had very good results.

jp