Idle Air Solenoid disconnected & it runs better
 

Recently I resorted to having our '91 300E checked on the new Sun computer system. Symptoms: Starts well when completely cold engine, but has a little roughness at idle, and if idled for an extended time with warm engine will occasionally bring on CE light, but starting the engine after warmed up will take a time before rpm stabilizes, and might even die when placed in gear. Get the car moving however, and with the exception of a little roughness at idle, everything is fine. Car has new fuel injectors, engine coolant temperature sensor, plug wires, Copper plugs, rotor and cap, and is not leaking around the fuel distributor's vertical metering shaft (whatever it is called). The idle air solenoid was also replaced along with the rubber hoses to and from it.

The roughness on idle seems improved, and the hot start, rpm hunting, and dying when first placed in gear, all these are not a problem when the idle air is disconnected.

Any suggestions would certainly be appreciated. Maybe the new idle air was defective, but it does operate in both directions, when checked on the bench.

What model?

I had a similar problem with my 85 500SEL. The reason it runs better when you disconnect the idle stabilizer is because it does not have any way to control it then. If the rpm's increase when you disconnect it, then you know it is not the idle stabilzer that is the problem. If you have a multimeter, you can check the circuit to see if the electronic idle control box is working.

I would check the operation of the vacuum control solenoid. It is the the little thing that is next to the fuel distributor that "buzzes" when the car is running. It creates a controlled vacuum leak in the intake. If it is buzzing then it probably is fine.

The big thing I would look at is vacuum leaks. that turned out to be my trouble.