The car is Canadian spec., 1986 190E 2.3-16V. I've had this same problem ever since I bought the car a year ago. The car idles between 1200 and 1500rpm. I have been to the dealer with no success in addition to "specialists", etc. The car has been totally refurbished from the ground up but this problem remains. The last fix was to place a rubber bushing to act as "restrictor" after the idle stability valve. This causes warm-up to take 15 minutes in the morning but keeps the idle at 900rpm. Everyone says the problem is the ECU. So I exchanged ECU with a friend with a German spec. 16V and the same problem remained (but with slightly more power available). It must be noted that the car now runs on leaded fuel with cat removed. Today we installed an O2 sensor but there was no difference in idle. The car brochure says that for converting from unleaded to leaded or vice versa, two plugs must be changed in the engine compartment to inform the ECU in which mode to run. These plugs are unavailable anywhere here. One of my plugs has a resistor and the other is just short circuited.
A final note is that idle goes down when the A/C is turned on. When idling at 1500rpm, it drops to 1200rpm. When idling at 900rpm with the restrictor, it idles at 600rpm and stalls when coming to a halt.
After exhausting all technical capabilities in this country, I have only the net to trun to. Please help me rid this car of this disease. Thanks.