Most if not all 103s will show a hint of uneven idle roughness, and it's probably more pronounced on manual transmission cars when idling in neutral. The slight load that an auto trans places on the engine along with the damping effect of the torque converter will smooth out the idle, and the additional load of the A/C compressor will further smooth the idle.

The source of this -what I call a "designed in feature" - is two fold. First is the KE injection system. Continuous flow systems tend to have more cylinder to cylinder fuel distribution variance than solenoid injectors, especially at idle, and I also expect this is a root cause of the 103s higher than average emissions.

The second problem is that maintaining the average idle mixture at stoichiometric (14.7:1) rather than the 12.5:1 that the engine would prefer (and was typical on pre-emission cars) can create greater cyclic or cylinder to cylinder variation in the pressure-volume diagrams. In other words, there is enough cylinder to cylinder torque variation over the two revolution for all six cylinders to deliver a power stroke that the engine can develop a slight uneven torsional vibration that we feel as a slight uneven idle roughness. The combination of the "lean" idle mixture and cylinder fuel distribution variance conspire to create the slight uneven roughness that we often feel.

I noticed on both my former '84 2.3 and current 2.6 that both developed this characteristic slight idle roughness at about 40K miles. It varies in intensity from time to time, and always seems to idle better after a good "Italian tuneup". The slight roughness is primarily a cosmetic issue, but the emission issue is real. There is no "fix" - you just have to know how to play the emission test game.

The idle roughness is never noticeable until the engine temp achieves about 60C, which is about the time is goes into closed loop operation and the average mixture is maintained at stoich. When cold the idle is always butter smoother because the mixture is richer.

It's just the nature of the beast and why Mercedes eventually abondoned the KE system in favor of a full EFI system with solenoid type injectors.

Duke