Good question. My 450 cabrio occasionally does this as well. My only assumptions are a bad ground or a bad alternator, but I'm really not sure.
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That’s possible solenoid failure.known issue for rough idle
www.evilution.co.uk
453 Rough Idle | Evilution
www.evilution.co.uk
The cars above are not 453s.... 
My bad then thought it was like mine 453 0.9T 😁The cars above are not 453s....
My 2010 Smart 451 slowly developed rough idle over the last 5 years. It is getting worse with time, thus something must be wearing out or slowly failing over 5 years. It only has 110,000 Km (66,000 miles), so that seems too young to be having such issues.That’s possible solenoid failure.known issue for rough idle
453 Rough Idle | Evilution
After reading many forum posts, I tried SeaFoam, MMS, Spark Plugs, Coils, Crankshaft Position Sensor, ECU (ME) replacement, Upstream Oxygen Sensor, Compression Test/Leak-Down okay, but NONE of these items fixed the rough-idle issue.
By observing the Timing Advance Waveform with OBD2 Car Scanner Pro App, I see the timing (at idle) is erratic and goes from -5 deg (retard) to +30 deg (advance), so I am thinking there may be some connection with Camshaft Hall Sensor or Adjustable Camshaft Timing Solenoid. So today, I looked at the PWM signal driving Adjustable Camshaft Timing Solenoid and it appears to be reasonably stable, certainly not showing a 30 degree jump in PWM to effect timing adjustment. Also, I look at the Camshaft Hall Sensor and its waveform is quite stable too. But, the problem with any closed-loop control system is to figure out what component is disturbing the loop: Is the unstable RPM causing the Hall Sensor to change, or vice-versa.
So now I am thinking that Hall Sensor may be okay, because it is giving good timing information to the ECU, which leaves theTiming Solenoid to be suspect. Perhaps the solenoid is erratic on its own behalf, because the PWM input signal is not changing the solenoid's position. Sounds unlikely, but perhaps it is "sticky" and over-compensates in jumps to different angles.
The only other alternatives include loose timing chain or loose tensioner, both mechanical issues outside of the ECU control domain.
If anybody has any ideas, I would like to hear them.
Thanks,
Garry Anderson
Mobile 604-805-2555
Rough Idle begins after Secondary Air Pump turns off, indicating ECU beings its "closed-loop" mode. Some other sensor must be monitored after the first 25 seconds, and that sensor is causing rough idle. Is that possible?
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