The smart 453 3-cylinder petrol engine utilises a camshaft control solenoid that is located in the top of the timing cover on the right hand side of the engine (easy to spot due to its green 2-pin connector and single 8mm retaining bolt) that controls the flow of oil to the cam adjuster.
Part Number: A2811530100
It is very common for this solenoid to become clogged (especially on a car with poor service history) which will cause a lumpy idle and some times timing (crankshaft/camshaft correlation) faults.
The sensor can be removed (simply unplug the loom, remove the 8mm bolt and pull the sensor out) and cleaned (with brake or carb cleaner) or replaced.