Our MY 2010 S40 has a very unsmooth idle even though it's steady at 750rpms. Sounds like a Diesel engine. How is the idle controlled on drive by wire? I see no idle control valve to speak of anywhere. Is it controlled strictly by ECU at Throttle body? I've cleaned the TB but still not smooth. Could it be just the nature of the 5 inline design?

 

As a rule on any electronic throttle setups, start by cleaning the throttle body, both sides. This requires removing the throttle body.

 

Miles on the engine? Have you changed plugs in the last 100,000 miles? The ECU will do its best to keep the idle speed correct, even if the idle is lumpy.