Okay, (B20f will correct me if I'm wrong!). Mine is B18ft. Idle probs are notorious & I THINK the same for you.
First port of call is Idle Control Valve - Take a peek at this brill 'how to':
http://volvo480.dragons.org.uk/forum...ic.php?t=15569
If yours has a throttle position sensor, clean it up with contact cleaner & Vaseline, then refit.
You may have an engine temp sensor & an engine COOLANT temp sensor (mine does). The latter if dead can cause rough idle as it tells the ecu that the coolant is still cold (continously) so "give me more fule please" causing rich running etc..............
Good Luck
