rough idling

It could be many things but it is hard to comment without being able to do some basic diagnostic tests or experience the problem first hand. Your car has one ignition coil per cylinder and a spark plug per cylinder either of which could be failing. A clogged or failing fuel injector or vacuum leak could also cause this type a problem. Is the check engine light on? Attach a scan tool to see if any mis-fire or pending codes are stored in the computer in your car.

Great advise from Patric. I replaced many fuel pressure regulator and ignition coils on S40s. Hook up a diagnostic computer - if possible Vadis, Vida or DICE. Generic scanners could give false info on these cars. With this problem I'd check if the fuel pressure regulator. Remove the vacuum hose and check if there is any fuel in it. Zee

ignition coils is my guess .....my car had the same problem this car has 2 coils 1 every 2 cylinders.......i changed mine and now car runs fine