I have been having a weird idle on my 2001 v70, while the car is in park or shifted into drive sitting at a stop light. The engine will idle regular then it begins a patern where the rpm's drop for a second then runs fine for about 3 seconds and continues in a rhythmic pattern, as long as the car is sitting still. About a month ago The engine started miss firing and had a horrible shake while the car was in park, drive sitting still, or while the car was attempting to be driven. I Had the code of Po304 and took it to a local shop, Not a Volvo shop... He told me exactly what it was and corrected the problem of the horrible shaking and the pulsating idle for total price of $200.00. The mechanic changed the spark plugs and and replaced one ignition coil... The car worked great for about a week and the weird, rhythmic, patterned idle returned... Can someone tell me what the problem may be creating this pulsating idle before I bring it to the shop again.

 

my 99 v70 GLT had similar symptoms (shaking when coming to or at a full stop like the car was going to die) the problem turned out to be a plugged PCV system - dealer charged $800 to clean and replace the parts. Almost four years later the same symptoms happened again and I had to have the PCV system cleaned again ($600 indy shop). A couple of weeks later I had a problem with erratic rpms (engine's rpm going up and down on its own) indy mechanic told me the the electronic throttle module needed to be cleaned ($270). It's been a month now, so far so good.

 

most likely the ETM is failing, common failure in these cars.

Do a search on ETM failures.