So i replaced the radiator in my car cuz i bew a hole in the bottom of it. Everythign was replaced correctly. ran fine for about a day and then My car does not idle. It drops below 500 and jumps to about 1200 constantly. I have put in fuel injector cleaner and also 3 tanks of premium gas.
Im also only getting 9mpg as i was geting 24/21.
Not sure whats going on please help me

 

Does the car shake when it's idle?

 

very much so. Its a very hard idle. It kind of jumps. Also i was told it could be the IAC sensor which im hoping not cuz they are expensive

 

Pretty much everything with Volvo is expensive unfortunately. You can save some by having your own trusted specialist mechanic though. I had this problem a little while ago, and it left me stranded at a parking lot, so watch out and get it taken care of. It sounds very similar to what I had, which was a failing mass air flow sensor. I had mine replaced at Volvo, which charged me $450 with parts and labor. It was steep, but it had to be done. Sounds like that's what it is, but I'm not 100%. Definitely get it checked out though.