grey smoke is coming out of my tailpipe.Noticed the day before that the car idles very low when first started almost to point of shutting off until i tap gas and it idles back to normal.When i hit the gas i see more smoke.hot or cold it continues to smoke.Afraid to drive it.Is this a costly problem ?If so how much and can i fix it myself or should i take it to my mechanic?PLEASE HELP SCARED TO DEATH!!!!