ok my sister has a 02 dodge neon 2.0 liter and when you first crank the car up you can a hear a really loud air noise coming from the left side of the motor but when it warms up the sound is faint but stil there and its making the car get half the gas mpg than it was before this happened. its sounds like a vaccum leak. i was wondering if anybody knows what it is. any help would be appreciated.

 

try replacing the PCV valve

 

take off the rubber accordion breather tube that goes from airbox to throttle body and look for splits. when they do split its on the bottom where you cannot see without taking it off.