I have just returned from a 3 year stay in Italy, and when my Cobalt arrived back in the US I suddenly had issues with it. first it ran great for about a week then out of the blue it started to rough really rough. I replaced the Mass air flow sensor with no change. Then a replaced the ignition coil and spark plugs, a little better but still a rough idle. I have also cleaned the throttle body which there is still no change. Does anyone have an idea what else i can do?
Is the Check Engine light on?
Maybe it is pining for a lost Italian love, a sexy Alpha Romero say, and your Cobalt just “has the blues”! lol
Seriously, my first guess would be an air leak. Unmetered air is getting into the system and causing a lean mixture. My first suspect is a ill-fitting vacuum line or a vacuum operated device that is leaking vacuum. The air then has a direct path to the intake manifold, bypassing the MAF. This will show up as a rough idle, but not be noticed or less noticed at highway speeds. Good idea to bring all the routine maintenance up to date so you are not wasting your time looking for something which should be done as part of preventive maintenance suggested in the owner’s manual. Also check that the PCV and EGR systems are working correctly, as they can cause rough idling too.
Clean the throttle body.