So recently I had a friend of a friend replace the thermostat in my Alero. The cars temp is just fine but since he did it, the car runs extremely rough and became 50x louder than before. Before, you could barely tell the car was running because of how smooth and quiet it was. Like a night and day difference now since he replaced it. From what I can tell, he disconnected the exhaust to reach the thermostat. I believe the flange gasket fell apart and was never replaced before he reconnected the exhaust. My question is would a faulty flange gasket cause so many issues (serious misfire, terrible fuel economy, erratic idle, inconsistent acceleration power, extremely loud compared to previous condition) and also since he had replaced the thermostat, is this a good deduction of what happened?