db13

My son came home the other day(2001 Chevy Cavalier 2.2 OHV 3spd auto) with the exhaust flange that bolts the cat/res pipe together broken.I fixed that and he ran that night but the next day said he was going up a long hill when it lost power (says he smelled rotten eggs)back fired from pipe and ran like crap then it started running fine.Then Sunday he said it ran fine.Monday he takes it to work only to make it about six miles from home when it starts to act up...no power at all,shutting off,rough idle when it would start...so I gave him my truck to go to work and started the long journey home at 15mph....but no luck...about 3 miles out it shuts off and won't start...turns over,bucks,fires,won't start.I scanned for codes and got P0440 EVAP malfunction (gas cap)and P0108 MAP/BARO CIRCUIT HIGH INPUT.Going out later to clear codes and try to get it started.Any how my question is when I hooked his exhaust back together and started the car for the first time I heard gravel/rust whatever going down to the pipe,would a plugged muffler cause this?Sorry this is so long but trying to post the whole story.