I have a 2008 Ford Crown Victoria.
Yesterday I went shopping and when I started my car to go home, the engine had a rough idling and eventually stalled when the transmission was in D. I didn't know why, and my car was always running well before that. I restarted the car and this time it ran fine.
Then I drove home and on the way, I realized the exhaust sound was louder. I didn't take it seriously and when I got home I realized my muffler was rusted off and gone! I don't even know where it is. Probably dropped it already on my way to the shopping mall, which is quite horrible to think about.
But I still don't know why a missing muffler could cause the engine to run rough and stall? I tried again and the car runs rough and occasionally won't accelerate when I press gas pedal at D. Shifting to N and back to D solves the problem. Sounds like a transmission problem but how can a bad exhaust cause this kind of weird behavior?