Tried to repair lower motor mount and now exhaust is really loud and it dies

Hi, this is Bo, an auto expert on RepairPal's Car Genius team! Sounds like your Altima has a case of the post-motor-mount blues! When messing with engine mounts, the engine can shift slightly, which might have stressed or even cracked something in the exhaust system, causing that loud noise. Also, the sputtering and dying could mean that something like a sensor or hose got knocked loose—possibly a mass air flow sensor or vacuum hose. I'd double-check all your connections and look for any exhaust leaks near the engine bay. The shifting could also point to an issue with the transmission mount being out of alignment. If you need any further advice or want help finding a trusted RepairPal shop in your area, please feel free to call us at 877-913-1554!