The answer to 'why is my sump pump foul smell' is that a foul odor coming from your sump pump is often caused by bacteria and organic buildup in the pump or the sump pit. This can happen when the pump isn't regularly maintained or if there is a lot of debris and organic matter in the sump pit.
Some common causes of a foul sump pump smell include: standing water in the sump pit, a clogged or blocked impeller, a cracked or damaged pump, or a lack of regular cleaning and maintenance. The smell is typically a sign that the pump and pit need to be cleaned and disinfected to remove any buildup or debris.