A couple of things come to mind from when my 12v battery went south, usually from cold when I move off the car moves off on electric and then starts the engine a little while later switches to petrol ,this didn't happen so I was revving a cold engine to get moving I assume running richer than normal. A lot of functional errors came up as well maybe the engine ecu could be affected by low voltage and cause the car to run in a richer mode.Seems like a lot of sensors are run by the 12v system such as the 02 sensors etc so could this cause the engine to run richer?