2011 Mini Clubman
 

Replaced the thermostat with an OEM product in my 2011 Mini. Original thermostat was apparently stuck open, no heat, I drove it for a couple of months with the engine light on. It developed a major leak and the plastic housing was melted. The car is now idling rough, showing all 4 cylinders misfiring. Seems to have power during test drive. No smoke from exhaust. Never had this problem before replacing the thermostat. How would changing the thermostat cause this?