blown coolant seal the rough idle and exhaust smell are the exact problems i had with mine. its almost guaranteed if you overheat a rotary and don't catch it in time ur gonna lose a coolant seal. If the mileage on the engine isn't too high u can just buy a soft seal kit and tear down the block and fix provided u didn't warp a housing.

mine went bad after 250miles on breakin with a mazda north america reman. it sounded like a four wheeler untill it warmed up and smoked when cold.

 

^ been there, on my FC though, drove it for 3 months puking coolant, lolol

 

good luck ... i hope u do find the problem...