Also see, if it has a catalyst converter installed and what were the contents of exhaust as exhaust analyzer tells a lot if car is running rich or lean and is used instead of STFT, LTFT in latest vehicles. How about injectors ultrasonic cleaning, plugs and fuel pressure test. One has to look into every aspect if one is suspecting a problem with rich and lean condition.
P.S: computer testing to see coolant temperature sensor values and intake temperature sensors values. See if thermostat valve is installed or not and if not then you found your culprit.