if you block the engine air intake it will make the car run richer - (some cars have hot/ cold - summer/ winter air intakes) - better to fit a fresh air filter if you want to get it to run less rich/ leaner

i presume you have not tried blocking' venting the breather or running hotter plugs as i have suggested?

the actual engine temprature is one thing (controlled by the mechanical thermostat/s) the temprature the ECU "thinks" it is can be totally different (as measured by the electronic sensor/s)

somtimes you can get "bogged down" with sensor readings/ fault codes - the problem and solution can be deceptively simple - like a loose connection