If you have had diesel in the tank can take a while to clear, and could just be this, run the tank out then refill full this is what I did when I accidentally put diesel in a petrol once.
You can normally tell if it is diesel smoke as it will burn in a petrol engine a bit more white than blue ?
Don't forget diesel is an oil so burns like one.
It took the car I did it to a good 250/300 miles to get rid of EVERY trace.
tabetha