incomplete combustion occurs. mtbe is (was) one of the primary components of this unburnt emission. it was one of the ‘heavy’ toxic chemicals which spewed out of exhausts all over the roads. and it was NOT burnt up in the catalytic convertors either (hence the need to eradicate mtbe.)
i am sorry i can’t recall where i read this bit, but i swear it was in a technical pub. i just can’t recall it now.