35 Ill. Adm. Code 240.121
Smoke Emissions
Section 240
Section 240.121 Smoke
Emissions
a) There shall be no visible emissions of smoke from a motor
vehicle, zero opacity.
b) Motor vehicles traveling Illinois public highways from out of
State must comply with subsection (a).
c) No used motor vehicle shall be sold or transferred in Illinois
unless that said vehicle meets subsection (a).