I have not smelt it before, and can't really describe it, but it is similar to burning oil. My cleaning cycle is up in about 300 miles. I do not notice any missing oil, transmission fluid, or supercharger oil. I haven't checked the radiator lately, but i will check it tomorrow. i also ran half a bottle of Reline SI-1 through the car a few weeks back. I think that is about the time that i started paying attention to what it was doing and smelling like.