Engine exhaust is not odourless or necessarily colourless. It contains numerous gasses including both odourless, the carbons, and pungent, the nitrides and sulphides. The latter are the tell-tale smell for the odourless. Exhaust might also have dark smoke due too rich and/or blue smoke due oil burn. Fume smell is more noticeable in old tech engines compared to modern engines due more efficient burning and catalyst converter. Exhaust gas only becomes dangerous if characteristic smell is not acted on.