Why do some exhausts smell bad?
The most common reasons for an exhaust system to smell bad is the result of the thermal reaction that occurs within the catalytic converter. The temperatures inside catalytic converters run around 1200F to 1600F. The front substrate inside the catalytic converter is used to convert carbon monoxide (a byproduct of combustion) and hydrocarbons (unburned fuel) into carbon dioxide and water vapor. The rear substrate inside the catalytic converter reduces oxides of nitrogen into nitrogen and oxygen. The catalytic reaction between the fuel being used and the noble metals in the substrates creates the smell that you're concerned about. This smell is significantly present on brand new catalytic converters when they light off for the first time. If you are noticing that the smell is becoming overwhelming, this could be an abnormal issue, and you may want to have the smell inspected to make sure there is not something unusual going on.
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