Most cars have minimal odor to their exhaust. It smells like, well, exhaust, but it’s not terribly strong. This is thanks in large part to the catalytic converter. However, some cars seem to have really smelly exhaust fumes due to a number of different reasons (depending on the smell in question).
If an engine is burning oil (due to leaking seals), then you’ll definitely notice the car has really smelly exhaust fumes. You may also notice that it creates smoke from the tailpipe (bluish in color).
Failing catalytic converter
Ever been riding along and suddenly smell rotten eggs? If so, you’re behind (or beside) a car with a failing or failed catalytic converter. Sulfur is found in gasoline, and is turned into hydrogen sulfide in the combustion process. However, the catalytic converter changes it into sulfur dioxide. This has no odor at all. As the cat fails, it stops changing hydrogen sulfide into its odorless counterpart and the result is a strong smell of rotten eggs from the exhaust.
Burnt antifreeze (hot and sweet)
If what you’re smelling is sort of “hot and sweet,” it’s probably engine coolant. There are many ways this can happen. A pinhole leak in a coolant hose can spray antifreeze onto a hot engine component, which boils the coolant away and creates that smell. It can also contaminate oil and be burned in the engine that way.
It’s a diesel
Diesel exhaust smells much stronger than the exhaust from a gasoline engine. You’ll also notice that the exhaust is darker, particularly on large trucks with powerful diesel engines.
If the exhaust smells like gasoline, it’s a sign that the air/fuel mixture is off in the engine and too much fuel is being added to too little oxygen.
If you notice any of these smells coming from your exhaust, you should have the problem diagnosed as soon as possible. If you notice there’s a strong smell of exhaust in the cabin, you should also have this diagnosed and repaired quickly. It’s a sign that your exhaust system is damaged. Keep the windows down to prevent carbon monoxide from building up in the cabin.
This article originally appeared on YourMechanic.com as Why Do Some Cars Have Really Smelly Exhaust Fumes? and was authored by Valerie Johnston.