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What’s that smell in my car?

Even in the era of cars that are basically microchips on wheels, your nose can be a very powerful diagnostic tool. Unusual car smells can be the sign of serious and potentially costly trouble for your car. Whether you're driving around town or taking a summer road trip, here are a few of the top smells that your nose should know.

Man plugging nose due to bad smell

Any of these smells are worthy of seeking professional help from a certified mechanic. So, listening to your nose could help you avert major repair bills and keep your car running safely. Note: Make sure you wear protective eyewear before looking under the hood.

Gym socks

If turning on the air conditioning or heater blasts a smell like a gym locker, it's likely a sign of mildew growing inside the A/C system (usually in the evaporator). One easy fix is to run the fan for a few miles without the A/C on to dry out the system. A mildew odor that happens even when the car is not running could indicate water leaking onto the carpet around a sunroof or window seal, or a plugged A/C condensation drain. Check for damp spots and see what could be allowing water into the car.

Burnt rubber

The pungent odor can indicate slipping drive belts or loose hoses that might be rubbing against rotating drive pulleys. Open the hood and listen to the engine run; you may be able to pinpoint the cause. Note: Don't touch anything under the hood while the engine is running or if it's still hot.

Sweet syrup

The smell akin to maple syrup usually means that toxic antifreeze (AKA ethylene glycol) is leaking. If the odor is strong inside the car, that means the heater core may have a leak.

Outside the car, the usual suspects include a radiator or heater hose, an intake manifold gasket or cylinder head. It could also be dripping from the radiator cap or the radiator, especially if you smell it outside the car. There may be telltale droplets under the car.

Hot oil

If your car has an oil leak, the sharp burning smell when it's hot will let you know. Cars can leak for a multitude of reasons, and none of them are good. Oil leaks from a bad crankshaft seal, or even an overflow from an oil change, will find their way to the red-hot exhaust manifold and create a nose-wrinkling aroma.

If you suspect an oil leak, you can look for drips on the driveway. Unfortunately, a leak that gets fully vaporized won't leave spots underneath. So, let the car warm up and look under the hood to see if you can spot any smoke to help you hone in on the problem.

Burning carpet

This smell usually emanates from the wheels due to overheated brakes. This can be normal during a trip through the mountains. But if it's happening during around-town driving, the vehicle could have brake problems, such as a seized caliper piston. The smell is the brake pad material being worn off at a much higher than normal rate.

Rotten eggs

To be clear, the smell of rotten eggs is not natural for a car. A best-case scenario involves a fuel injection problem, which can often be fixed with adjustments by a mechanic. A worst-case scenario involves a faulty catalytic converter that's failing to properly convert hydrogen sulfide in the exhaust to sulfur dioxide. This smell can also be a symptom of an engine that's not up to speed, overloading the catalytic converter with unconverted particles. The bad news is that catalytic converters are expensive to replace.

Gary Wollenhaupt is an experienced automotive writer, writing for Ford, GM, Saab and others. He’s also a shade tree mechanic tasked with keeping the family fleet on the road.

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