What Might That Smell Mean?

By Eric Peters, Automotive Columnist

You can guesstimate with some accuracy what might be wrong with your car just by using your nose. If you’re hip to what various smells suggest. Let’s have a look – or rather, a whiff!

* Rotten egg smell –

This is indicative of a problem downstream – in your car’s exhaust system. The egg smell derives from sulfur, and is byproduct of the combustion process. To be precise, it is a byproduct of an overly rich air-fuel mixture, which in a modern car is often caused by a problem with the oxygen sensor located in the exhaust piping upstream from the catalytic converter.

The O2 sensor samples the chemical composition of the exhaust gasses and sends a signal to the engine’s computer controller, which in turn maintains an optimum air-fuel mixture. But when the O2 sensor is not working properly, the result is sometimes too much fuel in the mix (a “rich” condition) and – in a car with a catalytic converter (which is every car made since 1975) – the result is often that stinky sulfurous smell.

It’s important – if you want to avoid a big bill – to get the car checked out as soon as possible, because running it in this condition may cause the converter to overheat, which can result in the honeycomb lattice inside the converter to melt and fuse, creating excess backpressure – which will result in progressively worse performance and declining gas mileage.

Be advised that replacing just one tits up catalytic converter can cost a couple hundred bucks. And many new cars have two – or more – cats.

* A sickly sweet smell –

This is usually leaking (and sometimes, burning) anti-freeze, also known as engine coolant. The neon green (and in some cars, orange-red) fluid that goes in the radiator. This fluid is used to carry heat away from the engine and radiate the heat – via the radiator – to the surrounding air. It is also used to carry heat into the passenger compartment, to keep you warm in winter.

Places to look for leaks include all the rubber hoses that plumb the radiator (at the front of the car, in most cars) to the engine. There are also hoses that (typically) run from the engine to the cowl area (just below the base of the windshield). These are the hoses (usually, two medium sized ones) that circulate warm engine coolant to the cabin, to provide heat.

The radiator itself is also a potential suspect.

Provided the leak is small – you usually have time to get the problem (such as a leaking hose) fixed before it leaves you stuck by the side of the road. But don’t ignore the problem – or it will eventually leave you stuck by the side of the road. Small leaks can become big ones, in part because your car’s cooling system is pressurized. Lose enough coolant and the car will overheat – and you’ll be calling AAA (NMA, if you’re Libertarian-minded!)

If you smell that sickly sweet coolant smell inside the car, consider it an emergency situation – because coolant is leaking from the heater into the passenger compartment and if not found and fixed, you’ll be dealing with an epic mess that might require ripping out and replacing the carpets. Just for starters. A leaking heater core can ruin your car, which will definitely ruin your day.

* Acrid, burnt smell –

This is typically one of two things (hopefully not both of them at the same time): Clutch – or brakes. It stands to reason that the smell is similar in both cases because the smell derives from the heating up (and burning up) of the friction material used in both.

Usually, context will provide the clue as to which of the two it is.

If you’ve been “riding” the brakes (keeping consistent pressure on the brake pedal) to avoid over-speeding the car as you’re descending a steep grade and begin to smell that smell, it is probably your brakes. This tends to happen more often in automatic-equipped cars – whereas in a car with a manual transmission, you can gear down to limit the car’s speed. Of course, you can also gear down in an automatic-equipped car (move the selector one notch down from “Drive” – or depress the “overdrive off” button that most modern automatic-equipped cars have) but people sometimes just keep pressure on the brakes instead. Overheating the brakes – pads and rotors – can cause permanent damage, so try to avoid it by not riding the brakes.

Riding the clutch – keeping it partially engaged for too long, as beginner drivers just learning to work a stickshift often do – will also produce that funky acrid smell. It is the smell of the friction material going up in smoke – literally. If you do smell it, look out (as OJ used to say). It’s the smell of trouble headed your way.

Concerned you’ve cooked the clutch?

There’s an easy way to know for sure: At about 25 or 30 MPH, with the transmission in a higher gear (fourth or fifth in a car with a five speed manual) floor the accelerator. If the clutch is toasted, it will slip – and you will know it. The engine speed will increase noticeably as you press down on the accelerator – but the car’s speed will not increase along with it. And you will probably smell that smell.

Time to see The Man.

* Moldy, musty smell –

This one means moisture. Water . . . inside the car. Water that’s been inside the car for long enough to grow things. Like mold. This is very bad news. It means the car leaks. Or – much, much worse – it has been playing U-boat.

Flood-damaged cars can often be sussed out by their smell. But you might not smell the moldy smell – because it’s been covered up by odor-maskers such as Febreze (or similar). If you are checking out a used car and it reeks of being spray-bombed with something, the seller might be up to something. Touch the carpets with your bare hand, especially the areas up above the footwells. If you feel any dampness, run – do not walk – away from that car. Ditto water stains on the door panels, or rust-covered fasteners inside the passenger compartment. If the car is older and the carpets/seats are new, find out why. It could just be the owner wanted to fix ratty carpets and seats to make the car more appealing.

But it could be something much worse.

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