Smell is the fastest diagnostic instrument you own and the
one nobody thinks to use. It works through closed panels, it works before a
warning light does, and it costs nothing. This is a field guide: ten odors,
what each one usually is, and how quickly it matters.
Do this on a car you are considering: open the door before the seller does,
and breathe in once, deliberately, before you say anything. That first lungful
is the only honest one. Thirty seconds later your nose has adapted and you
will not smell it again until you have been out of the car for ten minutes.
Then drive it, park it, and — this is the part people skip — walk around to
the front and smell the engine bay while it is hot. Cold cars hide almost
everything on this page.
Three urgency levels run down the guide. They are ours, not a standard, and
they are about decisions rather than danger ratings.
Note itWorth knowing, worth mentioning, not worth walking away
over on its own.
Diagnose before buyingSomething is leaking, slipping or soaked.
Get it identified before money changes hands.
Stop drivingPull over somewhere safe, shut it off, and do not
continue the drive.
Sweet and syrupy, faintly like warm maple
Diagnose before buying
- Where you notice it
- At the front of a hot car with the
hood up, or through the vents with the heat running — and if it is
coming through the vents, look at the inside of the windshield for an
oily film that wipes but does not clear.
- What it usually is
- Engine coolant. A leaking hose,
radiator, water pump or heater core. Coolant inside the cabin points at
the heater core specifically, which is behind the dash and is one of the
most labor-expensive small parts on a car.
- How fast it matters
- Coolant loss ends as overheating, and
overheating ends engines. If the temperature gauge is also climbing or
there is steam, this becomes stop driving immediately.
Hot, acrid, like something left on a griddle
Diagnose before buying
- Where you notice it
- Outside the car after a drive,
strongest at the front. Often accompanied by a thin haze off the engine
that is not steam.
- What it usually is
- Engine oil reaching hot metal.
A valve cover gasket, a cam or crank seal, a turbo oil line. The oil
itself is cheap; the labor to get to the leak is the whole cost, and it
varies enormously by engine.
- How fast it matters
- A weep can run for years. A drip onto
an exhaust manifold is a fire risk and is not something to think about
for a week.
Raw gasoline
Stop driving
- Where you notice it
- Anywhere. Inside, outside, at rest,
moving. A faint whiff for a few seconds right after a cold start is
ordinary; a smell that persists is not.
- What it usually is
- A fuel leak — an injector seal,
a cracked line, a rusted tank strap, a failed filler neck. On older cars
in New Jersey, the rusted line under the car is the common one.
- How fast it matters
- There is no version of this that
waits. Shut it off, get out, and have it towed rather than driven.
Sharp and hot, like scorched carpet
Diagnose before buying
- Where you notice it
- At a wheel, after braking. Put the
back of your hand near each wheel after a drive — near, not on — and one
hot corner among four cool ones is the answer.
- What it usually is
- A brake dragging. A seized
caliper slide, a sticking piston, a parking brake cable that is not
releasing, or a collapsed flexible hose acting as a one-way valve.
- How fast it matters
- A dragging brake destroys its own pad
and rotor quickly and can boil the fluid, which is how a pedal goes to
the floor. See reading brake
wear for the measurement that confirms it.
Burning rubber
Diagnose before buying
- Where you notice it
- Engine bay, usually with a squeal
that appears on cold start or when the steering is at full lock.
- What it usually is
- A belt slipping on a pulley it
should be gripping, a hose resting against something hot, or a
tire rubbing a liner — the last one is common on a car that has been in
a front-corner impact and put back together approximately.
- How fast it matters
- Depends entirely on which. The
rubbing tire is the one worth finding, because it is rarely rubbing for
an innocent reason.
Rotten eggs, sulfur
Note it
- Where you notice it
- From the exhaust, usually under hard
acceleration or right after. Occasionally from under the hood, which is
a different and more urgent thing.
- What it usually is
- From the tailpipe: an engine running
rich and a catalytic converter working harder than it should have to.
From under the hood: a battery being overcharged and venting,
which is both a failing battery and a charging-system fault.
- How fast it matters
- Tailpipe sulfur is a "why is it
running rich" question and a likely emissions-inspection problem. Battery
sulfur is a get-out-and-open-the-hood-carefully problem.
Hot plastic, or a faint fishy sharpness
Stop driving
- Where you notice it
- Inside the cabin, often near the
steering column or behind the dash, and frequently while a specific
accessory is running.
- What it usually is
- Overheating wiring insulation or a
failing connector. The fishy note some people describe comes from
certain plastics and epoxies breaking down under heat.
- How fast it matters
- This is the one smell we would stop
the car for without debate. Electrical heat that you can smell has
already been going on for a while.
Exhaust, inside the cabin
Stop driving
- Where you notice it
- Inside, worst at idle, at a light,
or with the tailgate of a wagon or SUV open.
- What it usually is
- A leak in the exhaust ahead of the
rear of the car, or a body seal — a tailgate gasket, a rusted floor
pan — letting the car's own exhaust back in.
- How fast it matters
- Exhaust carries carbon monoxide,
which is colorless and does not announce itself. Open the windows, shut
the car off, and do not treat this as a smell problem.
Musty, like a basement
Diagnose before buying
- Where you notice it
- Strongest in the first seconds of the
air conditioning, or when you press a hand into the carpet in the
footwells and the trunk, under the spare.
- What it usually is
- Best case, a blocked evaporator
drain — cheap. Worse case, water getting in: a cowl drain, a
sunroof tube, a windshield seal, a taillight gasket. Worst case, the car
has been deep in water at some point.
- How fast it matters
- Before you buy, not after. Standing
water ruins connectors and control modules on a delay of months, and a
history report does not always know about it — see
what the history
report misses.
Overwhelming air freshener
Note it
- Where you notice it
- The moment the door opens. Cherry,
vanilla, "new car scent" from an aerosol, or the distinct smell of an
ozone treatment.
- What it usually is
- Sometimes nothing — plenty of cars get
detailed with a heavy hand. But masking is a choice somebody made, and
the three things most often being masked are smoke, mildew and pets.
- How fast it matters
- Not urgent, but re-testable: come
back in a few days, or ask to drive the car with the windows up and the
fan off for ten minutes. Perfume fades faster than what is under it.
How we use it
Every car we take in gets driven until it is hot and then parked with the
hood open, and somebody stands at the front of it doing nothing for a minute.
It is not sophisticated. It has caught a heater core, a fuel line and more
than one mouse nest that a scan tool would never have mentioned, because none
of those three set a code.
We also do not deodorize cars before you see them. If something smells, we
would rather you smell it and ask, because the alternative is that you smell
it on day four in your own driveway and wonder what else was sprayed over.
This is a general field guide to common causes, not a
diagnosis. Odors overlap, cars differ, and several of the entries above have
benign explanations as well as serious ones. Anything on this page that you
actually smell should be identified by a qualified technician before you buy
or keep driving.
