A car in good condition has no smell of its own. If you sit behind the wheel and notice something that wasn't there before, sweet, burning, fuel, oil, damp, that's the car trying to tell you something is wrong. The type of smell usually points fairly accurately to where the problem is coming from and how urgent it is. Here's how to read what the engine is telling you through your nose.
A sweet smell through the vents is almost always coolant
A sweet, syrupy smell that comes in through the ventilation and gets stronger when you turn on the heater almost always means coolant is leaking. The most common culprit is the heater core, the small radiator behind the dashboard that coolant flows through to heat the cabin air. When it starts leaking, fluid evaporates into the cabin, the windows fog up from the inside (especially the windscreen), and a damp patch may appear on the carpet on the passenger side that smells just as sweet.
Don't ignore this smell. If the cooling system is losing fluid, the engine can overheat, and overheating seriously damages the cylinder head and head gasket. Check the coolant level in the reservoir under the bonnet when the engine is cold. If it's low, or if you constantly have to top it up, the system is leaking somewhere. In the workshop this is confirmed with a pressure test of the cooling system, which shows where the fluid is being lost in about half an hour.
Smell of petrol in the cabin or around the car, stop right away
The smell of petrol is never normal. Not in the cabin, not around the car while it's parked, not after starting the engine. The most common causes are cracked fuel lines, a leaking injector, a problem with the evaporative emissions system (the system that prevents vapours from the tank escaping into the atmosphere) or damage to the tank itself.
If you smell strong petrol fumes while driving, the procedure is clear, pull over, switch off the engine, open the bonnet to let the vapours out and don't ignite anything, no cigarette, no phone flash near the car. Petrol leaking onto a hot engine or exhaust manifold is a real fire risk. Don't try to drive on "just to get home". Call a tow truck or the workshop.
Drivers of LPG-powered vehicles need to make a distinction here, LPG has a characteristic smell because of the odorant added precisely so that leaks can be detected. That smell is sharper and more chemical than petrol. Both demand that the engine be shut off immediately, but the source of the leak is a completely different system.
Burning smell, from the clutch to the wiring
The burning smell is perhaps the hardest to read because there are several possible sources. What helps is when it shows up.
- When pulling away, especially uphill and under load, it's most often a slipping clutch. The friction disc wears, overheats and gives off a sharp smell similar to scorched paper. If you also feel the engine "revving" more than the car is accelerating, the diagnosis is almost certain.
- While stopped or right after stopping, the suspects are the brake system and the exhaust manifold. A seized caliper or an old, half-on handbrake keeps the pads pressed against the disc, which heats up to red. The smell is sharp and metallic, sometimes accompanied by smoke around the wheel.
- Anytime, with a hint of plastic or rubber, that usually points to the wiring. An overheated cable, a poor connection or a faulty fan motor can smell like melting plastic. This isn't something to put off, a short circuit in a car can easily end in a fire.
If you can't reliably locate the source, the safest thing is to stop, let the engine cool and call the workshop. Driving with an "unidentified burning smell" is a risk that isn't worth taking.
Smell of burnt oil, leaking onto hot parts
If you notice a thick, suffocating smell of burnt oil under the bonnet, oil is leaking somewhere and dripping onto hot parts, most commonly the exhaust manifold, the turbo housing or the oil cooler. A typical scenario is a leaking valve cover gasket, with oil slowly dripping onto the manifold and evaporating with every drive. At first it's just an unpleasant smell, later a faint bluish smoke can appear under the bonnet.
This isn't a cosmetic issue. Oil that sprays onto hot engine parts for years is a real cause of fires, especially on older diesels with a turbo running at high temperatures. Open the bonnet when the engine is cold and look for traces of oil on the engine, around the valve cover, down the block, around the turbo. Wet, dark patches on otherwise dry parts are a clear sign that an inspection is needed.
Damp and mould smell, cabin filter and water drains
A stale, musty smell that hits you the moment you turn on the ventilation usually isn't an engine fault, it's a problem with the cabin filter or the drainage channels. A pollen filter that hasn't been changed for two seasons turns into a damp sponge where bacteria and fungi develop. The fix is to replace the filter and, if needed, chemically clean the AC evaporator.
The other version is that the water drains in the area below the windscreen, where the air intake sits, are clogged. Leaves and dust fill the channels, water can't drain and overflows into the cabin, usually onto the carpet under the passenger. If the carpet is wet and the smell isn't sweet, it's not coolant, it's plain water sitting where it shouldn't. Often it's enough to clean the drains and dry the carpet.
There's also a third variant, the smell of eggs from the exhaust, which can sometimes be noticed in the cabin too. On petrol cars this usually means a problem with the catalytic converter or the oxygen sensor. It's not urgent here, but it is a reason for diagnostics, since a faulty cat converter eventually leads to higher fuel consumption and trouble at the technical inspection.
When it's urgent, and when it can wait
As a rough split:
- Stop immediately, smell of petrol, smell of LPG, strong burning smell with visible smoke, smell of burnt oil with smoke under the bonnet.
- Drive carefully to the workshop the same day, sweet coolant smell (along with checking the coolant level), burning smell when driving uphill, smell of hot brakes after a long descent.
- Book a service during the week, damp and mould through the vents, mild oil smell with no smoke, egg smell from the exhaust.
It's also useful to tell where the smell is coming in from. If it disappears when you switch to recirculated air, the source is outside, a truck in front of you, another car in traffic. If the smell stays the same with the outside air intake closed, the source is in your car, most likely in the heating, ventilation system or under the bonnet.
In the workshop we first locate the source, we don't treat the symptom. A visual inspection of the engine bay, a pressure test of the cooling system, checking cables and insulation, looking for fuel leaks, that handles 80% of cases in half an hour. If you're not sure what you're smelling, stop by the workshop or book an appointment, it's better to check while the fault is small than to wait until it gets expensive.