White steam with a sweet smell from under the hood usually means coolant is hitting hot parts or the engine is overheating; blue-grey smoke with a burning-oil smell means oil is dripping onto the exhaust; and acrid black smoke with a burning-plastic smell points to an electrical fault, which carries a real fire risk. Pull over safely, switch off the engine, and keep clear of the bonnet. This is different from smoke from the tailpipe — see white smoke from exhaust for that.
First: Stop Safely
Pull over somewhere safe and switch the engine off. Don’t open the bonnet if you see flames or heavy smoke — get everyone out and well away from the car, and call the emergency services. If it is only steam, let the engine cool for at least 30 minutes before opening the bonnet, and never remove the radiator cap from a hot engine. Don’t pour water on an electrical fire.
If the temperature gauge was rising before this happened, the cause is probably overheating — see our car overheats while driving guide and temperature warning light guide.
Reading the Colour and Smell
| What you see and smell | Likely cause |
|---|---|
| White steam, sweet smell | Coolant leaking onto hot parts, or overheating — see sweet smell from coolant and coolant leaking under car |
| Thin wisps, no smell, after rain or a wash | Water evaporating from hot surfaces — usually harmless |
| Blue-grey smoke, burning-oil smell | Oil leaking onto the exhaust manifold — see oil leaking from the bottom of the engine |
| Acrid black or grey smoke, burning plastic or electrical smell | A short circuit or overheated wiring — fire risk |
| Burning-rubber smell, light smoke | A slipping belt or hose touching a hot part — see car smells like burning rubber |
| Smoke and a smell inside the cabin too | See burning smell from dashboard or vents |
Common Causes, Most Likely First
- Overheating with coolant boiling over — check the level only when cool, per our coolant level guide.
- A coolant leak from a hose, radiator or water pump dripping onto hot parts.
- An oil leak — commonly a valve cover gasket — dripping onto the exhaust.
- An electrical fault — a damaged wire, a failing relay or a short.
- A belt or hose rubbing on a pulley or hot component.
- Spilled fluids after a service, burning off the engine — typically stops within a short drive.
What to Check Once It’s Safe
- Let it cool fully before touching anything.
- Look underneath the car for puddles — see the colour guide in our coolant leaking under car guide.
- Check the coolant and oil levels — see coolant level and oil level.
- Look for burn marks or melted insulation around wiring and the fuse box.
- Don’t restart if you smelled burning plastic or saw any sign of an electrical fault.
Let the engine and exhaust cool before touching hoses, sensors or connectors, and never open a hot cooling system. Keep hands, hair and loose clothing away from belts and fans, which can start without warning.
Possible Fixes
| Cause | Fix | Typical cost |
|---|---|---|
| Loose or split coolant hose | Replace the hose, refill and bleed | $100 – $350 |
| Radiator or water pump leak | Replace the part | $300 – $1,000 |
| Valve cover or other oil leak | Replace the gasket, clean the exhaust | $150 – $600 |
| Damaged wiring or relay | Repair the harness / replace the relay | $100 – $800 |
| Overheated engine damage | Depends on the damage — can be severe | $500 – $3,000+ |
Can I Keep Driving With This?
No. Stop, switch off and investigate. Driving with a coolant leak or an overheated engine can destroy it, and smoke with a burning electrical smell is a fire risk. If it turns out to be harmless evaporation after rain or a wash, you can carry on; otherwise have the car recovered to a workshop rather than risking it.
When to See a Mechanic
Straight away for anything other than a thin, odourless wisp of vapour. Tell them the colour, the smell and whether the temperature gauge rose first. More in our engine section and car problems hub.
Frequently Asked Questions
Is it steam or smoke coming from under my hood?
Steam is white, thin and often has a sweet smell if it is coolant. Smoke tends to be blue-grey, black or grey with a burning smell of oil, rubber or plastic.
What should I do if smoke is coming from under the hood?
Pull over somewhere safe, switch off, keep clear of the bonnet and, if you see flames, move away and call the emergency services.
Is it normal to see vapour from the hood after rain?
A thin wisp of water vapour from hot surfaces after rain or a wash can be normal and harmless.
Why does smoke come from under the hood after an oil change?
Spilled oil burning off the exhaust is a common reason and should stop soon. If it doesn’t, check for a leak.
Can I drive with smoke coming from under the hood?
No — stop and find the cause. Overheating and electrical faults can cause serious damage or fire.
