What do the Different Colours of Exhaust Smoke Mean?

Jack Dreyer | Friday 9th December 2022 9:30am

Cars sit in traffic as exhaust smoke drifts between them.

There are few things worse than driving behind a large vehicle, usually an old tractor or bus, while it’s spewing out exhaust fumes and smoke. In cinema, smokey exhausts are used as an almost comic image of something really wrong with a car – or at least the driver who neglects to do anything about them.

But what do the colours mean?

Let’s find out.

Exhausts tell you a lot

To begin with, it’s worth addressing that exhausts can tell you a lot about the health of a car’s engine system. Because it’s the component responsible for taking all the burnt gases and particulate matter out of an engine safely – what’s coming out of it can be a really useful indicator about what’s going on underneath the hood.

For example, the flames you’ll see from exhausts of modified cars are actually due to modifying the injectors to inject more fuel into the engine. As the fuel doesn’t get entirely burned, it’s burned further down the line and comes out as flames at the end of an exhaust.

While we never recommend modifying your car in this way, if this is happening unintentionally then you should book your car into an engine service and check up immediately.

While often less dramatic than boy racer flames, the colour of smoke is just as good an indicator of where problems lie – after all, there’s no smoke without fire!

White smoke

White smoke, likely condensation, coming out of a car's exhaust during cold weather.

If this dissipates quickly in the air, then it’s likely just steam that’s created by the heat of the exhaust. A lot of air and air moisture gets into the exhaust line when your car’s off so it’s normal to create condensation until it’s warmed up more.

But if it’s clearly white rather than simple condensation in the cold then you could have a problem with coolant leaking into the engine system. The coolant will often be overheated to the point of burning off – creating a consistent white exhaust smoke. While they’re so close, the coolant and engine systems shouldn’t ever mix.

If this is excessive, then you could have a blown head gasket.

Blue or grey smoke

Blue-grey smoke coming out of a car's exhaust, indicating oil being burned at some point in the line.

While blue sounds startling, the colour is often in between blue-grey. The colours caused by oil being burned at some point in the system and varies in severity and cause. The lighter the smoke, the less oil is likely to be getting burned. Darker blue-grey smoke could indicate worn seals – often the valve seals or piston rings.

Black smoke

Black smoke comes out of a car's exhaust – possibly indicating an issue with fuel not being burned properly.

If the smoke is black, then that’s usually a fuel problem – too much fuel is being burned, or not burned properly, and this results in sooty carbon coming out of the exhaust.

This is reasonably normal on diesel vehicles – because diesel is a thicker liquid, it burns less efficiently and therefore creates more byproducts (namely, carbon emission). But this usually stops after a minute or so, once the engine has warmed up. It can be caused by a build-up of soot in the filter too, so going for a drive on the motorway can help to clear things out.

If this continues to happen, then there could be an issue with the diesel particulate filter.

On petrol cars, black exhaust smoke indicates that there’s a problem with the fuel injectors or somewhere in the air intake line. While a small amount of black smoke doesn’t necessarily indicate an immediate problem with a petrol car, it’s likely to get worse without attention.

Trust the experts at your local Kwik Fit

If you have any smoke coming from your car exhaust that you’d like checked out, the experts at your local Kwik Fit are always on hand to help. Find your local centre.

Any facts, figures and prices shown in our blog articles are correct at time of publication.

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