What do the Different Colours of Exhaust Smoke Mean?
Jack Dreyer | Friday 9th December 2022 9:30am
There are few things worse than driving behind a large vehicle, usually an old tractor or bus, while itís spewing out exhaust fumes and smoke. In cinema, smokey exhausts are used as an almost comic image of something really wrong with a car Ė or at least the driver who neglects to do anything about them.
But what do the colours mean?
Letís find out.
Exhausts tell you a lot
To begin with, itís worth addressing that exhausts can tell you a lot about the health of a carís engine system. Because itís the component responsible for taking all the burnt gases and particulate matter out of an engine safely Ė whatís coming out of it can be a really useful indicator about whatís going on underneath the hood.
For example, the flames youíll see from exhausts of modified cars are actually due to modifying the injectors to inject more fuel into the engine. As the fuel doesnít get entirely burned, itís 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 Ė after all, thereís no smoke without fire!
If this dissipates quickly in the air, then itís likely just steam thatís created by the heat of the exhaust. A lot of air and air moisture gets into the exhaust line when your carís off so itís normal to create condensation until itís warmed up more.
But if itís 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 Ė creating a consistent white exhaust smoke. While theyíre so close, the coolant and engine systems shouldnít ever mix.
If this is excessive, then you could have a blown head gasket.
Blue or grey smoke
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 Ė often the valve seals or piston rings.
If the smoke is black, then thatís usually a fuel problem Ė 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 Ė 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 thereís a problem with the fuel injectors or somewhere in the air intake line. While a small amount of black smoke doesnít necessarily indicate an immediate problem with a petrol car, itís 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 youíd 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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