If the engine is the heart of your car, think of the exhaust pipe as the digestive tract – the exhaust smoke output can provide information on the overall health of the car.
However, deciphering what has gone wrong can be a whole different ball game. There are a few different colours of smoke that can come out of your car’s exhaust pipe, with some spelling an expensive trip to the mechanics if you don’t turn your car off and call a tow truck immediately.
We became aware of the lack of understanding surrounding smoke from the exhaust when a van (pictured above) was spotted with white smoke billowing from its exhaust pipe, and the driver seemed to be carrying on as if nothing were amiss.
But with that being said, here’s how you can decipher what your exhaust smoke colour says about your car.
What does it mean when there is white smoke upon engine start?
White smoke on initial start-up is not uncommon and is no reason to be alarmed. If it goes away once the car is warmed up, then you have nothing to worry about.
This is caused by condensation building up in the exhaust pipe while it has been sitting. Once the exhaust pipe heats up, it converts the sitting condensation into steam, which is then expelled out the tip.
There shouldn’t be a lot of it, and it should stop after 10 to 30 minutes (depending on how cold the climate is).
What does it mean when there is black smoke coming from my car?
Black smoke can be a bit more concerning to the overall health of your vehicle, but it isn’t as urgent to fix as a few others, so long as the amounts are not excessive.
The most common black smoke you'll see from an exhaust is typically associated with diesel vehicles; it often indicates an issue with the diesel particulate filter (DPF).
A DPF takes the shape of a metal tube integrated into a vehicle's exhaust system. Key to the DPF's operation is a large honeycomb-like filter, through which exhaust gases are passed to trap harmful particulates (i.e. minute particles and soot) and prevent them from being released into the atmosphere.
Through a process known as regeneration, the particulates trapped in the filter are then burned off at a high temperature (around 350 to 500 degrees Celsius), releasing small amounts of carbon dioxide (among other less harmful substances) out of the vehicle's exhaust pipe.
This process can occur through 'spontaneous regeneration' created by passive heat build-up on long highway drives, through a manual button activated by the driver, or by the car's computers themselves, which can inject more fuel into the exhaust stream to oxidise the fuel, create heat, and clear the DPF.
If your DPF isn’t working effectively, it will cause plumes of black smoke to exit the exhaust pipe when you accelerate.
It can also signal an issue with your fuel system – things like a blocked air filter, boost leak, and oxygen sensor failure can cause the car’s engine control unit to overcompensate and send too much fuel to the cylinder.
When compression occurs, the engine may not burn all of the diesel in the cylinder, and unburnt fuel in the form of black smoke, is sent out of the tailpipe.
While less common, black smoke from petrol cars can also indicate that the car is sending too much fuel to the cylinder. The most common culprits are a faulty oxygen sensor, a boost leak, or a clogged air filter.
This is unburnt fuel making its way out of the exhaust port, through your exhaust pipe, and out the rear. You will likely experience a decline in performance, and newer vehicles may also trigger a “check engine light” or enter limp mode when the air-fuel mixture is too rich.
What does it mean when there is blue smoke coming from my car?
Blue smoke is one of the more concerning colours that can come from your car; it often indicates that excessive oil is being burned.
All internal combustion cars, to some extent, burn small amounts of oil while the combustion chambers are lubricated; however, this is minuscule and often does not produce smoke.
Blue smoke is commonly associated with older, tired engines when the valve stem seals or piston rings have worn out, allowing too much oil to enter the cylinder. This heated oil then produces a blueish-grey smoke.
However, if your car has relatively low kilometres, i.e. 100,000km or lower, it usually means that a piston ring or valve seal has failed on the engine.
For higher-kilometre engines it's not overly urgent for repairs, as at best you’ll be in for a rebuild of the top end of your engine, or at worst a full rebuild to replace the piston rings. Still, oil levels will need to be monitored closely, and thicker oil may be recommended as a temporary fix. Although, the issue will only get worse over time.
If your car is relatively new and producing excessive levels of this colour smoke, then it is imperative to book it in for a warranty claim or take it to a mechanic as soon as possible.
What does it mean when there is white smoke when the engine is warm?
Perhaps one of the worst kinds of smoke you can find coming from your exhaust are plumes of white clouds. It is imperative that you switch the car off and call a tow truck as soon as possible if this happens.
The most common culprit of white smoke is a blown head gasket or cracked cylinder head, which allows coolant/water to enter the cylinder and burn off, producing the white smoke. The problem is that it also mixes with the oil in your engine, and ignoring it will likely damage your engine beyond a monetarily responsible repair.
Turning the car off before too much damage is done is imperative, and the engine can likely be saved, requiring only the cylinder head to be machined/replaced and a new head gasket to be installed.
White smoke can also be caused by a failure of the exhaust gas recirculation system (EGR), and this is likely what happened to the van in the first image.
The EGR is a piece of emissions equipment that recirculates exhaust gases back into the engine to reduce nitrogen oxide levels by reducing the combustion temperature. Most modern cars are equipped with this system, but as emissions laws became more stringent, EGR valves became more complex and began to be cooled by the engine coolant system.
Sometimes, cooled EGR valves can fail, allowing coolant to enter the exhaust gas section and inject water directly into your engine.
This has the same effect on the engine as blowing a head gasket, but can also go further by causing something called a “hydrolock”, where the cylinders fill up with liquid and prevent the engine from completing a revolution – locking it in place. Again, this could result in a brand-new engine.
Another less common occurrence of white smoke can be over-fuelling from a stuck fuel injector. Again, this should not be ignored.
Zane Dobie comes from a background of motorcycle journalism, working for notable titles such as Australian Motorcycle News Magazine, Just Bikes and BikeReview. Despite his fresh age, Zane brings a lifetime of racing and hands-on experience. His passion now resides on four wheels as an avid car collector, restorer, drift car pilot and weekend go-kart racer.
