At some point you may notice an unusual amount of exhaust smoke coming from the back of your car. But what do the different colours of exhaust smoke mean and what are the potential causes? Here we’ll help you understand what your exhaust smoke is trying to tell you.
Black Smoke
Dark or black exhaust smoke is generally nothing to be worried about and is linked to the balance of combustion controlled by the ECU (Engine Control Unit).
Black smoke from your exhaust is usually due to an overly rich fuel mixture, created when the air fuel mixture drops below the optimum ratio. The spark during ignition can only combust a certain amount of fuel, so unburnt fuel is expelled through the exhaust system and combusted downstream of its intended combustion chamber.
There are several potential reasons why this could be happening:
- A leaking fuel injector
- A blocked fuel return pipe
- A broken oxygen or air flow sensor
- A fuel pressure regulator being stuck closed
A dirty air filter can also prevent clean air from reaching the combustion chamber. This can constrict the volume of air entering the cylinders, resulting in less efficient combustion.
Blue Smoke
Blue coloured smoke can be found in vehicles which have unwanted oil combining with the air fuel mixture. This means there is a contaminate in the cylinder and it is mixing the air & fuel.
This cross-contamination can be caused by engine wear to:
The differing pressure within the combustion chamber throughout the engine cycle will lead to oil being sucked through any leaks, in the same way that the fuel is sucked in through the opening of the inlet valve.
This can lead to a lack of compression within the cylinders and increased pressure within the crankcase. This will result in a decreased power output from the engine. Blue smoke is most common in modified cars due to the additional stresses on the engine from additional power output.
White Smoke
White smoke probably isn’t smoke but steam, which can potentially be harmless, or you could be a few miles away from an engine replacement.
You’ll probably notice white smoke from your exhaust during a cold start, this is nothing to be worried about as it’s created by vapour from combustion. If however your car is warmed up, the outside temperature is reasonable and your car is still producing steam, you could be looking at head gasket failure or cracked cylinder head.
A split in your head gasket will send liquids into places they shouldn’t be and could lead to coolant entering the cylinders. The engine will try to compress the coolant, leading to potentially disastrous engine failure and lots of steam pouring out of the exhaust. The size of the leak will depend on the amount of smoke produced, however if this does happen you probably won’t be able to see out of the back window due to your car becoming a steam engine. If that suddenly happens to you then pull over as soon as possible and hope the damage isn’t too bad.
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