Reasons why your car exhaust is smoking

You may have encountered this before, with your own car or with other cars on the road. Conventional cars are powered by an Internal combustion engine that uses either petrol or diesel that gets ignited in a series of sequences in order to power the engine. After the combustion process is completed the engine releases the waste gases through the exhaust pipe in forms of smoke . 

While it is normal for a car exhaust to release smoke every now and then, the exhaust gases of a well maintained car should barely be noticeable. Visible emissions are often a sign that there is something wrong within the engine.

In this article we are going to list the different types of smokes and their main causes to help you quickly identify and diagnose them, before they cause any harm to your vehicle.

Types of exhaust smoke and their causes

There are four common types of car exhaust smoke and they are classed according to their colours. The colour of the smoke leaving the exhaust pipe should help you diagnose their potential causes.

White Exhaust Smoke

It’s often common for a car to generate white smoke when started on a cold day or under very humid conditions. This smoke is also known as condensation burn off, where the hot exhaust pipe burns off condensation (water) build up inside the exhaust pipe and generates a steam like smoke. This is something you shouldn’t be worried about.


This only becomes concerning when the white smoke is continuously coming through the exhaust pipe. This means that there could be a coolant/water leak inside one of the cylinders, which can be due to either a cracked cylinder head, blown head gasket or a cracked engine block.

Black Exhaust Smoke

Black smoke is often a result of a bad/incorrect fuel and air mixture inside the combustion chamber. One of the factors to take into account is excess fuel supply caused by a faulty fuel injector. This means that the injector is either spraying for too long or leaking fuel leading the engine to run rich.


An engine runs rich when there is either too much fuel or too little air coming through the intake valves. This could be due to a faulty/leaking fuel injector, a bad air filter or a faulty fuel regulator.

Black exhaust smoke can be very common on diesel engines, due to soot build-up from either bad quality fuel, unburnt fuel, or a clogged-up DPF (Diesel Particulate Filter).

Blue Exhaust Smoke

When you see blue smoke out of the exhaust pipe it is mostly a result of the oil being burnt along with fuel inside the combustion chamber. It is usually accompanied with a burning smell. This means that there is oil entering the system somewhere. Often caused by oil overfill , unburnt excess fuel which should be fine if it only appears for a short period of time. Blue smoke can be fairly common in high mileage cars, it often indicates that there is a potential failure within some internal components of the engine such as the valve seal or piston rings resulting in oil mixing with the fuel inside the cylinder. Engine oil is designed to lubricate moving components inside the engine, whilst avoiding any contact with the fuel system.

Grey Exhaust Smoke

Like blue smoke, grey smoke could be a sign of excess oil burning in the engine. It can also be caused by oil leak within the turbocharger or a faulty PCV (Positive Crankcase Ventilation). The PCV is an engine emission component that draws unburnt fuel back from the lower part of the engine to the top to avoid excess fluid build up to burn incorrectly.


The main causes of smoke from an exhaust can vary depending on your car and its condition. If your car is well maintained and serviced regularly, then any visible emissions from your exhaust are unlikely to be anything of concern, but it is well worth checking anyway.

We recommend that you visit your nearest/trusted service center if your car shows any of the concerning smoking signs mentioned above, to prevent any further damages.

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