Private used car buying checklist: how to check exhaust smoke colours

By Chad Lückhoff

Smoking is frowned upon in many places, and it's no different when it comes to your car. There are very few instances when a car is allowed to smoke and throwing out a James Bond-style smoke screen should have you at least a little concerned.

The one benefit of seeing smoke from an exhaust is that it can help you diagnose a problem, much like the way a doctor will ask you for a sample. The smoke being ejected from the car's exhaust pipe can tell a story and let a mechanic know where to look. You can self-diagnose by checking the colour and consistency of the smoke coming from the exhaust as well. This guide is for petrol engine cars and diesel engine cars are somewhat different.

Here's what the different colours mean:

Light, thin, white smoke:

- Little-to-no concern

It's somewhat normal to see a bit of vapour-like emission from the exhaust of a car, especially first thing in the morning on startup. This is water vapour and is a result of a condensation build-up in the exhaust that is being heated up and turning into vapour. This should go away after a few minutes as all of the moisture is heated up and burned off.

Should this vapour persist, it may be a small amount of coolant that is bleeding into the combustion chamber and being burned off. This will gradually deteriorate and result in a more serious issue.

Blue-grey smoke:

- Large concern

A blue or grey tint to the smoke is the sign of burning oil. This one needs to be monitored carefully. Overfilling the oil (which is just as dangerous as too little oil) can result in the oil being forced through a breather tube and into the intake deliberately, to burn off excess.

A blue/grey smoke under acceleration points to worn out oil and compression rings on the pistons, allowing oil to pass from the sump and into the combustion chamber, where it is then burned off. Blue/grey smoke under deceleration points to worn out valve stem seals or guides. It could also point to a blown head gasket where the seal between the oil channels and the combustion chamber has failed.

In turbocharged cars, blue/grey smoke can point to a worn out or fault turbocharger that is leaking lubricating oil into the intake of the engine. It may present itself under acceleration or deceleration.

Black smoke:

- Mild concern

Black smoke indicated that fuel is being burned. This happens when an engine is running too rich, when there is too much fuel and not enough air. This can be the result of a clogged air filter, faulty injection system or even worn spark plugs.

Black smoke can usually be rectified easier than other forms of smoke and no major damage will occur if caught in a timeous manner. Prolonged over-fueling will damage the catalytic converter though. Most modern car will adjust the fueling according to the engine's requirements so you may not notice a reduction in fuel consumption but performance will be affected.

Thick, white smoke:

- Large concern

Thick, white smoke is an indication that the engine is burning coolant. This can be as a result of a blown head gasket, damaged cylinder head or worse, a cracked engine block. The latter is very costly to repair.

White smoke from the exhaust usually won't last long because the car will quickly overheat and this can exacerbate the problem, spreading cracks and warping the metal of the engine. It is advised to have this seen to as soon as possible to avoid any further damage to the engine.