what do the different colours of exhaust fumes mean?

Your car’s engine has an inbuilt exhaust system made up of pipes and chambers that are designed to channel fumes away from the engine and out of the vehicle. Its other functions are to filter out noxious chemicals from the fumes in order to reduce harmful emissions into the atmosphere, as well as diminish the noises made by an engine during the burning of fuel. Because of the work that an exhaust system is doing, it’s likely that from time to time you will be able to see visible gas exiting through your vehicle. Sometimes it is completely normal and nothing to worry about, however on other occasions the colour, smell and quantity of the exhaust fumes can indicate a serious underlying problem with your car that needs to be checked out by a professional. In this article we will outline the various kinds of smoke your engine might produce and what it could mean:

Light and slightly translucent white smoke

If you can see light white smoke coming from your car, this is usually water vapour (steam) and nothing to worry about. You’ll be able to notice it when you first start your car and it is more visible in cold weather. After your car has warmed up fully, the white steam should stop. The reason that your car is producing steam is that when a cold engine is started and begins to heat up, the condensation that normally sits in exhaust pipes is turned into steam as the water vapour heats.

White steam is usually not a cause for concern however it can lead to corrosion and rusting in the exhaust pipes if you only use your car for very short journeys and don’t allow the engine to heat up fully.

Blue Smoke

Blue smoke coming out of your car’s exhaust could mean that some of the engine oil has entered into the fuel system somewhere. It’s not uncommon for blue smoke to also have a burning smell, which is a tell-tale sign that there’s a problem that needs to be fixed.

Sometimes this will happen right after your car has been serviced, which is usually because too much oil has been put back into the system and the excess is simply being burnt off. If the smoke stops after a while, then you won’t need to take it to a garage to get checked.

In cars that have been heavily used, the seals around the valves and piston heads may have become worn down, allowing oil to seep into the fuel system. In this case, you’ll need to get all seals replaced, which can be a costly procedure.

Grey Smoke

Grey smoke coming from your engine could also be an indicator that there has been an oil leak somewhere. Alternatively, it could suggest that the PCV (positive crankcase ventilation) valve is defective or worn out as the purpose of this part is for emission control. Fortunately, repairing a PVC valve is a fairly straightforward procedure. If your car is an automatic, then grey smoke could be a sign that the transmission fluid has escaped into the engine via a leak, in which case, repairs are essential and usually quite costly.

Black Smoke

In a petrol car, black smoke indicates that too much fuel is being burnt which could be because the air filter needs replacing. It could also mean that the fuel injectors are blocked or the pressure regulator needs a clean, so to be sure that this has been properly examined it is best to take it to a garage.

In a diesel car, black smoke can be caused by a build up soot, which occurs when there is an excess of unburnt fuel. Diesel cars are fitted with a particulate filter in order to trap this soot however it is possible for deposits to build up gradually if you only use your car for short and slow journeys. Luckily, this can be dealt with without having to go to a garage; simply take your car onto a dual carriageway or motorway and accelerate up to 70mph which should help the air and fuel pass through the system faster. By doing this, you’ll help dislodge some of the soot and the warning light should disappear.

Thick white smoke

A large quantity of thick white/grey smoke coming from the tailpipe could be an indication that there is a leaking head gasket, which can lead to an overheated engine. Alternatively, heavy white smoke might mean that your car has a cracked block or cylinder head which both need to be seen to as a matter of urgency.

In any case that you are concerned about the emissions coming from your car, it is best to be safe and visit a garage for a mechanic to have a look. If you leave it too long, you could cause more damage to your car and rack up a huge bill for repairs so get it checked promptly.


    Over 4 Years

    Best available rate
    XX% APR
    Total cost of credit
    Total repayment
    XX Monthly Payments of

    motorly is a credit broker, not a lender. Rates start from 6.9% APR. The rate you are offered will depend on your individual circumstances. Representative Example: Borrowing £5,500 over 48 months with a representative APR of 22.9% the amount payable would be £287 a month, with a total cost of credit of £1406 and a total amount payable of £6906.