Car exhaust smoke: what do the different kinds and colours of smoke mean?

Is your car suffering from a smoky exhaust? Our troubleshooting guide tells you the causes and what you need to do to fix it

British Steam Car

Most of the time, your car’s exhaust should barely even be noticeable, quietly pumping out spent gasses from the combustion process from the back of your vehicle. But if a problem were to arise, then blue, white or black smoke could start to appear from the exhaust pipe.

This unusual exhaust smoke could be coming out of your exhaust when the car is stationary or accelerating, and it may well cause an immediate sense of dread that an expensive repair bill is on the horizon. While exhaust smoke from petrol or diesel cars could indeed be a sign of a serious mechanical issue, however, this is not always the case. 

So what exactly does a smoking exhaust mean? There are a number of potential causes of pollutants coming from your car's rear end, and our handy guide is here to help you figure out what's going on with your car.

Diagnosing a smokey exhaust: what to look for

The first step is to identify exactly when the smoke is coming out of your car’s exhaust. If it only appears when you are accelerating, that might indicate a different problem than if it only appears when stationary, or is a constant when the engine is running.

Once you've worked out when the smoke occurs, next you need to try and determine what kind of smoke it is. Is it white, grey, or with a hint of a blue tinge to it? Or is it thick and black? Does it evaporate as soon as it's in the air? Or does the smoke linger and come with an acrid smell? If it's black smoke, then does it leave soot on the road under the exhaust tip when the car is stationary?

BMW X1 - exhaust

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.

The real issue is with cars that have been poorly maintained, as smoke from the exhaust could be a clear sign of neglect. This can be a useful clue when buying a used car, because if the service history is incomplete, a smoking exhaust can be a tell-tale sign of underlying problems.

The smoke you see can be caused by different things, depending on whether your car has a petrol or diesel engine. If you drive a hybrid, you too could see your exhaust putting out something that you might not be prepared for.

We've separated the different kinds of smoke you'll come across to help you quickly identify what problem causes which smoke. We’ve also indicated whether the problem will need to be fixed and whether it's a costly repair. The summaries for each exhaust smoke type are directly below but you can scroll down the page for a full explanation…

White smoke from the exhaust: This could be steam caused by condensation in the exhaust pipe or a more serious issue caused by an engine coolant leak. Excessive amounts of white smoke could indicate head gasket failure.

Blue smoke from the exhaust: Oil is being burned. There are a number of potential causes, with the more severe being worn valve seals, piston rings or turbochargers.

Grey smoke from the exhaust: This could be excess oil, a PCV valve failure or a transmission fluid leak on automatic cars.

Black smoke from the exhaust: In a petrol car, this suggests too much fuel is being burned and could be a sign of air filter or fuel injector problems. In diesel cars it’s more likely to be soot build-up or the diesel particulate filter cleaning itself. A longer drive - ideally on a motorway to enable higher speed and revs - should give the filter a chance to clean itself, resolving the problem.

White smoke from an exhaust

The most common form of smoke from an exhaust isn't actually smoke at all. When a cold engine is started, it soon begins to heat up, and a byproduct of this is water vapour. This creates condensation within the exhaust system, which then turns into steam as temperatures rise within the engine.

Exhaust emissions

Once a car has warmed up, this steam soon evaporates. However, problems can occur if you only use a car for short journeys, meaning that the exhaust system doesn't get completely warm all the way to the exhaust tip. If condensation forms in the system and doesn't clear, it can corrode the inside of the bare steel of the exhaust, leading to rust that could develop into exhaust leaks and blowing from the system - where exhaust gases leak out. This could lead to an MOT failure due to incorrect readings on the emissions test.

Because hybrid cars (especially plug-ins) don't always fire up the engine straight away, this can delay you from seeing steam coming from your exhaust. Depending on how much battery range you drive on, you could see steam from your exhaust well into your journey. But this again is nothing to worry about, it's just a delayed reaction to the same heating process mentioned above.

If the white smoke coming from the exhaust is thicker and doesn't stop, it is still steam, but also an indicator of a more severe issue that will be caused by your car's coolant leaking into the engine. This can be caused by a failed head gasket (the seal between the engine block and the head that sits on top), which could be a pricey repair, but not as expensive as needing to repair a cracked engine block or cylinder head.

If the leak is minor, but won't go away, then it will need repairing because leaving it will only make matters worse. It could even result in engine failure, which will be a very expensive repair. This could mean replacing the engine entirely, although if it's too expensive to do this, the car will probably be written off.

Blue smoke from an exhaust

If you see blue smoke from your car's exhaust, it will probably be accompanied by a burning smell. It means that there is oil entering the system somewhere. Engine oil is designed to lubricate moving parts, and it doesn't enter the fuel system if a car is running properly.

If your car has been serviced recently - or you've serviced it yourself - it could be that too much oil has been put back into the system, and this excess oil is burning off. If the smoke stops after a while, then there's no problem. Another cause could be that some oil has spilled onto the exhaust while being topped up, only to be heated and burnt off by the hot exhaust while you're driving.

If the blue smoke is happening in a car that's covered a high mileage, it might mean that the valve seals or piston rings are worn, which is allowing oil to enter the fuel system around the cylinders or valves. The remedy for this is new seals all round, and as you'd expect, this is a costly job as it means taking the engine apart to get to them.

Check engine oil - Nissan sponsored

Burning oil will also mean you're using more oil than normal, so it's worth checking the dipstick more regularly to see how much oil loss is occurring. If it's minor, then simply topping up with oil will be easier than repair, as long as the oil loss doesn't get any worse.

If you're seeing blue smoke from a turbocharged car, then another reason for it could be a worn-out turbocharger. This will need rebuilding or replacing, which is also an expensive job.

Grey smoke from an exhaust

Like blue smoke, grey smoke could be a sign of excess oil burning somewhere in the engine, or a turbo needing attention, but there are other causes. One might be a faulty PCV valve. PCV stands for Positive Crankcase Ventilation, and this device is one of the most basic forms of emissions control, as it draws unburnt fuel back from the lower part of the engine to the top.

This helps reduce vehicle emissions, especially when the engine is cold, but over time it can wear out. Smoke caused by PCV failure will look serious, but PCV repair is a relatively straightforward job when compared to some of the other causes on this list.

If you drive a car with an automatic gearbox, then another reason for grey smoke might be that transmission fluid is being drawn into the engine through a leak in the system. Again, this could be a costly repair and will definitely need attention from a garage.

Black smoke from an exhaust

Like the other types, there are also different causes of black smoke, and one of the main factors to take into account is whether your car is petrol or diesel-powered. In a petrol car, black smoke is a sign that too much fuel is being burnt. To remedy this, you should first check or replace your car's air filter - this should be easy to find but check your owners’ manual if you are unsure. If that's fine, then the next step is to check whether the fuel injectors are clogged and the fuel pressure regulator is clean, but this is a job better suited to a professional.

Twin test - Audi A5 - TDI badge

A cause of black smoke in diesel cars could be soot build-up from unburnt diesel. The diesel particulate filter (DPF)  is designed to trap soot from unburnt diesel before it pumps out of the car's exhaust, but if you only ever drive a diesel car at low speeds, the soot deposits can build up over time, and could result in a warning light flashing on the dashboard.

To remedy this, the best course of action is to get the fuel/air mixture flowing through the car faster, by driving it faster. Find a nearby dual carriageway or motorway and accelerate your diesel car to 70mph briskly, this should dislodge the soot which will appear as a ball of black smoke behind the car, and probably leave soot deposits on the road, too.

This should clear the warning sign on the dash, and might help your car feel a bit faster to respond, too. If the warning light flashes up regularly, then a diesel car might not be right for the kind of driving you do, and it might be worthwhile changing to a petrol or hybrid model instead.

If you're looking at a used diesel car that seems to pump out black smoke in regular driving, then get under the car to make sure that the DPF is still part of the system. Some unscrupulous garages have been known to remove the DPF from a troublesome car to stop issues arising, but this will cause the car to permanently pump out diesel soot, and will mean a potential MOT failure in the future.

Want to keep problems like this at bay? Take a look at our list of the most reliable cars to buy

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