Most of the time, your car exhaust is nothing to worry about. Its position at the back of the car means it’s easy to forget, and it spends the majority of its time happily pumping out the spent gases from your engine as you’re driving along.
But if you’ve ever seen smoke coming from your car’s exhaust – whether you’re accelerating and see it in the rear-view mirror, or if you’re walking around your car when it’s stationary – there is sure to be a sense of dread that something bad (and expensive to repair) has happened to your car’s mechanicals. But while a smoky exhaust can be a sign of a mechanical problem, that isn’t necessarily the case with every type of smokey emission from your car’s tail pipe.
So what exactly does a smoking exhaust mean? Well, there can be a number of 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 thing you need to do is identify when the smoke comes from your car’s exhaust. If it comes from the exhaust when accelerating, that might indicate a different problem than if you see smoke when your car is stationary.
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 like clouds? Is it grey 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, does it leave soot on the road under the exhaust tip when your stationary?
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 will be worth checking anyway.
The real issue is with cars that are 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 a service history is incomplete, a smoking exhaust can let you know if a potential buy has issues that will need resolving.
The smoke you see can be caused by different things, depending on whether your car has a diesel engine or petrol power. And if you drive a hybrid, then you could see your exhaust putting something out that you might not be prepared for, too.
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.• Blue smoke from the exhaust: Oil is entering the system. It could be excess oil being burned or may point to 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.
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 the water vapour is heated up by the rising temperatures in the engine and exhaust that follow.
Once a car has warmed up, this steam soon evaporates completely. Problems can occur if you only use a car for short journeys and 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, rather than exiting at the tail pipe. 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, it can delay when you see this 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’s still steam, but it’s an indicator of a more severe issue. It 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, although if it’s too expensive to do this, the car is likely to 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 spilt 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.
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 will be a worn-out turbocharger. This will need rebuilding or replacing, which is likely to be as expensive as changing the seals in the engine.
Grey smoke from an exhaust
Like blue smoke, grey smoke could be a sign of excess oil burning somewhere in the engine, or if it’s a turbocharged car, the turbo needs 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 back 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.
If you drive a car with an automatic gearbox, then another reason for grey smoke might be that transmission fluid might be 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
Again, there are different causes of black smoke, and one of the main issues 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 towards the front of your car’s engine bay in a black plastic case. 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 garage or advanced DIY mechanic.
A cause of black smoke in diesel cars could be soot build-up from unburnt diesel. You may have heard about the diesel particulate filter (DPF), which 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 going through the car faster, by driving it faster. Find a nearby dual carriageway and accelerate your diesel car to 70mph briskly, and 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 know to remove the DPF from a troublesome car to stop the issues arising, but this will cause the car to permanently pump out diesel soot, and will mean a potential MoT failure in the future.
Now read about Real-world MPG and emissions testing: What is it and how does it work?