Why Is Smoke Coming Out of My Exhaust & What The Different Colours Mean

Why Is Smoke Coming Out of My Exhaust & What The Different Colours Mean

Smoke from your car's exhaust can be a worrying thing to see, regardless of the colour, as it’s usually an indication of something wrong.  

The one upside of seeing said smoke, however, is that it can be very useful in diagnosing the issue, for differing problems cause different colours. 

In this blog, we’ll explain the potential causes for any colour exhaust smoke you’re likely to see.  

Should I see Smoke From my Exhaust? 

Even if a car is running perfectly, you may still see some light white smoke or steam from the exhaust, particularly on cold days, for the first few miles of driving.  

Older diesel-engined vehicles may also emit a small puff of black smoke upon hard acceleration, but not constantly.  

Other than those two cases, an engine that is running well should not produce visible smoke. If your car is producing smoke, pay close attention to the colour of it, as this can often tell you a lot about the nature of the underlying problem.  

What Coloured Smoke Means From an Engine 

If your car is producing smoke from the exhaust pipe, keep an eye out for what colour it is, this can either be done by having a friend rev the engine slightly whilst stationary, or checking behind you whilst driving.  

What causes white smoke from your exhaust? 

White smoke is likely to be steam or water vapour.  

A little white smoke at the very start of a journey is normal, but thick consistent clouds of white smoke mean your engine is burning coolant, which tends to indicate a head gasket failure or worse. 

For more information on white smoke, we have an entire blog article about the causes and symptoms. 

What causes blue smoke from an engine? 

Blue smoke is caused by the engine burning oil. There are a few different ways this can happen, as there are multiple sealing points stopping oil from entering the engine. Likely causes include: 

Worn turbo seals 

In cars with turbocharged engines, this is a very common cause of oil being burnt, turbochargers use oil to lubricate the spinning shaft, they have seals to prevent this oil from entering the air stream into the engine. 

Clogged PCV valve 

This is a valve that redirects exhaust gases that get past the pistons and into the crankcase (bottom half of the engine), when the system gets blocked oil can be pushed through instead of gas, and be burnt in the cylinders 

Worn piston rings 

Piston rings seal the piston to the cylinder wall when worn from very high mileage and/or poor oil servicing, they then allow oil into the combustion chamber 

Worn valve guides 

Another sealing component, valve guides hold the valves that allow air into the engine. When worn they allow oil into the combustion chamber.

What causes black smoke from a petrol engine? 

Large amounts of black smoke tend to indicate the fuel-air ratio is too rich - that is there is too much fuel for the amount of air entering the cylinders. This can be from either too much fuel, or not enough air.  

Worn or faulty injectors 

Too much fuel entering can be caused by the injectors not properly metering the fuel when spraying into the engine, allowing excess fuel to go unburnt and appear as black smoke.

Turbo faults 

The main job of the turbo is to increase the amount of air entering the cylinders, a fault with the turbo e.g. worn bearings or a stuck actuator, will have a dramatic effect on the amount of air entering the combustion chambers, causing air/fuel mixture that is far too rich. 

What causes black smoke from a Diesel engine? 

Diesel engines are different in their operation to petrol engines, whilst all of the issues listed above can be present in a diesel-engined car, there are other causes only a diesel car will experience.  

Can DPF issues cause black smoke? 

The diesel particulate filter is a diesel-specific emissions device used to reduce the amount of soot present in an engine's exhaust gases. Soot is created as part of the diesel engine cycle, but in cars fitted with a DPF, this is trapped and converted to ash.  

A problem with the DPF system can cause visible soot to be emitted, the removal of the DPF entirely is also another cause for soot to exit the exhaust. This will usually occur on heavy acceleration.  

Every diesel car from 2009 will have a DPF - and some made before then too - check out our guide on how DPF systems work and what problems you may experience. 

What causes grey smoke from your exhaust? 

Grey smoke can be caused by a few different things, if a small amount of engine oil is being burnt, this may not present as thick black smoke but a lighter colour that looks grey in daylight.  

Grey smoke from an automatic car 

Some automatic gearboxes use a modulator system that is powered by negative pressure. As the engine's intake manifold experiences negative pressure in most situations, gearbox modulators are sometimes connected to the intake manifold.  

If a problem occurs with the modulator, the vacuum in the intake manifold can suck transmission fluid into the intake, causing it to be ignited along with the fuel. This isn’t a very common problem and will only happen if the following conditions are met: 

  • The car has a traditional automatic transmission, not a twin-clutch/DSG one 
  • The car does not have a turbo - this rules out most diesel cars too 
  • The grey smoke has a burnt rubber smell to it 

Can I drive my car if the exhaust is smoking? 

If the smoke dissipates quickly after the engine has warmed up, it's probably just water vapour and the car will be okay to drive. However, if the smoke is constant, regardless of the colour, you should not drive any further than necessary, ideally to a garage or repair centre.  

Not only could the smoke-causing issue damage your engine, but it could also get you in trouble with the police if you’re obstructing the view of other drivers.  

Exhaust smoke smell in the car 

If you can smell exhaust or fuel fumes inside the car, refrain from driving as soon as possible, finishing the journey with the windows open. Being able to smell exhaust fumes inside the car is an indication there is a significant exhaust leak.  

If you can smell exhaust or fuel fumes inside your car, take it to a mechanic or garage as soon as possible, continuing to drive could cause coughing, breathing difficulties and other long-term effects. 

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