Thick smoke is billowing out of my car’s exhaust, can I tell what’s wrong from the colour?

I was driving back from work in the evening and in my rear view mirror noticed excessive smoke coming out of my car’s exhaust.

I didn’t notice the engine behaving differently, but I was fortunately close to home so decided to drive back very slowly in case there was a major issue.

Now I’m concerned there is a serious issue with the car and it’s going to cost me a fortune to get it fixed.

I told a friend who knows about cars and he asked what colour the smoke was. Is the colour really an indicator of the severity of the issue?

I really don’t want to be ripped off by a garage telling me this is something more serious than it truly is.

White smoke from the exhaust could just be a sign of the engine warming up on a cool day. But if persistent, it might suggest something more serious

Rob Hull, Daily Mail and This is Money motoring editor, replies: A thick cloud of smoke emerging from their exhaust is something likely to get a driver worried.

It can be a sign of a serious problem, but it can also be a minor issue. Getting it checked out properly is advisable and if in doubt visit more than one garage to get a second opinion.

We asked experts from MotorEasy, which powers the This is Money Motoring Club, what you can tell from the colour and for advice on what to do 

MotorEasy replies: Yes, the colour and thickness of a smoking exhaust can be a very strong indicator of what’s going on under the bonnet.

The cause of a smoking exhaust can vary depending on a car’s fuel type and mechanical condition so it’s always best to get persistent issues checked out to avoid engine damage, MOT failure or emissions fines. 

To make it easier for you to distinguish, there are the four main colours to look for.

White smoke 

White smoke can be a sign of nothing to worry about to something very sinister. 

At the less stressful end of the spectrum, white smoke, or condensation, is expelled if the car has been sitting in the rain for a while or the weather is especially cold. This is not usually anything to be concerned over. 

However, excessive amounts of white smoke that do not disappear once the engine temperature has increased could suggest something far more concerning.

This is often a sign of the head gasket failing, which can be expensive to fix. The head gasket is designed to seal the internal combustion process and keeps coolant and oil from mixing together.

A ‘blown’ head gasket typically occurs when the engine is overheating; when coolant flows into the combustion chamber from a blown head gasket it evaporates into white smoke.

There are telltale signs of a blown head gasket that’s easy for non-mechanically-minded drivers to check.

Firstly, the temperature gauge will move beyond the middle point and into the red to show the engine is operating too hot. 

And if you unscrew the oil filler cap and see a light-brown, creamy in texture, deposit underneath, this is a sign that oil is mixing with water in your engine.

It too could also indicate a cracked engine block – which is almost impossible to repair and would need a full engine replacement – or warped cylinder heads.

If white smoke does persist after the engine has had time to warm up, it’s best to take the car to a mechanic. 

Blue-grey smoke is typically a sign that oil is burning – and that’s not usually a good sign

Blue-grey smoke

If it’s a thick cloud of blue-grey smoke pluming out of the exhaust, this is an indication that  your car is burning oil, which may have escaped onto hot points around the engine.

There is a risk that the oil could leak into your car’s combustion chamber, which would could damage the catalytic converter in petrol and hybrid models.

This is often linked to a turbo charger issue or worn piston ring but would need to be investigated by a trusted garage.

While black smoke might appear most concerning, for diesel drivers it might just be the exhaust system’s particulate filter cleaning itself. For petrol owners, it could be a sign of something a little more serious

Black smoke 

Black smoke from the exhaust in petrol cars, usually means too much fuel is burning, often due to a faulty air filter or injector. 

If this is the case with your petrol car, book an appointment with a trusted mechanic. 

In diesels, the trigger of black smoke from the exhaust – especially if you’re accelerating hard on a motorway for the first time – is usually soot build-up or the diesel particulate filter (DPF) cleaning itself. 

If the DPF is clogged, a warning light may appear, but taking the car for a longer, faster drive can often clear it.