Why is white smoke coming out of my exhaust?

Why is white smoke coming out of my exhaust?

Cars are complex things, and because of this, they have many different ways of telling us as drivers, when something is going wrong. Often this appears in an easy-to-see light on the dash or a written warning, but sometimes it’s more subtle.  

The colour of the smoke from your exhaust can be a real indicator of the health of the engine, if you have white smoke coming from your exhaust, be sure to read on. 

What Should Exhaust Emissions Look Like 

In an ideal world, and with a car that’s running well, you should never expect to see much from the exhaust pipe, except sometimes a little steam.  

Light-coloured white smoke/steam is normal especially when starting the engine on a cold day, the moisture from the air condenses in the exhaust as it heats up. The force of the exhaust gases then expels this as steam from the tailpipe. 

What Causes White Smoke from the Exhaust 

Large amounts of thick white smoke from the exhaust are usually caused by the engine burning water in the cylinders, producing steam that looks just like smoke. This can be caused by a few different issues, one of which is a failing head gasket.  

Unfortunately, a blown head gasket is the best-case scenario for an engine producing white smoke, other causes can be from a cracked or warped engine block, or cracked cylinder head. 

White smoke coming from the exhaust in cold weather  

A small amount of white smoke when just starting the car, that dissipates into the air quickly, is usually not a sign of anything serious, it’s actually just water vapour and is usually more noticeable on colder days. 

Minimal white ‘smoke’ when the engine is cold is caused by a similar principle to us being able to see our breath on a cold winter's day; the cold humid air condenses inside the exhaust and is then turned into steam by the extremely hot exhaust gases.  

Once the exhaust has warmed up fully, the smoke will stop.

How to Check if White Smoke is Caused by a Blown Head Gasket 

If your car is producing white smoke, it may be caused by a blown head gasket. There are a few key points to check, to help diagnose the problem 

Does the smoke continue after a few minutes of driving?

If so then there’s a good chance the engine is burning coolant, through either the head gasket or a cracked cylinder head and/or block. 

Does your coolant level drop?

Coolant should stay at a constant level, with only very minimal losses over many thousands of miles. If you’re having to top up your coolant regularly, and have constant white smoke, it’s likely your head gasket has blown. 

Does your coolant tank bubble?

If you can see bubbles rising inside your expansion tank, along with the two previous symptoms, there is a strong chance the pressure inside the cylinder is pressurising the coolant system, through a break in the head gasket. 

Try a Block Testing Kit 

An inexpensive test you can do at home is a block leak test, which uses a simple device to check if exhaust gases are escaping through the coolant system, a surefire sign the head gasket is blown.  

The kits are around £20 and involve adding some of the included liquid into the plastic device, then this is held over the coolant expansion tank with the engine running. Be sure to follow the instructions on the product, and never open the coolant tank when the engine is already hot.  

For more information on what a blown head gasket means for your car, read our guide here. 

How to Fix White Smoke Coming From Your Exhaust 

As we’ve discussed, if the white smoke or steam is thick and continues even after the engine is warm, it is likely to be a blown head gasket. You can follow the steps above to check for this.  

How to fix a blown head gasket 

In the event of a blown head gasket, the engine will need substantial work at a garage, thankfully many engines can be stripped and rebuilt without the need for expensive machining work, as long as the engine hasn’t overheated severely. 

If it has overheated, though, the block and head may have been warped - resurfacing of one or both parts will then be needed, adding more time and expense to the job. 

How to fix a cracked engine block 

Unfortunately, there are no real solutions to a cracked engine block that doesn't require a new engine. The cheapest way to achieve this is often with a second-hand engine from a breaker’s yard or dismantler.  

Is White Smoke from the Exhaust in Winter Normal? 

White smoke isn't always a serious issue, if the smoke/steam dissipates easily and stops after the engine is warm, it is caused purely by condensation in the exhaust.  

This is much more prominent in winter, due to the water vapour being easier to spot - just like being able to see your breath.

To conclude, white smoke from your exhaust is often seen on cold days and is generally harmless, but it’s good to be aware of what to look out for if it persists. For more information on car maintenance and car repair tips, check our blog. 

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