Smoke and Mirrors – Demystifying Exhaust Smoke Coloration

I’m certain at some point you’ve seen a vehicle in traffic producing colored smoke from the exhaust, and wondered what might be causing it. Or perhaps your own vehicle was once fogging out the entire intersection sitting at a red light. In today’s computer-controlled automotive environment, engines run more efficiently than ever before in the history of the car. Several decades ago, seeing a car fuming from the tailpipe was not uncommon, but today’s cars are much less likely to have any visible exhaust (provided the temperature isn’t low enough for the moisture in the exhaust to condense to steam, which is perfectly normal).The color of the smoke from the exhaust of a car can tell you a lot about what might be wrong with it mechanically, so we wanted to provide a brief explanation about the concerning colors that may appear at your tailpipe.

white smoke coming from tailpipe

White Smoke

Thick white smoke is caused by coolant entering the combustion chamber and burning with the fuel and air, which is typically a sign of a blown head gasket. When a head gasket blows, it allows coolant and oil to mix, and may result in a coolant leak inside the engine or an oil leak into the cooling system. This can lead to sludge in the radiator, white froth under the oil fill cap, and white exhaust smoke. A blown head gasket should be repaired as soon as possible, as coolant mixing with the oil thins the oil’s viscosity and makes it less capable of protecting the engine’s internal parts.

Other causes of coolant leaking into the cylinder may also include a damaged cylinder head or a cracked engine block, both of which are costly repairs that should likely be undertaken by a professional service center.
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black smoke coming from tailpipe

Black Smoke

Black exhaust smoke is caused by overly-rich fuel mixtures, meaning there is more fuel than air combusting in the cylinder. A clogged air filter that limits the amount of air available to the engine can cause rich fuel mixtures, but a variety of electronic parts, including oxygen sensors, mass air flow sensors, and even the fuel injectors can also cause overly-rich conditions if they fail. Check the condition of your air filter, and stop by O’Reilly Auto Parts to have your Check Engine light codes scanned to figure out the source of the problem.
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blueish tint smoke coming out of tailpipe

Blue Smoke

Blue smoke is an indicator that the engine is burning oil in the cylinders. Worn valve guide seals, as well as piston rings that are not sealing in the cylinder well, can allow oil to leak into the cylinder. When oil combusts with the fuel and air mixture, it creates blue smoke. If you haven’t seen any puddles of oil under your car, but have still noticed that you’re having to add quite a bit of oil between oil changes, oil being consumed in the cylinder is one possible explanation.

Check your oil regularly if your exhaust is emitting blue smoke, as running your car low on oil could cause engine damage and seizing. It’s also possible that the oil could contaminate the spark plug electrodes, making it impossible for them to create spark. This may lead to a rough idle, misfiring, or hard starting. Blue smoke from your exhaust will likely require you or a qualified service center to replace the valve guide seals, or worse, the piston rings, which would require a complete engine teardown in most cases.
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grey smoke coming out of a tailpipe

Gray Smoke

Gray smoke may also be indicative of an oil leak, but it can also be caused by a stuck PCV (positive crankcase ventilation) valve. When the PCV valve sticks, the pressure may build in the crankcase and cause oil leaks, as well as allowing oil into the combustion chamber. Transmission fluid burning in the engine is also a possible cause of gray smoke, which may be caused by a damaged transmission vacuum modulator that allows the transmission fluid to be pulled out of the transmission and into the engine via the vacuum lines, where it is burned in the cylinder.

Replacing a PCV valve is a relatively straightforward job, and is good practice purely from a maintenance standpoint. PCV valves are typically inexpensive and simple parts to replace, and can help preserve your engine gaskets and prevent damage. Check out our video (link) on how to replace a PCV valve. A transmission vacuum modulator, depending on the application, may be a job more suited for a professional service facility, but is by no means impossible for the DIY mechanic to replace as well.
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If you have any colored smoke from your exhaust, it’s a sign that your engine is suffering from a problem. Some may be more pressing than others, but it’s still extremely important that you fix the issues (or have them fixed) before they cause permanent damage to your engine. O’Reilly Auto Parts can help by scanning any Check Engine codes your car may have stored, as well as providing a list of possible solutions.


If you decide the repairs to your car are more than you’re comfortable tackling, we’d be happy to provide a local Shop Referral that can help fully diagnose and repair your exhaust smoke issues.


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