The Color of Exhaust Smoke and What Problem It Might Indicate

On a cold Seattle morning, every car puts out a little white vapor for the first few minutes. That’s condensation burning off, and it fades as the engine warms up. Nothing to worry about.

Smoke is different. It hangs in the air, it has a color, and it usually has a smell. A healthy engine makes very little of it, so if you’re seeing smoke in the mirror, your car is telling you something. The color is the first clue.

White smoke

Thin white wisps on a cold start are normal. Thick white smoke that keeps coming after the engine is warm usually means coolant is getting into the combustion chamber. It often has a sweet smell.

The usual cause is a failing head gasket. A cracked cylinder head is less common but more serious. Either way, check your coolant level and your temperature gauge, and don’t keep driving a car that’s losing coolant or running hot. This is one to get looked at right away.

Blue smoke

Blue or blue-tinted smoke means the engine is burning oil. Where and when you see it narrows things down:

• A puff at startup often points to worn valve stem seals.
• Smoke under acceleration can mean worn piston rings.
• On turbocharged Volvos, tired turbo seals can let oil through.
• A clogged crankcase breather (the flame trap or oil trap, depending on your Volvo) builds pressure and pushes oil where it doesn’t belong.

Keep an eye on your oil level between changes. An engine that’s burning oil can run low before the next service is due.

Gray smoke

Gray is the hardest one to read. It’s often oil, just less of it, and the same causes as blue smoke apply. It can also come from the crankcase breather system or the turbo. On some older automatics, it can even be transmission fluid.

If you’re seeing gray smoke, note when it happens and bring the car in. This one usually takes a look under the hood to sort out.

Black smoke

Black smoke means the engine is getting more fuel than it can burn. You’ll usually notice worse gas mileage along with it, and sometimes a fuel smell.

On a fuel-injected Volvo the cause can be a clogged air filter, a leaking injector, a fuel pressure regulator or a sensor giving the computer bad information. On a carbureted classic, it’s often a choke that’s sticking or a mixture that’s set too rich.

What to tell your mechanic

You don’t have to diagnose it yourself. A few notes make the job faster:

• What color was it?
• When did it happen: at startup, when you accelerate, or all the time?
• Did you notice a smell?
• Has your oil or coolant level been dropping?

All of this applies to nearly any car or truck. If yours is a Volvo and you’re in the Seattle area, bring it by and we’ll tell you what we see. Regular oil changes and scheduled service are also the best way to catch these problems before they ever show up as smoke.