What Black, Blue, or White Smoke from a Car’s Exhaust Means

White, black, or blueish colored smoke streaming from the back of a car is something that bothers the driver and everyone else on the road. While smoke is always cause for concern, the smoke's color can provide you with useful information that can help you figure out what is wrong with your vehicle. Read on to learn what black, white, and blue smoke coming from your exhaust means.

The Workings of Your Exhaust System

Several problems with your engine can cause excessive or abnormal smoke production.

The fuel that burns in each of your car’s engine cylinders produces exhaust gases. The gases leave your engine through valves before flowing through the exhaust manifold, the catalytic converter, and out the muffler / tailpipe. When all parts of the exhaust system work as they should, the gases that leave your tailpipe will have no color. But a problem with the process will result in a visible cloud of smoke behind your car.

Exhaust smoke from vehicles that use gasoline as a fuel source can appear in multiple colors. The most common colors for this smoke are black, white, or a grey/blue color. Diesel, because it has a higher oil base, can generally be black and sooty, especially when accelerating under high-load.

What Different Colors of Exhaust Smoke Mean

1. White Smoke

White smoke is water vapor or another chemical vapor. Because all exhaust systems produce some amount of water vapor, this will be more evident on a cold day, where you see cars all streaming a small amount of water vapor in the exhaust behind them. All exhaust has some level of water vapor, and this is especially true when you first start your car for the day. If the white smoke coming from your exhaust system becomes excessive and persists in all temperatures, coolant may be leaking into the engine’s combustion chambers.

In these cases, a head gasket failure or cracked block is normally the cause. There are other times that gaskets on a part like an intake manifold can cause a leak of coolant into the combustion chamber, but this is less likely. The function of a head gasket is to seal combustion chambers, but also act as a seal between the water passages of the block, and the head. Any problem with a head gasket will and can allow coolant into the combustion chamber or an oil passage. Once this happens, a sweet-smelling, white smoke is likely to exit your tailpipe in the form of coolant being burned. At the same time, your coolant level will fall with no visible leaks anywhere, which is a dead giveaway of a head gasket issue. To fix a head gasket is a major repair, and is outlined further in our article about head gasket repair. A cracked cylinder head is another possible cause of the white smoke coming through your tailpipe, which is similar to a head gasket in that a crack in a coolant passage will and can also leak coolant into the combustion chamber. In turbocharged vehicles, some turbos anyway, lack of coolant in the system can also cause overheating, which causes further damage and issues.

For DIY diagnosis, the next step would be to pressure-test the coolant system and determine where the coolant is going or coming from. This will determine if further engine tear-down is needed, and where.

2. Black Smoke

Black smoke from the exhaust appears when the air/fuel ratio entering the engine is too rich with fuel. This can be caused by too much fuel entering the combustion chamber, or not enough air. Usually, the issue revolves around fuel. This can be anything from a faulty fuel pressure regulator letting too high of fuel pressure pass through the system, to a leaking fuel injector. This could also be a faulty engine sensor, such as an O2 sensor. In most of these cases, a check engine light will follow, telling you that the system is running rich, and will tell you if a sensor isn’t functioning properly.

In older, carbureted vehicles, this same rich condition is caused from a carburetor not being properly adjusted, or defective parts within the carburetor.

In Diesel engines, fast acceleration and engine load is often met by the engine’s system over-fueling, which can lead to black smoke appearing on acceleration. In diesel engines, this is very normal, even to the point where high-performance turbo diesel engines can produce a ton of black smoke on acceleration, which led to the industry moniker of “rolling coal”.

In a gas engine though, a rich condition inside the combustion chamber will eventually foul out spark plugs, ruin the catalytic converter, and leave massive carbon deposits in the combustion chamber. Ultimately, engine performance will suffer, and eventually, more serious damage can and will follow. The good news with most modern engines is that chasing down the problem will be done with partial clues in the form of Check Engine Lights. Everything going on in the engine, the various engine sensors will know, and will report. At that point, it is a matter of pinpointing the problem and fixing it.

3. Blue, Gray, or Other Colors of Smoke

In addition to the black and white smoke from exhaust systems that can signal problems with your car, a mix of other smoke colors can exit through your tailpipe. Bluish gray or grayish-white smoke is an indicator that your engine is burning oil. This produces a foul burnt oil smell to all the cars behind you, and if you’re one of those cars following someone who’s burning oil, you’re quickly reaching for the A/C recirculation button in your car to stave off the horrible smell.

Burning oil can be caused by a few different problems. All engines consume a small amount of oil no matter what, but excessive consumption of oil is usually seen in the exhaust as bluish-tinted smoke. Bad valve seals are one of the biggest culprits, but worn-out piston rings, valve guides, a plugged PVC valve or crankcase ventilation system, and even over-filling the crankcase with oil can all lead to oil consumption. The easiest thing to check, of course, is the oil level. Over-filling your oil can be just as bad as under-filling, so be sure your oil is at the correct level. With all the other variants of oil consumption, you will need to take a deeper dive into the engine to determine what’s causing the problem.

For DIY diagnosis, the next step would be a compression and leakdown test to determine whether the piston rings, or valve guides are causing an issue. Inspection of the crankcase ventilation system, and the PCV valve must be done to determine that the system is properly ventilating between the crankcase and the intake. Valve seals can be visually inspected for wear, and if necessary, replaced even if the head/s remain on the engine.

Get Professional Help

With engine diagnosis, sometimes a professional hand is needed. If you need help, search our list of preferred shops in your area that can help determine what’s causing the smoke.

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