Other than flashing red and blue lights, there might be nothing worse to see behind your vehicle than smoke. Billowing fumes almost always mean there's something seriously wrong with your car. But the color of that smoke might help you understand what type of repair can help fix it.
How Your Exhaust System Works
Many engine problems create abnormal or excessive exhaust smoke. In your engine, fuel is burned in each cylinder, creating exhaust gases. These gases exit through valves in the engine and then flow through the exhaust manifold and into the exhaust pipes. Along the way, the gases travel through the catalytic converter, where any remaining fuel is converted to water to reduce emissions. Finally, the gases exit through the tailpipe.
When everything's working properly, this exhaust is colorless. But if something is amiss, you'll see a cloud of smoke coming out of the back of your car.
Color Can Point to the Cause of Exhaust Smoke
There are three common colors of exhaust smoke emitted from a gasoline-powered engine:
- Black
- White
- Blue-gray or gray-white
Black Smoke
Black smoke indicates an engine air-fuel mixture that is too rich — meaning there's too much fuel and not enough air. This is typically caused by one or more of the following problems:
Faulty fuel pressure regulator: As the name implies, this device regulates fuel pressure to the engine. The regulator accomplishes this by routing excess fuel back to the fuel tank. A faulty fuel pressure regulator can allow the engine to get more fuel than it needs, making it run rich. A restricted fuel return line can have the same effect. Many vehicles produced within the past decade have a returnless fuel system. Therefore, they don't have an external fuel pressure regulator.
Solution: Your mechanic can test the fuel pressure regulator. If it's found to be bad, it should be replaced. On most returnless fuel systems, this typically means the fuel pump assembly must be replaced.
Leaking fuel injector: A leaking fuel injector can allow excess fuel to enter the engine, causing a rich condition and black smoke.
Solution: If a fuel injector goes bad, it'll need to be replaced or remanufactured and tested. This is a big job that's best left to a professional, and will likely be pricey.
Faulty engine sensor: The engine control module (ECM) — in essence, the car's computer — receives input from sensors throughout the vehicle. It then uses this information to operate various parts, such as the fuel pump and fuel injectors. A faulty engine sensor can cause the ECM to make incorrect decisions, such as sending too much fuel to the engine. Some of the sensors that could affect this include the mass air flow sensor, manifold absolute pressure (MAP) sensor, oxygen sensors, and engine coolant temperature sensor.
Solution: Have the issue diagnosed, and replace the faulty sensor.
White Smoke
Some white exhaust smoke is normal, especially when you first start the car. Condensation can turn to vapor, providing what looks like white exhaust. But excessive white smoke likely means coolant is leaking into the engine combustion chambers. This is usually caused by one or more of the following problems:
Leaking or blown head gasket: The head gasket is located between the engine block and cylinder head. It seals the combustion chambers, as well as the coolant and oil passages. A leaking or blown head gasket can allow coolant to enter the combustion chamber. This results in white smoke coming from the tailpipe, usually accompanied by a sweet smell. It's also possible that your engine will overheat.
Solution: Your engine will need to be partially disassembled, have the gasket replaced, and then be reassembled. Other processes may be necessary if the engine has overheated. It's an expensive repair, and you'll have to decide whether it's worth it or if you should get another car instead. Get an estimate from your mechanic for the head gasket replacement. Then figure out what your vehicle will be worth, both with and without a functioning engine. If you have an older vehicle with lots of miles on it, it may not be worth fixing.
Cracked cylinder head: Like a leaking head gasket, a cracked cylinder head can allow coolant to enter the combustion chamber.
Solution: A cracked cylinder head will need to be replaced. The engine block may need to be checked as well if the engine has been overheated. If the block is found to be damaged or excessively warped, it will also need to be replaced. If the cylinder head failed because the engine was overheated — which is usually the case — the root cause of the overheating will need to be fixed. Otherwise, the problem may reoccur.
Cracked engine block: Coolant and oil passages run through the engine block. A cracked engine block can allow coolant from these passages to enter the combustion chamber, resulting in white smoke.
Solution: Usually, the entire engine will need to be replaced. Again, this may not be worth it, depending on the value of your car.
Blue-gray or Gray-white Smoke
Blue-gray or gray-white smoke usually means the engine is burning oil. If you have a turbocharged engine, a faulty turbo seal could be causing this, but if you have a more common engine, it's likely one or more of the following problems:
Oil is overfilled: If you recently changed your oil, or had it changed, it's possible you ended up with too much oil in the engine. This can cause severe damage to the engine if left unchecked, but you may first notice bluish smoke coming from your tailpipe as some of the excess is burned. You can check your oil level by driving the car for five or 10 minutes and then checking the oil dipstick. If it's up past the "full" mark, you'll need to address the situation.
Solution: If you changed the oil yourself, it's time to get back under the car. You'll need to loosen the oil drain plug so that some of the oil runs out. Don't loosen it too far — you don't want it to come flooding out. If you had the oil changed at a shop, take the car back there and explain the situation so they can fix it.
Worn valve guides or seals: The engine's valves must be lubricated without allowing oil into the combustion chamber. This is accomplished by maintaining a tight fit between the valves and guides, and by using valve stem seals. Worn valve guides or seals can allow engine oil to enter the combustion chamber, resulting in bluish or grayish smoke from the tailpipe.
Solution: If the oil-burning problem in your valve system is limited to your valve seals, some labor will be needed, but the seals can be replaced without tearing down the engine.
If you need to replace your valve guides, on the other hand, it'll be significantly more expensive. The amount of labor required is much greater, and the cost of the parts is higher as well. Another option is to replace the cylinder head.
Worn piston rings or cylinders: Piston rings seal the space between the piston and cylinder wall. If the piston rings or cylinder walls are worn, a proper seal can't be formed. As a result, oil will get sucked into the combustion chamber. The result is grayish smoke from the tailpipe.
Solution: Usually, worn piston rings or cylinder walls mean the engine should be rebuilt or replaced. Your mechanic can perform a cylinder leakdown test to determine which of these issues is causing the problem. Talk with your mechanic about your options.
Information provided by Repair Pal.