Exhaust coming from your vehicle's tailpipe should be clear. If you see smoke coming from your tailpipe, pay attention to the color of the smoke; you can use it to troubleshoot engine problems. It can tell you whether you have a leaky gasket or give you hints about your air/fuel mixture:
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You see white vapor on a cold morning: Disregard it if it stops after the vehicle warms up. If it continues after the engine is warm, a cracked engine block or cylinder head or a leaky head gasket may be letting coolant into the engine. You need professional help with this one.
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You see black smoke and you drive an older vehicle with a carburetor: The fuel/air mixture may need to be adjusted to a leaner setting or the carburetor float may have absorbed gasoline and is flooding the engine.
Black smoke from a vehicle with fuel injection usually requires special diagnosis and should be left to a repair facility.
Check to see whether the fuel/air mixture on either type of vehicle is too rich by running your finger around the inside edge of the tailpipe. (First make sure that it’s not hot.) If carbon comes off on your finger, the mixture is probably too rich.
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The smoke is light or dark blue: The vehicle is burning oil, which can indicate that oil is leaking into the combustion chambers and you may need to have your valve stem seals replaced or your engine rebuilt or replaced.
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The smoke is light gray: The car may be burning automatic transmission fluid. Check the transmission dipstick. Is the fluid dark and burned-looking? Does it smell burned? If so, changing the fluid may solve your problem.
A faulty transmission vacuum modulator, as found on very old cars, also can suck transmission fluid into the engine, where it’s burned in the cylinders and causes light gray smoke to come out of the tailpipe. Have the mechanic check the problem.
