There's a moment of panic when a thick cloud of black smoke rolls out of the tailpipe. Sometimes the cloud is gray or white. What does it all mean?
Puff of white smoke during cranking, before the engine starts; cold engine; gas or diesel. This smoke is unburned fuel vapor being pumped out the exhaust pipe. The white is tiny droplets of fuel. In a gas engine, the spark plugs aren't firing, or the fuel mixture is too rich to ignite. It may mean the choke
plate is stuck closed, or a load of bad gasoline. In a diesel, this cranking smoke indicates defective glow plugs, but may also come from low compression caused by worn rings or valves.
Gasoline engines typically don't belch black smoke under sudden load, but they can if the accelerator pump is set wrong, the choke plate is partially closed, or the air filter is filthy. If everything checks out mechanically with the engine, there may be something in the drive line that is causing excess load.
Black smoke in a steady stream; cold engine, gas or diesel. Black smoke contains carbon particles from fuel molecules that have broken down but have not burned completely. On gas engines, this can mean the
choke is not fully opened, the air filter is badly clogged, the ignition system is not firing every time or the carburetor or fuel injection system is set too rich. Normally fixable without tearing down the engine. On diesels, black smoke during warmup can result from a plugged air filter. It can also be a sign of weak compression in one or more cylinders or a maladjusted injection system. In gas and diesel engines, black smoke means excess fuel in the combustion chamber, which washes vital lubricating oil
off the cylinder walls. Get it fixed quickly.
Gray smoke seen only briefly when the engine first fires up; gas or diesel. This is engine oil that has gotten into the cylinder head combustion area or onto the back side of the exhaust valves as a result of worn valve stems or seals. The oil smolders and smokes at first, but then burns off when the engine gets running, and the smoke disappears. This can be an early warning sign of serious trouble. But if the engine isn't using a lot of oil, it's not a fatal condition.