Mechanics know that two things tell the tale of an engine's health more than any other: the condition of the the exhaust emissions and that of the spark plugs. Exhaust color, odor, pressure and even temperature can speak volumes about the engine's internal condition. Once you know what's contaminating the exhaust, the remainder of diagnosis consists of tracking down the source of that contamination.
Exhaust is generally colorless, but can take on a color if it's contaminated. White smoke is actually steam from water in the engine, black smoke is generally an excess of fuel, and blue or gray smoke is oil. Some white smoke is normal, particularly just after start-up and in cold weather. Black smoke generally comes with a powerful fuel odor and is not atypical for diesels. Oil smoke also has a telltale odor, and may occur at idle or higher in the rpm range, depending upon the source of the oil.
Leaking intake valve seals are among the most common sources of oil intrusion. An engine's intake valves pass through the top of the intake port and into the top-most portion of the head where the valvetrain rests. A rubber or plastic seal keeps oil from the upper cylinder head from being drawn down the valve stem under the force of engine vacuum. Leaking valve-stem seals will typically produce a greater volume of smoke at idle or under sudden deceleration, when intake manifold vacuum is the highest.
Even the best piston rings don't create a perfect seal; a certain percentage of combustion gasses will sneak past them and become "blow-by" in the crankcase. This blow-by increases pressure in the engine block, encouraging oil leaks and potentially blowing out an engine's gaskets. Blow-by is also extremely nasty from an emissions point of view, which is why we use positive crankcase ventilation systems to suck these gases out of the engine block and back into the engine's intake. PCV systems use a valve to prevent oil intrusion; if it sticks open, then oil will work its way through the PCV tube and into your intake. Oil smoke volume may increase or decrease by rpm, depending upon where the PCV tube attaches. Follow the hose from your valve cover to the intake tube or manifold then inspect the inside of the tube or manifold for streaks of wet oil.
Loose piston rings aren't necessarily worn out, they're just too small for the bore. Every engine experiences bore wear over time; the springy piston rings will compensate for a certain amount of it, but can only expand so much without sacrificing some of their tension on the bore wall. This is an important distinction to make, since bore wear not only decreases ring tension but increases the size of the ring gaps. Lower tension by itself will contribute to a certain amount of smoking, but the ring gaps are a more serious issue. To test for excess blow-by resulting from ring, bore or piston failure, remove the oil filler cap on the valve cover and put your hand over it. A regular puff of hot gas against the palm of your hand indicates excessive blow-by in one or more cylinders.
