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The specific color of the exhaust smoke can help diagnose the engine problem. According to Truck Parts & Service, problematic exhaust smoke comes in four different colors.

Blue exhaust smoke may indicate burning oil or a faulty fuel nozzle. "Blue smoke is mainly an indication of an oil consumption problem," says Peter Blonde, powertrain service engineer with Volvo Trucks North America. "Possible causes are leakage from the turbo bearing housing, with oil leaking into the charge air cooling system and/or worn cylinder liners and piston rings."

The smoke turns blue in color when the crankcase oil enters the combustion chamber and undergoes partial combustion. To diagnose the problem, start by making sure the crankcase ventilation system breather isn't plugged before checking into the more expensive potential problems such as valve guides, piston rings, cylinder walls, scored pistons, broken rings, turbocharger seal ring leakage and glazed cylinder liner walls.

Black and gray smoke usually indicates incomplete combustion, but the exact cause is sometimes difficult to determine. The problem could be incompletely burned fuel, excessive fuel or fuel distribution, the wrong grade of diesel fuel, air starvation or high exhaust back pressure. When checking for black or gray smoke, the engine should be run at a minimum operating temperature of 160 degrees F.

According to Blonde, black smoke often indicates over-fueling or excessive fuel. Check for the following: misadjusted throttle delay mechanisms or fuel modulators, bad fuel nozzles, wrong seal washer installed under nozzles, incorrect fuel injection timing, incorrect fuel settings, bad fuel injection pump, automatic timing advance not operating correctly and air in the fuel system.

High exhaust pressure and air starvation can lead to incompletely burned fuel. If high exhaust pressure is suspected, check for defective exhaust piping and muffler obstructions. To remedy air flow problems, look for air inlet piping damage or restriction, a dirty air filter, a valve leak or misadjusted valve.

Due to different temperatures outside and inside the engine, white smoke is most common upon cold start up. If the engine is in sound condition, it will disappear in a few minutes. It it doesn't disappear, it may indicate internal coolant leakage, possibly from a leaking cylinder head gasket, injector copper tube, crack head or liner or worn or broken O-rings on the injector sleeves.