A well-maintained vehicle should not emit visible exhaust. Preventing a vehicle from smoking often involves simple maintenance procedures.
Common Causes of Vehicle Smoke
|VISIBLE SIGN||DIAGNOSIS||POSSIBLE CAUSE|
|Black or Gray Smoke||Incomplete fuel combustion|
|Blue Smoke||Engine oil being burned|
|White Smoke||Coolant and/or water being vaporized in combustion chamber|
This is from http://www.salemboysauto.com/faqs/faq-38.htm and was written with automobiles in mind but, is helpfull anyway.
Black smoke is raw gasoline burning. A rich condition can be the cause by a heavy or mis adjusted float, a choke stuck shut, a bad oxygen sensor, a bad map sensor, a bad fuel pressure regulator, a plugged up air filter or a bad injector. Typically, if you only get black smoke first thing in the morning, it has to do with the choke or the fuel enrichment portion of your fuel injection system. If you get black smoke all the time, get it fixed NOW. If you don't, you run the risk of wiping out the catalytic convertor and doubling your repair bill.
Blue smoke is oil burning. The tailpipe will either smoke all the time or just once, first thing in the morning. If you get blue smoke all the time, that is a sign of impeding doom. That is the first sign of the beginning of the end. Broken rings, bad pistons, damaged cylinder walls, all high dollar items.
On the other hand, if all you get is a puff or two first thing in the morning and never again throughout the day, then your problem is probably bad value guides or value guide seals. It will cost between $350-800 to replace the seals, $750-1,500 to replace the guides and seals. I always tell my customers, wait until you begin to see other symptoms of this oil usage. Wait until you start fouling plugs, wait until the oil consumption is greater than one quart every 1,000 miles. And never assume that seals alone will totally fix this problem, if the engine has 100k miles plus.
Grey smoke can really be black or blue. You can usually tell which is it by the smell or by matching other symptoms you have to the color of the smoke is easier. Others will say blue is blue and all other is really grey to black and I would agree.
White smoke is either the transmission shift modulator is bad and allowing the engine to suck and burn transmission fluid (repair costs under $100) or it's really, really bad. White smoke could also be coolant or antifreeze that is either leaking, or being forced into the combustion chambers and being burned. The exhaust will look wet and have a sweet smell to it. A bad head gasket (75% of the time), a broken head (15% of the time) or a broken cylinder wall (10% of the time) will be the reason your engine is burning coolant or antifreeze. The repairs start at $400 and go to $4,000.