You could have a leaking head gasket into the combustion chamber, are you losing coolant? Pull the the plugs and look for a burnt "white" coloring on it. Coolant in the combustion chambers would yield white smoke from the exhaust, as well as a "sweet" smell due to the antifreeze being burned off. You may also experience loss of power and coolant loss depending on the severity of the head gaskets condition. Another thing you can do is have it checked on a scope, by a mechanic, for really HIGH hydrocarbon emissions.
