what color smoke? white blue black? if white could be condensation in the exhaust, coolant or the lovely one that i had...tranny fluid. the a4ld on my 93 4wd 3.0 ranger had this nice thing called a vacuum modulator on it. went out and pumped atf thru the vent lines to the heads and burned off thru the valve seals, 2 qts a wk. blue smoke is oil, could be valve seals or rings, and black is just rich. new plugs/pcv should take care of it, along w/ checking filter and usual maintenance stuff.