What was found
THICK SMOKE COMING OUT OF VENTS IN COCKPIT AND CABIN. RETURNED TO FIELD. LANDED WITHOUT INCIDENT. (FLT#4298) CLOSED RAN BOTH PACKS ON APU AND AS WELL AS BOTH ENGINES FOR AN EXTENDED PERIOD AT FLIGHT POWER IAW AMM. RAN ENGINES WITH DIFFERENT POWER SETTINGS, COLD, HOT AND WARM. SMOKED FOR ABOUT 5-10 MINUTES THEN WENT AWAY AND NEVER CAME BACK. NO SMOKE SMELL OF ANYTHING AFTER POWER RUN AS WELL. SMOKE WAS DUE TO GLYCOL (DE-ICE) FLUID RESIDUE.