Beech 1900 Captain reported after turning on the engine bleeds during climb; a foul smell and smoke began to fill the flight deck. The Captain asked for assistance from ATC and returned to the departure airport. Engine cleaning the night before was a suspected cause.

Narrative

On initial climb out of ZZZ; when bleed valves were opened an odd smell was noticed. Once the heat was turned on when climbing through 5;500 feet smoke entered the cockpit rapidly. Reacted by closing bleed valves but enough smoke had already entered cockpit to reduce visibility greatly and continued to enter the cockpit for some time after bleed valves were closed; until finally stopping on the approach back into ZZZ. Requested priority and returned to ZZZ. Landed and taxied to the ramp. First aware due to an odd smell during departure; then seeing smoke in the cockpit shortly after. Possibly an issue with the bleed system due to the reaction when turning on and off the bleed valves. After speaking with the Mechanic in ZZZ I believe it could have possibly been caused by improper cleaning of the engine during the routine maintenance performed the previous night. Shutting bleed valves; air returned back to ZZZ after requesting priority. Difficult to offer a solution to the event. Possibly entering the cleaning of the engine to be entered into the logbook rather than just being included as a 'weekly' check. This would allow the pilot to narrow the cause of the smoke down faster and react more effectively.