MD80 FLT CREW HAS SMOKE/FUMES IN COCKPIT AND CABIN DURING TAXI.

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MAINT PERFORMED A CHK OF THE L THRUST REVERSER BEFORE OUR DEP BASED ON THE INBOUND CREW WRITE-UP OF A NOISE IN THE L REVERSER. THEY DETERMINED IT WAS WORKING CORRECTLY AND SERVICED THE L HYDRAULIC SYSTEM WHICH WAS SLIGHTLY LOW. WE STARTED THE L ENGINE DURING THE PUSHBACK FROM GATE XX. THERE WAS NO DELAY IN OUR SINGLE ENGINE TAXI OUT OF SPOT Y. ONCE WE REACHED TXWY K WE STARTED TO SMELL SOMETHING LIKE A BURNING SMELL. WHEN WE BEGAN OUR L TURN ONTO TXWY L THE COCKPIT BEGAN TO FILL WITH SMOKE AND FUMES. I INSTRUCTED THE FO TO CALL THE RAMP BACK AND TELL THEM WE WERE RETURNING TO GATE XX. I TOLD GND CTL THE SAME AND CALLED THE FLT ATTENDANTS TO ASSESS THE CABIN SITUATION. THEY HAD THE SAME CONDITIONS BUT WERE IN AGREEMENT WE COULD RETURN TO THE GATE UNDER THE PRESENT CIRCUMSTANCES. OUR TOTAL TIME FROM OUT TO IN WAS 9 MINS. I DID NOT DECLARE AN EMER SINCE OUR RETURN TO THE GATE WAS UNOBSTRUCTED AND QUICK AND THE SITUATION IN THE CABIN SEEMED UNDER CTL. WHEN I ARRIVED AT THE GATE; THE SMOKE AND FUMES HAD DISSIPATED AND MY PLAN WAS TO LEAVE THE PAX ON; HOWEVER THE SMOKE AND FUMES STARTED TO FILL THE CABIN AGAIN AND I INSTRUCTED EVERYONE TO EXIT VIA THE FORWARD ENTRY DOOR IN A TIMELY MANNER. SUPPLEMENTAL INFO FROM ACN 707450: AFTER PUSH OUT WE STARTED THE L ENGINE AND BEGAN TO TAXI. AS SOON AS I TURNED THE PACKS ON AFTER ENGINE START WE COULD SMELL SMOKE. A FEW SECONDS AFTER THAT WE COULD SEE SMOKE IN THE COCKPIT AND OPENED OUR WINDOWS. THEN THE FLT ATTENDANTS CALLED AND SAID THEY HAD SMOKE IN THE CABIN. WE TURNED OFF THE AC PACKS AND DECIDED TO GO BACK TO THE GATE.