What was found
SMELLED A SMOKE ODOR IN THE COCKPIT. FIRST IMPRESSION WAS THAT IT WAS FUMES FROM ANOTHER AIRPLANE. CONTINUED TO THE HOLDING POINT AND RECEIVED A TAXI CLEARANCE. AT THIS POINT THE ODOR HAD BECOME MUCH STRONGER.IMMEDIATELY CANCELED TAXI CLEARANCE AND TURNED BACK INTO THE RAMP. THE ODOR AND VIEW OF WHAT WAS THOUGHT WAS SMOKE HAD BECOME MUCH MORE PRONOUNCED. IMMEDIATELY SHUT DOWN THE AIRCRAFT. THE ODOR AND SMOKE HAD DISSIPATED SLIGHTLY. TROUBLESHOT, MIGHT BE AN AIR CYCLE MACHINE FAILURE. AFTER ABOUT 20-30 MINUTES OF TROUBLESHOOTING DECIDED TO DO AN ENGINE START WHILE ISOLATING THE COCKPIT ACM. TAXIED OUT TO THE RUN UP AREA AIRPLANE WOULD FUNCTION NORMALLY WITHOUT THE ACM. ACFT CONTINUED TO DESINATION.