As reported to the FAA. Reports are unverified, and codes are left as the FAA publishes them.
What was found
WITH THE APU ON, BLEED AIR OFF, DURING THE NUMBER 2 ENGINE START (WHICH WAS FIRST TO BE STARTED) SMOKE STARTED TO COME INTO COCKPIT.PARTICULARLY THRU THE RT SIDE EYEBALL VENT BUT ALSO THRU BOTH WEMACS. IT CONTINUED DURING THE 2ND ENGINE START. WE SHUTDOWN BOTH APU AND ENGINES. NO OTHER ABNORMAL INDICATIONS NOTED. THIS WAS OUR SECOND LEG OF THE DAY WITH NO ISSUES ON EARLIER FLIGHT. R & R THE WATER SEPARATOR SOCK. R & R THE APU IAW MM 49-10-00. OPS & LEAK CHECKED GOOD.