What was found
APPROX 2 MINUTES AFTER THE APU WAS STARTED ON THE GROUND, THE COCKPIT AND CABIN BEGAN FILLING WITH ENVIROMENTAL TYPE SMOKE THE COCKPIT PAC WAS IN THE OFF POSTION AS REQUIRED BY THE MEL. THE CABIN PAC WAS ON AND THE ISOLATION VALVE WAS OPEN. THE COCKPIT SUPPLY TEMP WAS NOTICED TO BE 187 AND CLIMBING RAPIDLY THE PAC AND APU ANS ENTIRE AIRCRAFT WERE ALL QUICKLY SHUT DOWN. REMOVED WATER SEPARATOR COALESCER CLOTH AND CLEANED. REINSTALLED RAN APU AND OPS CHECKED. NORMAL OPS WORK IAW CESSNA 750 M/M CHPT 49-10-00.