What was found
5-10 MINUTES AFTER ENGINE SHUTDOWN, WITH THE APU RUNNING AND THE APU BLEED AIR ON, THE COCKPIT STARTED FILLING WITH SMOKE. THE SMOKE DID NOT SMELL ELECTRICAL. I IMMEDIATELY SHUTDOWN THE APU AND RAN THE SHUTDOWN CHECKLIST. AT THAT TIME THE SMOKE DISSIPATED. A FBO EMPLOYEE OBSERVED SMOKE COMING OUT OF THE ACM INLET ON TOP OF THE FUSELAGE IN FRONT OF THE VERTICAL STABILIZER. WE RECEIVED NO ASSOCIATED CAS MESSAGES. REMOVED DEFECTIVE HP VALVE PN 9914520-2 SN D1-92 AND INSTALLED NEW HP VALVE PN 9914520-2 SN D3060. PERFORMED OPS AND LEAK CHECKS WITH NO DEFECTS NOTED. ALL WORK PERFORMED PER CESSNA AMM 36-10-00.